Abstracts of Interest
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Abstract: 2209.12905
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Title:The evolution of the internal structure of massive star forming regions in the Milky Way as revealed by ALMA
Download PDFAbstract: We analyze the structure of 15 protocluster forming regions in the Milky Way using their $1.3$ mm continuum emission maps from the ALMA-IMF large program. The analysis of the clouds structure is performed using the delta-variance spectrum technique. The calculated spectra display a self-similar regime on small scales as well as the presence of a prominent bump on larger scales and whose physical size, $L_{\rm hub}$, falls in the range of $\approx 7000$ au to $60000$ au. These scales correspond to the sizes of the most compact clumps within the protocluster forming clouds. A significant correlation is found between $L_{\rm hub}$ and the surface density of the free-free emission estimated from the integrated flux of the H41$\alpha$ recombination line $\left(\Sigma_{\rm H41\alpha}^{\rm free-free}\right)$ as well as a significant anti-correlation between $L_{\rm hub}$ and the ratio of the 1.3 mm to 3 mm continuum emission fluxes $\left(S_{\rm 1.3 mm}^{\rm cloud}/S_{\rm 3 mm}^{\rm cloud}\right)$. Smaller values of $\left(S_{\rm 1.3 mm}^{\rm cloud}/S_{\rm 3 mm}^{\rm cloud}\right)$ and larger values of $\Sigma_{\rm H41\alpha}^{\rm free-free}$ correspond to more advanced evolutionary stages of the protocluster forming clumps. Hence, our results suggest that the sizes of the densest regions in the clouds are directly linked to their evolutionary stage and to their star formation activity with more evolved clouds having larger protocluster forming clumps. This is a an indication that gravity pays a vital role in regulating the size and mass growth of these clumps with ongoing gas accretion.
Abstract: 2209.12961
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Title:Modelling the energy distribution in CHIME/FRB Catalog-1
Download PDFAbstract: We characterize the intrinsic properties of any FRB using its redshift $z$, spectral index $\alpha$ and energy $E_{33}$ in units of $10^{33} \, {\rm J}$ emitted across $2128 - 2848\; {\rm MHz}$ in the FRB's rest frame. Provided that $z$ is inferred from the measured extra-galactic dispersion measure $DM_{\rm Ex}$, the fluence $F$ of the observed event defines a track in $(\alpha, E_{33})$ space which we refer to as the "energy track". Here we consider the energy tracks for a sample of $254$ non-repeating low dispersion measure FRBs from the CHIME/FRB Catalog-1, and use these to determine $n(E_{33} \mid \alpha)$ the conditional energy distribution i.e. the number of FRBs in the interval $\Delta E_{33}$ given a value of $\alpha$. Considering $-10 \le \alpha \le 10$, we find that the entire energy scale shifts to higher energy values as $\alpha$ is increased. For all values of $\alpha$, we can identify two distinct energy ranges indicating that there are possibly two distinct FRB populations. At high energies, the distribution is well fitted by a modified Schechter function whose slope and characteristic energy both increase with $\alpha$. At low energies, the number of FRBs are in excess of the predictions of the modified Schechter function indicating that we may have a distinctly different population of low-energy FRBs. We have checked that our main findings are reasonably robust to the assumptions regarding the Galactic Halo and Host galaxy contributions to the dispersion measure.
Abstract: 2209.13004
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Title:A light in the dark: searching for electromagnetic counterparts to black hole-black hole mergers in LIGO/Virgo O3 with the Zwicky Transient Facility
Download PDFAbstract: The accretion disks of active galactic nuclei (AGN) are promising locations for the merger of compact objects detected by gravitational wave (GW) observatories. Embedded within a baryon-rich, high density environment, mergers within AGN are the only GW channel where an electromagnetic (EM) counterpart must occur (whether detectable or not). Considering AGN with unusual flaring activity observed by the Zwicky Transient Facility (ZTF), we describe a search for candidate EM counterparts to binary black hole (BBH) mergers detected by LIGO/Virgo in O3. After removing probable false positives, we find nine candidate counterparts to BBH mergers mergers during O3 (seven in O3a, two in O3b) with a $p$-value of 0.019. Based on ZTF sky coverage, AGN geometry, and merger geometry, we expect $\approx 3(N_{\rm BBH}/83)(f_{\rm AGN}/0.5)$ potentially detectable EM counterparts from O3, where $N_{\rm BBH}$ is the total number of observed BBH mergers and $f_{\rm AGN}$ is the fraction originating in AGN. Further modeling of breakout and flaring phenomena in AGN disks is required to reduce our false positive rate. Two of the events are also associated with mergers with total masses $> 100M_\odot$, which is the expected rate for O3 if hierarchical (large mass) mergers occur in the AGN channel. Candidate EM counterparts in future GW observing runs can be better constrained by coverage of the Southern sky as well as spectral monitoring of unusual AGN flaring events in LIGO/Virgo alert volumes. A future set of reliable AGN EM counterparts to BBH mergers will yield an independent means of measuring cosmic expansion ($H_0$) as a function of redshift.
Abstract: 2209.13061
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Title:Abundances and Transients from Neutron Star-White Dwarf Mergers
Download PDFAbstract: We systematically investigate the mergers of neutron star-white dwarf binaries from beginning to end, with focus on the properties of the inflows and outflows in accretion disks and their electromagnetic emissions. Using population synthesis models, we determine a subset of these binaries in which the white dwarf companion undergoes unstable mass transfer and complete tidal disruption, forming a large accretion disk around the neutron star. The material evolves according to an advection-dominated accretion disk model with nuclear burning, neutrino-emissions, and disk-surface wind ejection. The extreme dynamics of the entire process has proven difficult for analytic analysis, and thus currently the properties are poorly understood. The outflows from the mergers are iron- and nickel-rich, giving rise to optical and infrared emissions powered from the decay of the radioactive iron-type isotopes, calculated via the SuperNu light-curve code. We find these systems capable of powering bright, yet short-lived, optical transients with the potential to power gamma-ray bursts.
Abstract: 2209.13088
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Title:On the Localization Methods of High Energy Transients for All-Sky Gamma-Ray Monitors
Download PDFAbstract: Reliable localization of high-energy transients, such as Gamma-ray Bursts (GRBs) and Soft Gamma-ray Repeaters (SGRs), is the prerequisite for characterizing the burst properties (e.g. spectrum) and implementing the follow-up observations in the multi-wavelength and multi-messenger. Localization based on the relative counts of different detectors has been widely used for all-sky gamma-ray monitors, such as {\it CGRO}/BATSE, {\it Fermi}/GBM, POLAR, and GECAM. There are two major statistical frameworks for counts distribution localization methods: $\chi^{2}$ and Bayesian. Here, we studied and compared several localization methods based on these two statistical frameworks, by directly checking the localization probability map and credible region with comprehensive simulations. We find that the Bayesian method is generally more applicable for various bursts than $\chi^{2}$ method. We also proposed a location-spectrum iteration approach based on the Bayesian inference, which could not only take advantage of straightforward calculation but also alleviate the problems caused by the spectral difference between the burst and location templates.
