Abstracts of Interest
Selected by:
Simon Lee
Abstract: arxiv:2501.03831
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Title:Characterization of Markarian 421 during its most violent year: Multiwavelength variability and correlations
View PDF HTML (experimental)Abstract:Mrk 421 was in its most active state around early 2010, which led to the highest TeV gamma-ray flux ever recorded from any active galactic nuclei. We aim to characterize the multiwavelength behavior during this exceptional year for Mrk 421, and evaluate whether it is consistent with the picture derived with data from other less exceptional years. We investigated the period from November 5, 2009, (MJD 55140) until July 3, 2010, (MJD 55380) with extensive coverage from very-high-energy (VHE; E$\,>\,$100$\,$GeV) gamma rays to radio with MAGIC, VERITAS, Fermi-LAT, RXTE, Swift, GASP-WEBT, VLBA, and a variety of additional optical and radio telescopes. We investigated the variability and correlation behavior among different energy bands in great detail. We find the strongest variability in X-rays and VHE gamma rays, and PSDs compatible with power-law functions. We observe strong correlations between X-rays and VHE gamma rays. We also report a marginally significant positive correlation between high-energy (HE; E$\,>\,$100$\,$MeV) gamma rays and the ultraviolet band. We detected marginally significant correlations between the HE and VHE gamma rays, and between HE gamma rays and the X-ray, that disappear when the large flare in February 2010 is excluded from the correlation study. The activity of Mrk 421 also yielded the first ejection of features in the VLBA images of the jet of Mrk 421. Yet the large uncertainties in the ejection times of these radio features prevent us from firmly associating them to the specific flares recorded during the campaign. We also show that the collected multi-instrument data are consistent with a scenario where the emission is dominated by two regions, a compact and extended zone, which could be considered as a simplified implementation of an energy-stratified jet as suggested by recent IXPE observations.
Abstract: arxiv:2501.03889
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Title:Cosmic-ray acceleration and escape from supernova remnant W44 as probed by Fermi-LAT and MAGIC
View PDF HTML (experimental)Abstract:Context. The supernova remnant (SNR) W44 and its surroundings are a prime target for studying the acceleration of cosmic rays (CRs). Several previous studies established an extended gamma-ray emission that is set apart from the radio shell of W44. This emission is thought to originate from escaped high-energy CRs that interact with a surrounding dense molecular cloud complex. Aims. We present a detailed analysis of Fermi-LAT data with an emphasis on the spatial and spectral properties of W44 and its surroundings. We also report the results of the observations performed with the MAGIC telescopes of the northwestern region of W44. Finally, we present an interpretation model to explain the gamma-ray emission of the SNR and its surroundings. Methods. We first performed a detailed spatial analysis of 12 years of Fermi-LAT data at energies above 1 GeV, in order to exploit the better angular resolution, while we set a threshold of 100MeV for the spectral analysis. We performed a likelihood analysis of 174 hours of MAGIC data above 130 GeV using the spatial information obtained with Fermi-LAT. Results. The combined spectra of Fermi-LAT and MAGIC, extending from 100MeV to several TeV, were used to derive constraints on the escape of CRs. Using a time-dependent model to describe the particle acceleration and escape from the SNR, we show that the maximum energy of the accelerated particles has to be ' 40 GeV. However, our gamma-ray data suggest that a small number of lower-energy particles also needs to escape. We propose a novel model, the broken-shock scenario, to account for this effect and explain the gamma-ray emission.
Abstract: arxiv:2501.03082
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Title:A two-hump spectrum in the prompt emission of GRB 240825A
View PDF HTML (experimental)Abstract:An extra hard spectral component that extends to GeV energies, in additional to the typical sub- MeV Band component, appears in several gamma-ray burst (GRBs) detected by Fermi Large Area Telescopes (LAT). Only in one case (i.e., GRB 090926A), a spectral break feature at the high energy end is identified in the extra hard component, but the photon counts are not enough to distinguish between the cutoff model and the broken power law model for the spectral break. In this work, we report the detection of an extra hard component showing the spectral break in GRB 240825A. We find that a broken power-law model fits the spectral data of the extra component better than a single power-law with an exponential cutoff in the time resolved spectrum for the second emission pulse, with a break at about 50 MeV. This spectral feature disfavors the gamma-ray opacity to pair creation as the origin of the spectral break, but points to an intrinsic peak for the extra component. The low ratio between the peak of the extra hard component and that of the Band component challenges the synchrotron self-Compton origin for the extra component. Alternative scenarios, such as the inverse Compton scattering of the photosphere emission, are discussed. In addition, we find a clear transition from the prompt emission to afterglow emission at GeV energies in GRB 240825A, manifested by a temporal steep decay and an unique spectral evolution.
