Abstracts of Interest
Selected by:
Justin Albury
Abstract: 1910.03721
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Title:News from Cosmic Ray Air Showers (ICRC 2019 -- Cosmic Ray Indirect Rapport)
(Submitted on 8 Oct 2019)
Abstract: This rapport summarizes the cosmic-ray indirect (CRI) session of the 36th ICRC conference. Updated measurements from several air-shower arrays with higher precision lead to the discovery of new features in the energy spectrum: HAWC measures a softening of the light component (p+He) around $10^{13.5}\,$eV; measurements of the Pierre Auger Observatory show that the second knee is a smooth feature extending at least over the range of $100-200\,$PeV and that the energy spectrum between the ankle and the cut-off cannot be described by a simple broken power law. Measurements of the mass composition confirm that the composition is a varying mixture of protons and nuclei at least up to several $10\,$EeV. A joint effort of several collaborations helps to better assess deficiencies of hadronic interaction models, e.g., by quantifying the muon deficit in the models over the shower energy. Anisotropy measurements with higher precision generally confirm earlier results. A change of the amplitude and phase of the equatorial dipole indicates that in the energy range between the second knee and the ankle there likely is a transition from Galactic to extragalactic sources. However, neither the most energetic Galatic nor the extragalactic sources have been discovered, which remains a primary science goal. Next to more exposure, an increase of measurement accuracy and decrease of systematic uncertainties will provide future progress. It is particularly exciting that new experiments are built and existing experiments upgraded to increase the measurement accuracy of the energy and mass composition, e.g., by combining radio antennas with particle detectors. Last but not least, there is a trend that experiments are designed such that they can target cosmic rays, photons, and neutrinos at the same time, which will facilitate multi-messenger astrophysics at the highest energies.
Abstract: 1910.02704
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Title:Photons in the science of the Pierre Auger Observatory
(Submitted on 7 Oct 2019)
Abstract: In this paper, the connection between Pierre Auger Observatory measurements and photons is discussed. Three cases are presented: the search for photons in the ultrahigh-energy cosmic-ray radiation, the impact of the photon background in the cosmic ray propagation and the role of the ambient photon fields surrounding cosmic accelerators.
Abstract: 1910.05017
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Title:Expectation on LHAASO sensitivity to decaying dark matter signatures from dwarf galaxies gamma-ray emission
(Submitted on 11 Oct 2019)
Abstract: As a next-generation complex extensive air shower array with a large field of view (FOV), the Large High Altitude Air Shower Observatory (LHAASO) would be very sensitive to the 300 GeV $-$ 1 PeV gamma rays. Dwarf spheroidal satellite galaxies (dSphs) are some of the most DM dominated objects and the ideal laboratory for dark matter (DM) indirect detection. LHAASO will become a promising project to search for the gamma-ray signatures induced by heavy DM particles in the dSphs. In this paper, we make a forecast for the LHAASO sensitivity to the gamma-ray signatures of decaying DM particles from 19 dSphs within the LHAASO FOV. We make a joint likelihood analysis for the overall 19 dSphs and also take the uncertainties of dSphs' spatial DM distribution into consideration. We find that the LHAASO sensitivity to the DM decay lifetime will reach $\mathcal{O} (10^{26} s) \sim \mathcal{O} (10^{28} s)$ for several decay channels at the mass scale from 1 TeV to 100 TeV.