Abstract: 2209.13125
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Title:GOALS-JWST: Mid-Infrared Spectroscopy of the Nucleus of NGC 7469
Download PDFAbstract: We present mid-infrared spectroscopic observations of the nucleus of the nearby Seyfert galaxy NGC 7469 taken with the MIRI instrument on the James Webb Space Telescope (JWST) as part of Directors Discretionary Time Early Release Science (ERS) program 1328. The high resolution nuclear spectrum contains 19 emission lines covering a wide range of ionization. The high ionization lines show broad, blueshifted emission reaching velocities up to 1700 km s$^{-1}$ and FWHM ranging from $\sim500 - 1100$ km s$^{-1}$. The width of the broad emission and the broad to narrow line flux ratios correlate with ionization potential. The results suggest a decelerating, stratified, AGN driven outflow emerging from the nucleus. The estimated mass outflow rate is one to two orders of magnitude larger than the current black hole accretion rate needed to power the AGN. Eight pure rotational H$_{2}$ emission lines are detected with intrinsic widths ranging from FWHM $\sim 125-330$ km s$^{-1}$. We estimate a total mass of warm H$_{2}$ gas of $\sim1.2\times10^{7}$M$_{\odot}$ in the central 100 pc. The PAH features are extremely weak in the nuclear spectrum, but a $6.2\mu$m PAH feature with an equivalent width $\sim0.07\mu$m and a flux of $2.7\times10^{-17}$ W m$^{-2}$ is detected. The spectrum is steeply rising in the mid-infrared, with a silicate strength $\sim0.02$, significantly smaller than seen in most PG QSOs, but comparable to other Seyfert 1's. These early MIRI mid-infrared IFU data highlight the power of JWST to probe the multi-phase interstellar media surrounding actively accreting supermassive black holes.
Abstract: 2209.13285
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Title:LHAASO J2226+6057 as a pulsar wind nebula
Download PDFAbstract: The Large High Altitude Air Shower Observatory has reported the detection of cosmic-ray sources in Milky Way that can accelerate particles up to PeV (= 10$^{15}$ eV) energies. These sources, so called ``PeVatrons'', are mostly unidentified. Several classes of sources, such as supernova remnants, pulsar wind nebula, or young stellar clusters can potentially be the counterparts of these PeVatrons. The aim of this work is to study a pulsar wind nebula interpretation of one of these PeVatrons, LHAASO J2226+6057, which has a relatively well covered multi-frequency spectrum. We have performed a leptonic, time-dependent modeling of the pulsar wind nebula (PWN) associated with PSR J2229+6114 considering a time-energy-dependent diffusion-loss equation. Injection, energy losses, as well as escape of particles were considered to balance the time-dependent lepton population. We have also included the dynamics of the PWN and the associated supernova remnant (SNR) and their interaction via the reverse shock to study the reverberation phase of the system. We have considered different values of braking index ($n$) and true age ($t_{age}$) for the fitting of the multi-wavelength (MWL) spectral energy distribution (SED) of LHAASO J2226+6057. The best-fit PWN model parameters and their 1$\sigma$ confidence intervals were evaluated. We have also demonstrated the impact of reverberation on the MWL SED with increasing time. Additionally, we have discussed the resultant large radius and low magnetic field associated with the PWN in question, as caveats for the possible physical connection of the pulsar as the origin of this high energy source.
Abstract: 2209.13294
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Title:Gamma-ray Pulsar Halos in the Galaxy
Download PDFAbstract: Pulsar halos are extended gamma-ray structures generated by electrons and positrons escaping from pulsar wind nebulae (PWNe), considered a new class of gamma-ray sources. They are ideal indicators for cosmic-ray propagation in localized regions of the Galaxy and particle escape process from PWNe. The cosmic-ray diffusion coefficient inferred from pulsar halos is more than two orders of magnitude smaller than the average value in the Galaxy, which has been arousing extensive discussion. We review the recent advances in the study of pulsar halos, including the characteristics of this class of sources, the known pulsar halos, the possible mechanisms of the extremely slow diffusion, the critical roles of pulsar halos in the studies of cosmic-ray propagation and electron injection from PWNe, and the implications on the problems of the cosmic positron excess and the diffuse TeV gamma-ray excess. Finally, we give prospects for the study in this direction based on the expectation of a larger sample of pulsar halos and deeper observations for bright sources.
Abstract: 2209.13324
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Title:pulsar_spectra: A pulsar flux density catalogue and spectrum fitting repository
Download PDFAbstract: We present the pulsar_spectra software repository, an open-source pulsar flux density catalogue and automated spectral fitting software that finds the best spectral model and produces publication-quality plots. The Python-based software includes features that enable users in the astronomical community to add newly published spectral measurements to the catalogue as they become available. The spectral fitting software is an implementation of the method described in Jankowski et al. (2018) which uses robust statistical methods to decide on the best-fitting model for individual pulsar spectra. pulsar_spectra is motivated by the need for a centralised repository for pulsar flux density measurements to make published measurements more accessible to the astronomical community and provide a suite of tools for measuring spectra.
Abstract: 2209.13392
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Title:Muon measurements at the Pierre Auger Observatory
Download PDFAbstract: The Pierre Auger Observatory is the world's largest detector for observation of ultra-high-energy cosmic rays (UHECRs) (above the energy of $10^{17}$ eV). It consists of a Fluorescence Detector (FD) and an array of particle detectors known as the Surface Detector (SD). Observations of extensive air showers by the Observatory can be used to probe hadronic interactions at high energy, in a kinematic and energy region inaccessible to experiments at man-made accelerators and to measure the muon component of the shower. Air showers induced by different primaries have different muon contents. With increasing mass of the primary cosmic ray particle, it is expected that the muon content in the corresponding air showers should also increase. Recent results obtained from the Pierre Auger Observatory and other experiments indicate that all the shower simulations underestimate the number of muons in the showers compared to the data. This is the so-called muon deficit. In this paper we briefly review the muon measurements, and present in more detail recent results on fluctuations in the muon number. These results provide new insights into the origin of the muon deficit in air shower simulations and constrain the models of hadronic interactions at ultrahigh energies. With the current design of the surface detectors it is also difficult to reliably separate the contributions of muons to the SD signal from the contributions of photons, electrons, and positrons. Therefore, we also present a new method to extract the muon component of the signal time traces recorded by each SD station using recurrent neural networks. The combination of such algorithms, with the future data collected by the upgraded Pierre Auger Observatory, will be a major step forward, as we are likely to achieve an unprecedented resolution in mass estimation on an event-by-event basis.