Abstract: arxiv:2501.05518
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Title:A multi-frequency study of sub-parsec jets with the Event Horizon Telescope
View PDFAbstract:The 2017 observing campaign of the Event Horizon Telescope (EHT) delivered the first very long baseline interferometry (VLBI) images at the observing frequency of 230 GHz, leading to a number of unique studies on black holes and relativistic jets from active galactic nuclei (AGN). In total, eighteen sources were observed: the main science targets, Sgr A* and M87 along with various calibrators. We investigated the morphology of the sixteen AGN in the EHT 2017 data set, focusing on the properties of the VLBI cores: size, flux density, and brightness temperature. We studied their dependence on the observing frequency in order to compare it with the Blandford-Königl (BK) jet model. We modeled the source structure of seven AGN in the EHT 2017 data set using linearly polarized circular Gaussian components and collected results for the other nine AGN from dedicated EHT publications, complemented by lower frequency data in the 2-86 GHz range. Then, we studied the dependences of the VLBI core flux density, size, and brightness temperature on the frequency measured in the AGN host frame. We compared the observations with the BK jet model and estimated the magnetic field strength dependence on the distance from the central black hole. Our results indicate a deviation from the standard BK model, particularly in the decrease of the brightness temperature with the observing frequency. Either bulk acceleration of the jet material, energy transfer from the magnetic field to the particles, or both are required to explain the observations.
Abstract: arxiv:2501.03309
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Title:Little Red Dots are Tidal Disruption Events in Runaway-Collapsing Clusters
View PDF HTML (experimental)Abstract:I hypothesize a physical explanation for the "Little Red Dots" (LRDs) discovered by the James Webb Space Telescope (JWST). The first star formation in the universe occurs in dense clusters, some of which may undergo runaway collapse and form an intermediate mass black hole. This process would appear as a very dense stellar system, with recurring tidal disruption events (TDEs) as stellar material is accreted by the black hole. Such a system would be compact, UV-emitting, and exhibit broad H-alpha emission. If runaway collapse is the primary mechanism for forming massive black hole seeds, this process could be fairly common and explain the large volume densities of LRDs. In order to match the predicted number density of runaway collapse clusters, the tidal disruption rate must be on the order of 10^-4 per year. A top-heavy stellar initial mass function may be required to match observations without exceeding the predicted LambdaCDM mass function. The TDE LRD hypothesis can be verified with followup JWST observations looking for TDE-like variability.
Abstract: arxiv:2501.04912
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Title:Another piece to the puzzle: radio detection of a JWST discovered AGN candidate
View PDF HTML (experimental)Abstract:Radio observations can provide crucial insight into the nature of a new abundant and mysterious population of dust-reddened active galactic nuclei (AGN) candidates discovered by the James Webb Space Telescope (JWST), including ``Little Red Dots" (LRDs). In this study, we search for radio bright sources in a large sample of $\sim$700 JWST discovered AGN candidates ($z\sim2-11$) in the 0.144-3 GHz frequency range, utilizing deep radio imaging in COSMOS, GOODS-N, and GOODS-S. Only one source is significantly detected in our radio surveys, which is PRIMER-COS 3866 at $z=4.66$. Its radio properties are consistent with both an AGN and star formation origin with a spectral index of $\alpha=-0.76^{+0.11}_{-0.09}$ and a radio-loudness of $R\approx0.5$. The derived brightness temperature limit of PRIMER-COS 3866 of $T_b \gtrsim 10^{3}$ K is too low to confirm its AGN nature. Our stacking results yield non-detections in all fields with the most constraining 3$\sigma$ limit $L_{1.3\text{GHz}} < 1.3\times10^{39}$ erg s$^{-1}$ (rms of $\sim$0.15 $\mu$Jy beam$^{-1}$ at $z_{\text{median}}=6.1$) obtained for photometrically selected AGN candidates in the COSMOS field. This result is still consistent with expectations from the empirical $L_X - L_{\text{H}\alpha}$ and $L_X - L_R$ correlations established for local AGN. We conclude that current radio observations have insufficient depth to claim JWST discovered AGN candidates are radio-weak. We project that future surveys carried out by the SKA and ngVLA should be able to obtain significant detections within a few hours, providing crucial measurements of their brightness temperature, which would allow for distinguishing between AGN and starburst-driven origins of this new abundant population.