Abstract: 1910.04762
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Title:The Central 300 pc of the Galaxy probed by infrared spectra of H$_3^+$ and CO: part I. Predominance of warm and diffuse gas and high H$_2$ ionization rate
(Submitted on 10 Oct 2019)
Abstract: The molecular gas in the Central Molecular Zone (CMZ) of the Galaxy has been studied using infrared absorption spectra of H$_3^+$ lines at 3.5-4.0 $\mu$m and CO lines near 2.34 $\mu$m. In addition to the previously reported spectra of these lines toward 8 stars located within 30 pc of Sgr A$^\ast$, there are now spectra toward $\sim$30 bright stars located from 140 pc west to 120 pc east of Sgr A$^\ast$. The spectra show the presence of warm ($T\sim 200$ K) and diffuse ($n < 100 $cm$^{-3}$) gas with $N$(H$_3^+$) $\sim 3 \times 10^{15} $cm$^{-2}$ on majority of sightlines. Instead of our previous analysis in which only electrons from photoionization of carbon atoms were considered, we have developed a simple model calculation in which the cosmic ray ionization of H$_2$ and H are also taken into account. We conclude: (1) Warm and diffuse gas dominates the volume of the CMZ. The volume filling factor of dense gas must be much less than 0.1 and the CMZ is not as opaque as previously considered. The X-ray emitting ultra-hot 10$^8 $K plasma, which some thought to dominate the CMZ, does not exist over extended regions. (2) The cosmic ray ionization rate is $\zeta \sim 2 \times 10^{-14} $s$^{-1}$, higher than in Galactic dense clouds and diffuse clouds by factors of $\sim$1000 and $\sim$100, respectively. If the equipartition law stands, this suggests a pervading magnetic field on the order of $\sim$100 $\mu$G.
Abstract: 1910.04124
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Title:The SUrvey for Pulsars and Extragalactic Radio Bursts IV: Discovery and polarimetry of a 12.1-second radio pulsar
(Submitted on 9 Oct 2019)
Abstract: We report the discovery of PSR J2251$-$3711, a radio pulsar with a spin period of 12.1 seconds, the second longest currently known. It has a spin period derivative of $1.31 \times 10^{-14}~\mathrm{s~s}^{-1}$, corresponding to a characteristic surface magnetic field of $1.3 \times 10^{13}$ G and a spin-down luminosity of $2.9 \times 10^{29}~\mathrm{erg~s}^{-1}$. Its dispersion measure of 12.12(1) $\mathrm{pc}~\mathrm{cm}^{-3}$ leads to distance estimates of 0.5 and 1.3 kpc according to the NE2001 and YMW16 Galactic free electron density models, respectively. Some of its single pulses show an uninterrupted 180 degree sweep of the phase-resolved polarization position angle, with an S-shape reminiscent of the rotating vector model prediction. However, the fact that this sweep occurs at different phases from one pulse to another is remarkable and without straightforward explanation. Although PSR J2251$-$3711 lies in the region of the $P-\dot{P}$ parameter space occupied by the X-ray Isolated Neutron Stars (XINS) and some low-magnetic field magnetars, there is no evidence for an X-ray counterpart in our Swift XRT observation; this places a 99%-confidence upper bound on its unabsorbed bolometric thermal luminosity of $1.1 \times 10^{31}~(d / 1~\mathrm{kpc})^2~\mathrm{erg/s}$ for an assumed blackbody temperature of 85 eV, where $d$ is the distance to the pulsar. We discuss plausible evolution history scenarios for this source, but further observations are needed to determine whether it is a rotation-powered pulsar with a true age of at least several Myr, or a much younger object such as an XINS or a recently cooled magnetar. Extreme specimens like PSR J2251-3711 help bridge populations in the so-called neutron star zoo in an attempt to understand their origins and evolution.
Abstract: 1910.03614
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Title:A year-long plateau in the late-time near-infrared light curves of Type Ia supernovae
(Submitted on 8 Oct 2019)
Abstract: The light curves of Type Ia supernovae are routinely used to constrain cosmology models. Driven by radioactive decay of 56Ni, the light curves steadily decline over time, but >150 days past explosion, the near-infrared portion is poorly characterized. We report a year-long plateau in the near-infrared light curve at 150-500 days, followed by a second decline phase accompanied by a possible appearance of [Fe I] emission lines. This near-infrared plateau contrasts sharply with Type IIP plateaus and requires a new physical mechanism. We suggest a such as masking of the "near-infrared catastrophe," a predicted yet unobserved sharp light-curve decline, by scattering of ultraviolet photons to longer wavelengths. The transition off the plateau could be due to a change in the dominant ionization state of the supernova ejecta. Our results shed new light on the complex radiative transfer processes that take place in Type Ia supernovae and enhance their use as "standard candles."