Abstract: 2209.13462
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Title:Search for Gamma-Ray and Neutrino Coincidences Using HAWC and ANTARES Data
Download PDFAbstract: In the quest for high-energy neutrino sources, the Astrophysical Multimessenger Observatory Net- work (AMON) has implemented a new search by combining data from the High Altitude Water Cherenkov (HAWC) observatory and the Astronomy with a Neutrino Telescope and Abyss environ- mental RESearch (ANTARES) neutrino telescope. Using the same analysis strategy as in a previous detector combination of HAWC and IceCube data, we perform a search for coincidences in HAWC and ANTARES events that are below the threshold for sending public alerts in each individual detector. Data were collected between July 2015 and February 2020 with a livetime of 4.39 years. Over this time period, 3 coincident events with an estimated false-alarm rate of $< 1$ coincidence per year were found. This number is consistent with background expectations.
Abstract: 2209.13574
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Title:Long-term optical and $γ-$ray variability of the blazar PKS~1222+216
Download PDFAbstract: The $\gamma-$ray emission from flat-spectrum radio quasars (FSRQs) is thought to be dominated by the inverse Compton scattering of the external sources of photon fields, e.g., accretion disk, broad-line region (BLR), and torus. FSRQs show strong optical emission lines and hence can be a useful probe of the variability in BLR output, which is the reprocessed disk emission. We study the connection between the optical continuum, H$\gamma$ line, and $\gamma-$ray emissions from the FSRQ PKS~1222+216, using long-term ($\sim$2011-2018) optical spectroscopic data from Steward Observatory and $\gamma-$ray observations from $Fermi$-LAT. We measured the continuum ($F_{C,opt}$) and H$\gamma$ ($F_{H\gamma}$) fluxes by performing a systematic analysis of the 6029-6452 Å optical spectra. We observed stronger variability in $F_{C,opt}$ than $F_{H\gamma}$, an inverse correlation between H$\gamma$ equivalent width and $F_{C,opt}$, and a redder-when-brighter trend. Using discrete cross-correlation analysis, we found a positive correlation (DCF$\sim$0.5) between $F_{\gamma-ray>100MeV}$ and $F_{C,opt}$ (6024-6092 Å) light curves with time-lag consistent with zero at 2$\sigma$ level. We found no correlation between $F_{\gamma-ray>100MeV}$ and $F_{H\gamma}$ light curves, probably dismissing the disk contribution to the optical and $\gamma$-ray variability. The observed strong variability in the $Fermi$-LAT flux and $F_{\gamma-ray>100MeV}-F_{C,opt}$ correlation could be due to the changes in the particle acceleration at various epochs. We derived the optical-to-$\gamma$-ray spectral energy distributions (SEDs) during the $\gamma$-ray flaring and quiescent epochs that show a dominant disk component with no variability. Our study suggests that the $\gamma$-ray emission zone is likely located at the edge of the BLR or in the radiation field of the torus.
Abstract: 2209.12795
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Title:Multi-wavelength Observations of MWC 297: Constraints on Disk Inclination and Mass Outflow
Download PDFAbstract: MWC 297 is a young, early-type star driving an ionized outflow and surrounded by warm, entrained dust. Previous analyses of near- and mid-IR interferometric images suggest that the emission at these wavelengths arises from a compact accretion disk with a moderate ($i < 40$ degrees) inclination. We have obtained 5-40 micron images of MWC 297 with FORCAST on SOFIA, as well as near-infrared spectra acquired with SpeX on the IRTF and radio data obtained with the VLA and BIMA, and supplemented these with archival data from Herschel/PACS and SPIRE. The FORCAST images, combined with the VLA data, indicate that the outflow lobes are aligned nearly north-south and are well separated. Simple geometrical modeling of the FORCAST images suggests that the disk driving the outflow has an inclination of $55\pm 5$ degrees, in disagreement with the results of the interferometric analyses. Analysis of the SpeX data, with a wind model, suggests the the mass loss rate is on the order of $6.0 \pm ^{3.7}_{1.7} \times 10^{-7} M_\odot ~\mathrm{yr}^{-1}$ and the extinction to the source is $A_V \sim 8.1 \pm^{2.5}_{1.5}$ mag. We have combined our data with values from the literature to generate the spectral energy distribution of the source from $0.35~ \mu$m to $6$ cm and estimate the total luminosity. We find the total luminosity to be about $7900 ~ L_\odot$, if we include emission from an extended region around the star, only slightly below that expected for a B1.5V star. The reddening must be produced by dust along the line of sight, but distant from the star.
Abstract: 2209.12461
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Title:An update on Fermi-LAT transients in the Galactic plane, including strong activity of Cygnus X-3 in mid-2020
Download PDFAbstract: We present a search for Galactic transient gamma-ray sources using 13 years of the Fermi Large Area Telescope data. The search is based on a recently developed variable-size sliding-time-window (VSSTW) analysis and aimed at studying variable gamma-ray emission from binary systems, including novae, gamma-ray binaries, and microquasars. Compared to the previous search for transient sources at random positions in the sky, we included gamma rays with energies down to 500 MeV, increased a number of test positions, and extended the data set by adding data collected between from February 2020 to July 2021. These refinements allowed us to detect an additional three novae, V1324 Sco, V5855 Sgr, V357 Mus, and one gamma-ray binary, PSR B1259-63, with the VSSTW method. Our search revealed a gamma-ray flare from the microquasar, Cygnus X-3, occurred in 2020. When applied to equal quarters of the data, the analysis provided us with detections of repeating signals from PSR B1259-63, LS I +61 303, PSR J2021+4026, and Cygnus X-3. While the Cygnus X-3 was bright in gamma rays in mid-2020, it was in a soft X-ray state and we found that its gamma-ray emission was modulated with the orbital period.