Abstract: arxiv:2501.02923
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Title:Searches for primary photons and neutrinos with the Pierre Auger Observatory
View PDF HTML (experimental)Abstract:The Pierre Auger Observatory stands as the largest detector for ultra-high-energy (UHE) cosmic rays. The Observatory is also sensitive to UHE photons and neutrinos that can be produced along with UHE cosmic rays or in top-down processes, such as the decay of dark matter particles. The search for these neutral particles relies on the hybrid measurements of extensive air showers, combining a fluorescence detector with a surface detector array and an underground muon detector. We present an overview of the searches for UHE photons and neutrinos utilizing data from the Pierre Auger Observatory. Currently, no photon or neutrino candidates have been identified. Consequently, we report on the most stringent limits to the integral UHE photon and neutrino fluxes above 50 PeV and 100 PeV, respectively, from diffuse and point-like steady sources. These limits led to strong constraints on theoretical models describing the cosmological evolution of the acceleration sites and the nature of dark-matter particles. Lastly, we briefly comment on the searches for these neutral particles in coincidence with gravitational wave events, underscoring the pivotal role of the Observatory in the context of multi-messenger astronomy at the highest energies.
Abstract: arxiv:2501.04775
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Title:Constraining $γ$-ray dissipation site in gravitationally lensed quasar -- PKS 1830$-$211
View PDF HTML (experimental)Abstract:Variable $\gamma$-ray flares upto minute timescales reflect extreme particle acceleration sites. However, for high-redshift blazars, the detection of such rapid variations remains limited by current telescope sensitivities. Gravitationally lensed blazars serve as powerful tools to probe $\gamma$-ray production zones in distant sources, with time delays between lensed signals providing crucial insights into the spatial distribution of emission regions relative to the lens's mass-weighted center. We have utilized 15 years of Fermi-LAT $\gamma$-ray data from direction of PKS 1830$-$211 to understand the origin of flaring high-energy production zone at varying flux states. To efficiently estimate the (lensed) time delay, we used a machine learning-based tool - the Gaussian Process regression algorithm, in addition to - Autocorrelation function and Double power spectrum. We found a consistent time delay across all flaring activity states, indicating a similar location for the $\gamma$-ray emission zone, possibly within the radio core. The estimated time delay of approximately 20 days for the five flaring epochs was significantly shorter than previously estimated radio delays. This suggests that the $\gamma$-ray emission zone is closer to the central engine, in contrast to the radio emission zone, which is expected to be much farther away. A linear relationship between lag and magnification has been observed in the identified source and echo flares. Our results suggest that the $\gamma$-ray emission zone originates from similar regions away from the site of radio dissipation.
Abstract: arxiv:2501.03614
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Title:The joint detection rate of short gamma-ray bursts and gravitational waves from binary neutron star mergers
View PDF HTML (experimental)Abstract:In this work, we analyze the most recent short gamma-ray burst (sGRB) sample detected by the \emph{Fermi} satellite to reassess the sGRB luminosity function and formation rate. Using the empirical luminosity correlation, we first determine the pseudo redshifts of 478 sGRBs. Then, we use the maximum likelihood method to constrain the luminosity function and formation rate of sGRBs under various delay-time distribution models, finding the power-law delay model marginally preferred over the Gaussian and lognormal delay models based on the Akaike Information Criterion. The local formation rate of sGRBs is $1.73_{-0.45}^{+0.60}$ $\mathrm{Gpc^{-3}\,yr^{-1}}$, largely independent of the adopted delay-time distribution model. Additionally, we investigate the potential for joint detection of sGRBs and their gravitational wave (GW) counterparts from binary neutron star mergers using both current and future GRB and GW facilities. For sGRB detection, we consider three existing satellites: \emph{Fermi}, the Space-based multi-band astronomical Variable Objects Monitor (\emph{SVOM}), and the Einstein Probe (\emph{EP}). For GW detection, we examine two International GW Networks (IGWN): a four-detector network consisting of LIGO Hanford, Livingston, Virgo, and Kagra (IGWN4) and an upcoming five-detector network that includes these four detectors plus LIGO India (IGWN5). Our results indicate that for different delay-time distribution models, the joint sGRB and GW detection rates for \emph{Fermi}, \emph{SVOM}, and \emph{EP} with IGWN4 (IGWN5) lie within 0.09--0.31 $\mathrm{yr^{-1}}$ (0.55--1.98 $\mathrm{yr^{-1}}$), 0.03--0.11 $\mathrm{yr^{-1}}$ (0.26--0.80 $\mathrm{yr^{-1}}$), and 0.01--0.04 $\mathrm{yr^{-1}}$ (0.09--0.27 $\mathrm{yr^{-1}}$), respectively.
This page created: Tue Jan 28 10:10:32 ACDT 2025 by Simon Lee
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