Abstract: 1910.03459
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Title:NuSTAR and Chandra observations of new X-ray transients in the central parsec of the Galaxy
(Submitted on 8 Oct 2019)
Abstract: We report NuSTAR and Chandra observations of two X-ray transients, SWIFT J174540.7$-$290015 (T15) and SWIFT J174540.2$-$290037 (T37), which were discovered by the Neil Gehrels Swift Observatory in 2016 within $r\sim1$ pc of Sgr A*. NuSTAR detected bright X-ray outbursts from T15 and T37, likely in the soft and hard states, with 3-79~keV luminosities of $8\times10^{36}$ and $3\times10^{37}$ erg/s, respectively. No X-ray outbursts have previously been detected from the two transients and our Chandra ACIS analysis puts an upper limit of $L_X \lesssim 2 \times10^{31}$ erg/s on their quiescent 2-8 keV luminosities. No pulsations, significant QPOs, or type I X-ray bursts were detected in the NuSTAR data. While T15 exhibited no significant red noise, the T37 power density spectra are well characterized by three Lorentzian components. The declining variability of T37 above $\nu \sim 10$ Hz is typical of black hole (BH) transients in the hard state. NuSTAR spectra of both transients exhibit a thermal disk blackbody, X-ray reflection with broadened Fe atomic features, and a continuum component well described by Comptonization models. Their X-ray reflection spectra are most consistent with high BH spin ($a_{*} \gtrsim 0.9$) and large disk density ($n_e\sim10^{21}$ cm$^{-3}$). Based on the best-fit ionization parameters and disk densities, we found that X-ray reflection occurred near the inner disk radius, which was derived from the relativistic broadening and thermal disk component. These X-ray characteristics suggest the outbursting BH-LMXB scenario for both transients and yield the first BH spin measurements from X-ray transients in the central 100 pc region.
Abstract: 1910.02960
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Title:High angular resolution ALMA images of dust and molecules in the SN 1987A ejecta
(Submitted on 7 Oct 2019)
Abstract: We present high angular resolution (~80 mas) ALMA continuum images of the SN 1987A system, together with CO $J$=2 $\!\rightarrow\!$ 1, $J$=6 $\!\rightarrow\!$ 5, and SiO $J$=5 $\!\rightarrow\!$ 4 to $J$=7 $\!\rightarrow\!$ 6 images, which clearly resolve the ejecta (dust continuum and molecules) and ring (synchrotron continuum) components. Dust in the ejecta is asymmetric and clumpy, and overall the dust fills the spatial void seen in H$\alpha$ images, filling that region with material from heavier elements. The dust clumps generally fill the space where CO $J$=6 $\!\rightarrow\!$ 5 is fainter, tentatively indicating that these dust clumps and CO are locationally and chemically linked. In these regions, carbonaceous dust grains might have formed after dissociation of CO. The dust grains would have cooled by radiation, and subsequent collisions of grains with gas would also cool the gas, suppressing the CO $J$=6 $\!\rightarrow\!$ 5 intensity. The data show a dust peak spatially coincident with the molecular hole seen in previous ALMA CO $J$=2 $\!\rightarrow\!$ 1 and SiO $J$=5 $\!\rightarrow\!$ 4 images. That dust peak, combined with CO and SiO line spectra, suggests that the dust and gas could be at higher temperatures than the surrounding material, though higher density cannot be totally excluded. One of the possibilities is that a compact source provides additional heat at that location. Fits to the far-infrared--millimeter spectral energy distribution give ejecta dust temperatures of 18--23K. We revise the ejecta dust mass to $\mathrm{M_{dust}} = 0.2-0.4$M$_\odot$ for carbon or silicate grains, or a maximum of $<0.7$M$_\odot$ for a mixture of grain species, using the predicted nucleosynthesis yields as an upper limit.