Abstract: 2209.12264
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Title:Moderate Correlation between the Accretion Disk and Jet Power in a Large Sample of Fermi Blazars
Download PDFAbstract: We present the results of studying the accretion disk vs jet power for a large fraction of all the blazars detected by the Fermi Gamma-Ray Space Telescope. The disk power is inferred from the emission line luminosities obtained from published results. As indicators of jet power, we use low frequency radio luminosity from the extended jet, maximum speed of radio knots observed in the VLBA monitoring of the pc-scale jets, kinetic energy of electrons in the jet deduced from the best-fit theoretical models of their spectral energy distribution, and gamma-ray luminosity with and without beaming correction. We obtain a significant correlation in most of those cases. However, we find that the correlations are often driven by the common redshift dependence of the compared quantities. In order to remove the redshift bias and probe the intrinsic correlation between the disk and jet power, we compute the partial correlation coefficient as well as the correlation in small redshift bins, and find that the intrinsic disk-jet correlation is still present but weaker. In the cases, in which the common redshift dependence does not affect the result, we find that blazars do not exhibit high jet power for low disk luminosities while there are both high and low jet power for high disk luminosities. This result indicates that a powerful disk is a necessary but not sufficient condition to produce a powerful jet.
Abstract: 2209.12070
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Title:The Fourth Catalog of Active Galactic Nuclei Detected by the Fermi Large Area Telescope -- Data Release 3
Download PDFAbstract: An incremental version of the fourth catalog of active galactic nuclei (AGNs) detected by the Fermi-Large Area Telescope is presented. This version (4LAC-DR3) derives from the third data release of the 4FGL catalog based on 12 years of E>50 MeV gamma-ray data, where the spectral parameters, spectral energy distributions (SEDs), yearly light curves, and associations have been updated for all sources. The new reported AGNs include 587 blazar candidates and four radio galaxies. We describe the properties of the new sample and outline changes affecting the previously published one. We also introduce two new parameters in this release, namely the peak energy of the SED high-energy component and the corresponding flux. These parameters allow an assessment of the Compton dominance, the ratio of the Inverse-Compton to the synchrotron peak luminosities, without relying on X-ray data.
Abstract: 2209.11855
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Title:The Pursuit for Gamma-ray Emitting Pulsar Wind Nebulae with the Fermi-Large Area Telescope
Download PDFAbstract: There are at least 125 Galactic pulsar wind nebulae (PWNe) that have been discovered from radio wavelengths to TeV gamma-rays, the majority of which were first identified in radio or X-ray surveys. An increasing number of PWNe are being identified in the TeV band by ground-based air Cherenkov Telescopes such as HESS, MAGIC, and VERITAS such that they constitute the dominant source class of Galactic TeV emitters. Combining available MeV-GeV data with observations in the TeV band is critical for precise characterization of high-energy emission from the relativistic particle population in PWNe, thus revealing the capability to produce a significant fraction of the detected Galactic CR flux. However, MeV-GeV PWN counterparts are still largely lacking even after 12 years of continuous observation of the entire sky. Less than a dozen PWNe are currently identified by the Fermi-LAT in the MeV-GeV band. Most PWNe are located along the Galactic plane embedded within the prominent, diffuse Galactic gamma-ray emission, which makes these sources difficult to disentangle from the bright diffuse background. We present a systematic search for gamma-ray counterparts to known PWNe in the 300MeV - 2TeV energy band using 11.5 years of Fermi-LAT data. For the first part of this search, we target the locations of PWNe previously identified across the electromagnetic spectrum that are not powered by pulsars previously detected by the Fermi-LAT as pulsating gamma-ray signals, which includes 6 Fermi PWNe and 7 Fermi PWN associations. We report the analysis of 58 total regions of interest and provide all firm and tentative detections along with their morphological and spectral characteristics. There are 11 unidentified gamma-ray sources that we classify as firm PWN counterparts, which doubles the PWN population detected by the Fermi-LAT, and 22 gamma-ray sources that are PWN candidates.
Abstract: 2209.11847
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Title:Properties of the Prompt Optical Counterpart Arising from the Cooling of Electrons in Gamma-Ray Bursts
Download PDFAbstract: This work extends a contemporaneous effort (Panaitescu & Vestrand 2022) to study the properties of the lower-energy counterpart synchrotron emission produced by the cooling of relativistic Gamma-Ray Burst (GRB) electrons through radiation (synchrotron and self-Compton) emission and adiabatic losses. We derive the major characteristics (pulse duration, lag-time after burst, brightness relative to the burst) of the Prompt Optical Counterpart (POC) accompanying GRBs. Depending on the magnetic field life-time, duration of electron injection, and electron transit-time Dto from hard X-ray (GRB) to optical emitting energies, a (true) POC may appear during the GRB pulse (of duration dtg) or after (delayed OC). The signature of counterparts arising from the cooling of GRB electrons is that true POC pulses (Dto < dtg) last as long as the corresponding GRB pulse (dto ~ dtg) while delayed OC pulses (Dto > dtg) last as long as the transit-time (dto ~ Dto). If OC variability can be measured, then another signature for this OC mechanism is that the GRB variability is "passed" only to POCs but is lost for delayed OCs. Within the GRB electron cooling model for counterparts, POCs should be on average dimmer than delayed one (which is found to be consistent with the data), and harder GRB low-energy slopes bLE should be associated more often with the dimmer POCs The range of low-energy slopes bLE in [-1/2,1/3] produced by electron cooling and the average burst brightness of 1 mJy (with 1 dex dispersion) imply that POCs of hard GRBs can be dimmer than R=20 and difficult to detect by robotic telescopes (unless there is another mechanism that overshines the emission from cooling electrons) and that the POCs of soft GRBs can be brighter than R=10, i.e. as bright as the Optical Flashes (OFs) seen for several bursts.
Abstract: 2209.11798
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Title:Gravitational signal propagation in the Double Pulsar studied with the MeerKAT telescope
Download PDFAbstract: The Double Pulsar, PSR J0737-3039A/B, has offered a wealth of gravitational experiments in the strong-field regime, all of which GR has passed with flying colours. In particular, among current gravity experiments that test photon propagation, the Double Pulsar probes the strongest spacetime curvature. Observations with MeerKAT and, in future, the SKA can greatly improve the accuracy of current tests and facilitate tests of NLO contributions in both orbital motion and signal propagation. We present our timing analysis of new observations of PSR J0737-3039A, made using the MeerKAT telescope over the last 3 years. The increased timing precision offered by MeerKAT yields a 2 times better measurement of Shapiro delay parameter s and improved mass measurements compared to previous studies. In addition, our results provide an independent confirmation of the NLO signal propagation effects and already surpass the previous measurement from 16-yr data by a factor of 1.65. These effects include the retardation effect due to the movement of B and the deflection of the signal by the gravitational field of B. We also investigate novel effects which are expected. For instance, we search for potential profile variations near superior conjunctions caused by shifts of the line-of-sight due to latitudinal signal deflection and find insignificant evidence with our current data. With simulations, we find that the latitudinal deflection delay is unlikely to be measured with timing because of its correlation with Shapiro delay. Furthermore, although it is currently not possible to detect the expected lensing correction to the Shapiro delay, our simulations suggest that this effect may be measured with the full SKA. Finally, we provide an improved analytical description for the signal propagation in the Double Pulsar system that meets the timing precision expected from future instruments such as the full SKA.
Abstract: 2209.11458
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Title:Prospective Study on Observations of γ-Ray Sources in the Galaxy Using the HADAR Experiment
Download PDFAbstract: The High Altitude Detection of Astronomical Radiation (HADAR) experiment is a refracting terrestrial telescope array based on the atmospheric Cherenkov imaging technique. It focuses the Cherenkov light emitted by extensive air showers through a large aperture water-lens system for observing very-high-energy-rays and cosmic rays. With the advantages of a large field-of-view (FOV) and low energy threshold, the HADAR experiment operates in a large-scale sky scanning mode to observe galactic sources. This study presents the prospects of using the HADAR experiment for the sky survey of TeV {\gamma}-ray sources from TeVCat and provids a one-year survey of statistical significance. Results from the simulation show that a total of 23 galactic point sources, including five supernova remnant sources and superbubbles, four pulsar wind nebula sources, and 14 unidentified sources, were detected in the HADAR FOV with a significance greater than 5 standard deviations ({\sigma}). The statistical significance for the Crab Nebula during one year of operation reached 346.0 {\sigma} and the one-year integral sensitivity of HADAR above 1TeV was ~1.3%-2.4% of the flux from the Crab Nebula.
Abstract: 2209.11376
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Title:The jet formation mechanism of Gamma-ray Narrow-line Seyfert 1 Galaxies
Download PDFAbstract: Under the coronal magnetic field, we estimate the maximal jet power of the Blandford-\Znajek (BZ) mechanism, Blandford-\Payne (BP) mechanism, and hybrid model. The jet power of the BZ and Hybrid model mechanisms depends on the spin of a black hole, while the jet power of the BP mechanism does not depend on the spin of a black hole. At high black hole spin, the jet power of the hybrid model is greater than that of the BZ and BP mechanisms. We find that the jet power of almost all gamma-\ray narrow line Seyfert 1 galaxies (gamma-\NLS1s) can be explained by the hybrid model. However, one source with jet power 0.1~\1 Eddington luminosity can not be explained by the hybrid model. We suggest that the magnetic field dragged inward by the accretion disk with magnetization-\driven outflows may accelerate the jets in this gamma-\NLS1.
Abstract: 2209.10581
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Title:Stochasticity of Cosmic Rays from Supernova Remnants and the Ionization Rates in Molecular Clouds
Download PDFAbstract: Cosmic rays are the only agent able to penetrate into the interior of dense molecular clouds. Depositing (part of) their energy through ionisation, cosmic rays play an essential role in determining the physical and chemical evolution of star-forming regions. To a first approximation their effect can be quantified by the cosmic-ray induced ionization rate. Interestingly, theoretical estimates of the ionization rate assuming the cosmic-ray spectra observed in the local interstellar medium result in an ionization rate that is one to two orders of magnitude below the values inferred from observations. However, due to the discrete nature of sources, the local spectra of MeV cosmic rays are in general not representative for the spectra elsewhere in the Galaxy. Such stochasticity effects have the potential of reconciling modelled ionization rates with measured ones. Here, we model the distribution of low-energy cosmic-ray spectra expected from a statistical population of supernova remnants in the Milky Way. The corresponding distribution for the ionization rate is derived and confronted with data. We find that the stochastic uncertainty helps with explaining the surprisingly high ionization rates observed in many molecular clouds.
Abstract: 2209.10576
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Title:Accretion physics at high X-ray spectral resolution: New frontiers and game-changing science
Download PDFAbstract: Microcalorimeters have demonstrated success in delivering high spectral resolution, and have paved the path to revolutionary new science possibilities in the coming decade of X-ray astronomy. There are several research areas in compact object science that can only be addressed with energy resolution Delta(E)<~5 eV at photon energies of a few keV, corresponding to velocity resolution of <~a few hundred km/s, to be ushered in by microcalorimeters. Here, we review some of these outstanding questions, focusing on how the research landscape is set to be transformed (i) at the interface between accreting supermassive black holes and their host galaxies, (ii) in unravelling the structures of accretion environments, (iii) in resolving long-standing issues on the origins of energy and matter feedback, and (iv) to test mass-scaled unification of accretion and feedback. The need to learn lessons from Hitomi and to make improvements in laboratory atomic data precision as well as plasma modeling are highlighted.
Abstract: 2209.10819
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Full Text: [ PostScript, PDF]
Title:Structure in the Magnetic Field of the Milky Way Disk and Halo traced by Faraday Rotation
Download PDFAbstract: Magnetic fields in the ionized medium of the disk and halo of the Milky Way impose Faraday rotation on linearly polarized radio emission. We compare two surveys mapping the Galactic Faraday rotation, one showing the rotation measures of extragalactic sources seen through the Galaxy (from Hutschenreuter et al 2022), and one showing the Faraday depth of the diffuse Galactic synchrotron emission from the Global Magneto-Ionic Medium Survey. Comparing the two data sets in 5deg x 10deg bins shows good agreement at intermediate latitudes, 10 < |b| < 50 deg, and little correlation between them at lower and higher latitudes. Where they agree, both tracers show clear patterns as a function of Galactic longitude: in the Northern Hemisphere a strong sin(2 x longitude) pattern, and in the Southern hemisphere a sin(longitude + pi) pattern. Pulsars with height above or below the plane |z| > 300 pc show similar longitude dependence in their rotation measures. Nearby non-thermal structures show rotation measure shadows as does the Orion-Eridanus superbubble. We describe families of dynamo models that could explain the observed patterns in the two hemispheres. We suggest that a field reversal, known to cross the plane a few hundred pc inside the solar circle, could shift to positive z with increasing Galactic radius to explain the sin(2xlongitude) pattern in the Northern Hemisphere. Correlation shows that rotation measures from extragalactic sources are one to two times the corresponding rotation measure of the diffuse emission, implying Faraday complexity along some lines of sight, especially in the Southern hemisphere.
Abstract: 2209.10228
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Full Text: [ PostScript, PDF]
Title:Radio observations of the Black Hole X-ray Binary EXO 1846-031 re-awakening from a 34-year slumber
Download PDFAbstract: We present radio [1.3 GHz MeerKAT, 4-8 GHz Karl G. Jansky Very Large Array (VLA) and 15.5 GHz Arcminute Microkelvin Imager Large Array (AMI-LA)] and X-ray (Swift and MAXI) data from the 2019 outburst of the candidate Black Hole X-ray Binary (BHXB) EXO 1846-031. We compute a Hardness-Intensity diagram, which shows the characteristic q-shaped hysteresis of BHXBs in outburst. EXO 1846-031 was monitored weekly with MeerKAT and approximately daily with AMI-LA. The VLA observations provide sub-arcsecond-resolution images at key points in the outburst, showing moving radio components. The radio and X-ray light curves broadly follow each other, showing a peak on ~MJD 58702, followed by a short decline before a second peak between ~MJD 58731-58739. We estimate the minimum energy of these radio flares from equipartition, calculating values of $E_{\rm min} \sim$ 4$\times$10$^{41}$ and 5$\times$10$^{42}$ erg, respectively. The exact date of the return to `quiescence' is missed in the X-ray and radio observations, but we suggest that it likely occurred between MJD 58887 and 58905. From the Swift X-ray flux on MJD 58905 and assuming the soft-to-hard transition happened at 0.3-3 per cent Eddington, we calculate a distance range of 2.4-7.5\,kpc. We computed the radio:X-ray plane for EXO 1846-031 in the `hard' state, showing that it is most likely a `radio-quiet' BH, preferentially at 4.5 kpc. Using this distance and a jet inclination angle of $\theta$=73$^{\circ}$, the VLA data place limits on the intrinsic jet speed of $\beta_{\rm int} = 0.29c$, indicating sub-luminal jet motion.
Abstract: 2209.10011
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Full Text: [ PostScript, PDF]
Title:Prospects for detection of a Galactic diffuse neutrino flux
Download PDFAbstract: A Galactic cosmic-ray transport model featuring non-homogeneous transport has been developed over the latest years. This setup is aimed at reproducing gamma-ray observations in different regions of the Galaxy (with particular focus on the progressive hardening of the hadronic spectrum in the inner Galaxy) and was shown to be compatible with the very-high-energy gamma-ray diffuse emission recently detected up to PeV energies. In this work, we extend the results previously presented to test the reliability of that model throughout the whole sky. To this aim, we compare our predictions with detailed longitude and latitude profiles of the diffuse gamma-ray emission measured by Fermi-LAT for different energies and compute the expected Galactic neutrino diffuse emission, comparing it with current limits from the ANTARES collaboration. We emphasize that the possible detection of a Galactic neutrino component will allow us to break the degeneracy between our model and other scenarios featuring prominent contributions from unresolved sources and TeV halos.
Abstract: 2209.09929
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Full Text: [ PostScript, PDF]
Title:BASS XXXIII: Swift-BAT blazars and their jets through cosmic time
Download PDFAbstract: We derive the most up-to-date Swift-Burst Alert Telescope (BAT) blazar luminosity function in the 14-195 keV range, making use of a clean sample of 118 blazars detected in the BAT 105-month survey catalog, with newly obtained redshifts from the BAT AGN Spectroscopic Survey (BASS). We determine the best-fit X-ray luminosity function for the whole blazar population, as well as for Flat Spectrum Radio Quasars (FSRQs) alone. The main results are: (1) at any redshift, BAT detects the most luminous blazars, above any possible break in their luminosity distribution, which means we cannot differentiate between density and luminosity evolution; (2) the whole blazar population, dominated by FSRQs, evolves positively up to redshift z~4.3, confirming earlier results and implying lower number densities of blazars at higher redshifts than previously estimated. The contribution of this source class to the Cosmic X-ray Background at 14-195 keV can range from 5-18%, while possibly accounting for 100% of the MeV background. We also derived the average 14 keV-10 GeV SED for BAT blazars, which allows us to predict the number counts of sources in the MeV range, as well as the expected number of high-energy (>100 TeV) neutrinos. A mission like COSI, will detect 40 MeV blazars and 2 coincident neutrinos. Finally, taking into account beaming selection effects, the distribution and properties of the parent population of these extragalactic jets are derived. We find that the distribution of viewing angles is quite narrow, with most sources aligned within < 5° of the line of sight. Moreover, the average Lorentz factor, <$\Gamma$>= 8-12, is lower than previously suggested for these powerful sources.
Abstract: 2209.09926
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Full Text: [ PostScript, PDF]
Title:Orbital motion near Sagittarius A* -- Constraints from polarimetric ALMA observations
Download PDFAbstract: We report on the polarized light curves of the Galactic Center supermassive black hole Sagittarius A*, obtained at millimeter wavelength with the Atacama Large Millimeter/submillimeter Array (ALMA). The observations took place as a part of the Event Horizon Telescope campaign. We compare the observations taken during the low variability source state on 2017 Apr 6 and 7 with those taken immediately after the X-ray flare on 2017 Apr 11. For the latter case, we observe rotation of the electric vector position angle with a timescale of $\sim 70$ min. We interpret this rotation as a signature of the equatorial clockwise orbital motion of a hot spot embedded in a magnetic field dominated by a dynamically important vertical component, observed at a low inclination $\sim20^\circ$. The hot spot radiates strongly polarized synchrotron emission, briefly dominating the linear polarization measured by ALMA in the unresolved source. Our simple emission model captures the overall features of the polarized light curves remarkably well. Assuming a Keplerian orbit, we find the hot spot orbital radius to be $\sim$ 5 Schwarzschild radii. We observe hints of a positive black hole spin, that is, a prograde hot spot motion. Accounting for the rapidly varying rotation measure, we estimate the projected on-sky axis of the angular momentum of the hot spot to be $\sim 60^\circ$ east of north, with a 180$^\circ$ ambiguity. These results suggest that the accretion structure in Sgr A* is a magnetically arrested disk rotating clockwise.
Abstract: 2209.09913
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Full Text: [ PostScript, PDF]
Title:NuSTAR observations of a heavily X-ray obscured AGN in the dwarf galaxy J144013+024744
Download PDFAbstract: We present a multi-wavelength analysis of the dwarf Seyfert-2 galaxy J$144013+024744$, a candidate obscured active galactic nucleus (AGN) thought to be powered by an intermediate-mass black hole (IMBH, $M_\bullet \approx 10^{4-6} M_\odot$) of mass $M_{\bullet} \sim 10^{5.2}M_\odot$. To study its X-ray properties, we targeted J$144013+024744$ with NuSTAR for $\approx 100$ ks. The X-ray spectrum was fitted with absorbed power law, Pexmon and a physical model (RXTorus). A Bayesian X-ray analysis was performed to estimate the posteriors. The phenomenological and the physical models suggest the AGN to be heavily obscured by a column density of $N_{\rm H} = (3.4-7.0)\times10^{23}$ cm$^{-2}$. In particular, the RXTorus model with a sub-solar metallicity suggests the obscuring column to be almost Compton-thick. We compared the $2-10$ keV intrinsic X-ray luminosity with the inferred X-ray luminosities based on empirical scaling relations for unobscured AGNs using $L_{\rm [OIV](25.89\mu {\rm m})}$, $L_{[{\rm OIII}](5007 {\rm angstrom})}$, and $L_{6\rm \mu m}$ and found that the high-excitation $[{\rm OIV}]$ line provides a better estimate of the intrinsic $2-10$ keV X-ray luminosity ($L_{2-10}^{\rm int} \sim 10^{41.41}{\rm erg s}^{-1}$). Our results suggest that J$144013+024744$ is the first type-2 dwarf galaxy that shows X-ray spectroscopic evidence for obscuration. The column density that we estimated is among the highest measured to date for IMBH-powered AGNs, implying that a typical AGN torus geometry might extend to the low-mass end. This work has implications for constraining the black hole occupation fraction in dwarf galaxies using X-ray observations.
Abstract: 2209.10014
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Full Text: [ PostScript, PDF]
Title:The Synchrotron Low-Energy Spectrum Arising from the Cooling of Electrons in Gamma-Ray Bursts
Download PDFAbstract: This work is a continuation of a previous effort (Panaitescu 2019) to study the cooling of relativistic electrons through radiation (synchrotron and self-Compton) emission and adiabatic losses, with application to the spectra and light-curves of the synchrotron Gamma-Ray Burst produced by such cooling electrons. Here, we derive the low-energy slope b_LE of GRB pulse-integrated spectrum and quantify the implications of the measured distribution of b_LE. If the magnetic field lives longer than it takes the cooling GRB electrons to radiate below 1-10 keV, then radiative cooling processes of power P(gamma) ~ gamma^n with n geq 2, i.e. synchrotron and inverse-Compton (iC) through Thomson scatterings, lead to a soft low-energy spectral slope b_LE leq -1/2 of the GRB pulse-integrated spectrum F_eps ~ eps^{b_LE} below the peak-energy E_p, irrespective of the duration of electron injection t_i. IC-cooling dominated by scatterings at the Thomson--Klein-Nishina transition of synchrotron photons below E_p has an index n = 2/3 -> 1 and yield harder integrated spectra with b_LE in [0,1/6], while adiabatic electron-cooling leads to a soft slope b_LE = -3/4. Radiative processes that produce soft integrated spectra can accommodate the harder slopes measured by CGRO/BATSE and Fermi/GBM only if the magnetic field life-time t_B is shorter than the time during which the typical GRB electrons cool to radiate below 1-10 keV, which is less than (at most) ten radiative cooling timescales t_rad of the typical GRB electron. In this case, there is a one-to-one correspondence between t_B and b_LE. To account for low-energy slopes b_LE > -3/4, adiabatic electron-cooling requires a similar restriction on t_B. In this case, the diversity of slopes arises mostly from how the electron-injection rate varies with time and not from the magnetic field timescale.
Abstract: 2209.09785
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Full Text: [ PostScript, PDF]
Title:Gamma-ray observations of MAXI J1820+070 during the 2018 outburst
Download PDFAbstract: MAXI J1820+070 is a low-mass X-ray binary with a black hole as a compact object. This binary underwent an exceptionally bright X-ray outburst from March to October 2018, showing evidence of a non-thermal particle population through its radio emission during this whole period. The combined results of 59.5 hours of observations of the MAXI J1820+070 outburst with the H.E.S.S., MAGIC and VERITAS experiments at energies above 200 GeV are presented, together with Fermi-LAT data between 0.1 and 500 GeV, and multiwavelength observations from radio to X-rays. Gamma-ray emission is not detected from MAXI J1820+070, but the obtained upper limits and the multiwavelength data allow us to put meaningful constraints on the source properties under reasonable assumptions regarding the non-thermal particle population and the jet synchrotron spectrum. In particular, it is possible to show that, if a high-energy gamma-ray emitting region is present during the hard state of the source, its predicted flux should be at most a factor of 20 below the obtained Fermi-LAT upper limits, and closer for magnetic fields significantly below equipartition. During the state transitions, under the plausible assumption that electrons are accelerated up to ~ 500 GeV, the multiwavelength data and the gamma-ray upper limits lead consistently to the conclusion that a potential high-energy and very-high-energy gamma-ray emitting region should be located at a distance from the black hole ranging between 10^11 and 10^13 cm. Similar outbursts from low-mass X-ray binaries might be detectable in the near future with upcoming instruments such as CTA.
Abstract: 2106.03860
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Full Text: [ PostScript, PDF]
Title:Jet propagation in expanding media
Download PDFAbstract: We present a comprehensive analytic model of relativistic jet propagation in expanding homologous media (ejecta). This model covers the entire jet evolution as well as a range of configurations that are relevant to binary neutron star mergers. These include low and high luminosity jets, unmagnetized and mildly magnetized jets, time-dependent luminosity jets, and Newtonian and relativistic head velocities. We also extend the existing solution of jets in a static medium to power-law density media with index $\alpha<5$. Our model provides simple analytic formulae (calibrated by 3D simulations) for the jet head propagation and breakout times. We find that the system evolution has two main regimes: strong and weak jets. Strong jets start their propagation immediately within the ejecta. Weak jets are unable to penetrate the ejecta at first, and breach it only after the ejecta expands significantly, thus their evolution is independent of the delay between the onset of the ejecta and the jet launching. After enough time, both strong and weak jets approach a common asymptotic phase. We find that a necessary, but insufficient, criterion for the breakout of unmagnetized [weakly magnetized] jets is $E_{j,{\rm iso,tot}} \gtrsim 3[0.4]E_{ej,{\rm tot}}\left({\theta_j}/{0.1{\rm~rad}}\right)^2$, where $E_{j,{\rm iso,tot}}$ is the jet total isotropic equivalent energy, $\theta_j$ is its opening angle, and $E_{ej,{\rm tot}}$ is the ejecta energy. Applying our model to short GRBs, we find that there is most likely a large diversity of ejecta mass, where mass $ \lesssim 10^{-3}~{\rm M}_{\odot} $ (at least along the poles) is common.
Abstract: 2209.09566
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Full Text: [ PostScript, PDF]
Title:Constraining leptonic emission scenarios for the PeVatron candidate HESS J1702-420 with deep XMM-Newton observations
Download PDFAbstract: The unidentified TeV source HESS J1702-420 has recently been proposed as a new hadronic PeVatron candidate, based on the discovery of a small-scale emission sub-region with extremely hard gamma-ray spectrum up to 100 TeV (named HESS J1702-420A). Given the difficulty to discriminate between a hadronic or leptonic origin of the TeV emission, based on the H.E.S.S. measurement alone, we opted for a multi-wavelength approach. A deep X-ray observation was carried out using the XMM-Newton satellite, with the goal of probing a possible association with a hidden leptonic accelerator. No evidence of a clear counterpart for HESS J1702-420A was found in the X-ray data. After excluding an association with all nearby X-ray point sources, we derived strict upper limits on the diffuse X-ray emission and average magnetic field in the HESS J1702-420A region. We additionally report the serendipitous discovery of a new extended X-ray source, whose association with HESS J1702-420A is not obvious but cannot be ruled out either. A set of scripts dedicated to the multi-wavelength modeling of X-ray and gamma-ray data, based on Gammapy, Naima and Xspec, was developed in the context of this work and is made publicly available along with this paper.
Abstract: 2209.09305
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Full Text: [ PostScript, PDF]
Title:A Double Layered Water Cherenkov Detector Array for Gamma-Ray Astronomy
Download PDFAbstract: Ground-level particle detection is now a well-established approach to TeV {\gamma}-ray astronomy. Detection of Cherenkov light produced in water-filled detection units is a proven and cost-effective approach. Here we discuss the optimization of the units towards the future Southern Wide-field Gamma-ray Observatory (SWGO). We investigate a configuration in which each Water Cherenkov Detector (WCD) unit in the array comprises two chambers with black or reflective walls and a single PMT in each chamber. A shallow lower chamber with a PMT facing downwards enables muon tagging and the identification of hadron-induced air showers, which are the primary source of background in {\gamma}-ray astronomy. We investigate how {\gamma}/hadron separation power and achievable angular resolution depend on the geometry and wall reflectivity of the detector units in this configuration. We find that excellent angular resolution, background rejection power and low-energy response are achievable in this double-layer configuration, with some reflective surfaces in both chambers.
Abstract: 2202.02181
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Full Text: [ PostScript, PDF]
Title:Investigating the detection rates and inference of gravitational-wave and radio emission from black hole neutron star mergers
Download PDFAbstract: Black hole neutron star (BHNS) mergers have recently been detected through their gravitational-wave (GW) emission. BHNS mergers could also produce electromagnetic (EM) emission as a short gamma-ray burst (sGRB), and/or an sGRB afterglow upon interaction with the circummerger medium. Here, we make predictions for the expected detection rates with the Square Kilometre Array Phase 1 (SKA1) of sGRB radio afterglows associated with BHNS mergers. We also investigate the benefits of a multimessenger analysis in inferring the properties of the merging binary. We simulate a population of BHNS mergers and estimate their sGRB afterglow flux to obtain the detection rates with SKA1. We investigate how this rate depends on the GW detector sensitivity, the primary black hole (BH) spin, and the neutron star equation of state. We then perform a multimessenger Bayesian inference study on a fiducial BHNS merger. We simulate its sGRB afterglow and GW emission and take systematic errors into account. The expected rates of a combined GW and radio detection with the current generation GW detectors are likely low. Due to the much increased sensitivity of future GW detectors like the Einstein Telescope, the chances of an sGRB localisation and radio detection increase substantially. The unknown distribution of the BH spin has a big influence on the detection rates, however, and it is a large source of uncertainty. Furthermore, for our fiducial BHNS merger we are able to infer both the binary source parameters as well as the parameters of the sGRB afterglow simultaneously when combining the GW and radio data. The radio data provides useful extra information on the binary parameters such as the mass ratio but this is limited by the systematic errors involved. A better understanding of the systematics will further increase the amount of information on the binary parameters that can be extracted from this radio data.
Abstract: 2206.02054
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Full Text: [ PostScript, PDF]
Title:Searching for High-Energy Neutrino Emission from Galaxy Clusters with IceCube
Download PDFAbstract: Galaxy clusters have the potential to accelerate cosmic rays (CRs) to ultra-high energies via accretion shocks or embedded CR acceleration sites. CRs with energies below the Hillas condition will be confined within the cluster and will eventually interact with the intracluster medium (ICM) gas to produce secondary neutrinos and $\gamma$ rays. Using 9.5 years of muon-neutrino track events from the IceCube Neutrino Observatory, we report the results of a stacking analysis of 1094 galaxy clusters, with masses $\gtrsim 10^{14}$ \(\textup{M}_\odot\) and redshifts between 0.01 and $\sim$1, detected by the {\it Planck} mission via the Sunyaev-Zeldovich (SZ) effect. We find no evidence for significant neutrino emission and report upper limits on the cumulative unresolved neutrino flux from massive galaxy clusters after accounting for the completeness of the catalog up to a redshift of 2, assuming three different weighting scenarios for the stacking and three different power-law spectra. Weighting the sources according to mass and distance, we set upper limits at $90\%$ confidence level that constrain the flux of neutrinos from massive galaxy clusters ($\gtrsim 10^{14}$ \(\textup{M}_\odot\)) to be no more than $4.6\%$ of the diffuse IceCube observations at 100~TeV, assuming an unbroken $E^{-2.5}$ power-law spectrum.
Abstract: 2209.08106
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Full Text: [ PostScript, PDF]
Title:Limits on the Diffuse Gamma-Ray Background above 10 TeV with HAWC
Download PDFAbstract: The high-energy Diffuse Gamma-Ray Background (DGRB) is expected to be produced by unresolved isotropically distributed astrophysical objects, potentially including dark matter annihilation or decay emissions in galactic or extragalactic structures. The DGRB has only been observed below 1 TeV; above this energy, upper limits have been reported. Observations or stringent limits on the DGRB above this energy could have significant multi-messenger implications, such as constraining the origin of TeV-PeV astrophysical neutrinos detected by IceCube. The High Altitude Water Cherenkov (HAWC) Observatory, located in central Mexico at 4100 m above sea level, is sensitive to gamma rays from a few hundred GeV to several hundred TeV and continuously observes a wide field-of-view (2 sr). With its high-energy reach and large area coverage, HAWC is well-suited to notably improve searches for the DGRB at TeV energies. In this work, strict cuts have been applied to the HAWC dataset to better isolate gamma-ray air showers from background hadronic showers. The sensitivity to the DGRB was then verified using 535 days of Crab data and Monte Carlo simulations, leading to new limits above 10 TeV on the DGRB as well as prospective implications for multi-messenger studies.
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