Abstract: 1910.02429
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Title:Gamma-ray and Neutrino Emissions due to Cosmic-Ray Protons Accelerated at Intracluster Shocks in Galaxy Clusters
(Submitted on 6 Oct 2019)
Abstract: We examine the cosmic-ray protons (CRp) accelerated at collisionless shocks in galaxy clusters through cosmological structure formation simulations. We find that in the intracluster medium of simulated sample clusters, only $\sim7$% of the shock kinetic energy flux is dissipated by the shocks that are expected to accelerate CRp, that is, supercritcal, quasi-parallel shocks with sonic Mach number $M_s\ge2.25$; the rest is dissipated at subcritical shocks and quasi-perpendicular shocks, both of which may not accelerate CRp. Adopting the diffusive shock acceleration model recently presented in Ryu et al. (2019), we estimate the production of CRp in simulated clusters. The average fraction of the shock kinetic energy transferred to CRp is assessed at $\sim10^{-4}$. We then calculate diffuse $\gamma$-ray emissions from simulated clusters, resulting from the decay of neutral pions that are produced through inelastic collisions between CRp and thermal protons. The predicted $\gamma$-ray emissions lie mostly below the upper limits for observed clusters set by Fermi-LAT. We also estimate neutrino emission due to the decay of charged pions produced by the same process. The predicted neutrino fluxes towards nearby clusters are $\lesssim10^{-5}$ of the IceCube flux at $E_{\nu}=1$ PeV and $\lesssim10^{-7}$ of the atmospheric neutrino flux in the energy range of $E_{\nu}\leq1$ TeV.
Abstract: 1910.04435
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Title:Surveillance of Space using Passive Radar and the Murchison Widefield Array
(Submitted on 10 Oct 2019)
Abstract: In this paper we build upon recent work in the radio astronomy community to experimentally demonstrate the viability of passive radar for Space Situational Awareness. Furthermore, we show that the six state parameters of objects in orbit may be measured and used to perform orbit characterisation/estimation.
Abstract: 1910.02895
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Title:Science with the Murchison Widefield Array: Phase I Results and Phase II Opportunities
(Submitted on 7 Oct 2019)
Abstract: The Murchison Widefield Array (MWA) is an open access telescope dedicated to studying the low frequency (80$-$300 MHz) southern sky. Since beginning operations in mid 2013, the MWA has opened a new observational window in the southern hemisphere enabling many science areas. The driving science objectives of the original design were to observe 21\,cm radiation from the Epoch of Reionisation (EoR), explore the radio time domain, perform Galactic and extragalactic surveys, and monitor solar, heliospheric, and ionospheric phenomena. All together 60$+$ programs recorded 20,000 hours producing 146 papers to date. In 2016 the telescope underwent a major upgrade resulting in alternating compact and extended configurations. Other upgrades, including digital back-ends and a rapid-response triggering system, have been developed since the original array was commissioned. In this paper we review the major results from the prior operation of the MWA, and then discuss the new science paths enabled by the improved capabilities. We group these science opportunities by the four original science themes, but also include ideas for directions outside these categories.
Abstract: 1910.04550
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Title:On the beam properties of radio pulsars with interpulse emission
(Submitted on 8 Oct 2019)
Abstract: In the canonical picture of pulsars, radio emission arises from a narrow cone centered on the star's magnetic axis but many basic details remain unclear. We use high-quality polarization data taken with the Parkes radio telescope to constrain the geometry and emission heights of pulsars showing interpulse emission, and include the possibility that emission heights in the main and interpulse may be different. We show that emission heights are low in the centre of the beam, typically less than 3\% of the light cylinder radius. The emission beams are under-filled in longitude, with an average profile width only 60% of the maximal beam width and there is a strong preference for the visible emission to be located on the trailing part of the beam. We show substantial evidence that the emission heights are larger at the beam edges than in the beam centre. There is some indication that a fan-like emission beam explains the data better than conal structures. Finally, there is a strong correlation between handedness of circular polarization in the main and interpulse profiles which implies that the hand of circular polarization is determined by the hemisphere of the visible emission.
Abstract: 1910.04782
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Title:Explaining the AMS positron excess via Right-handed Neutrinos
(Submitted on 10 Oct 2019)
Abstract: We have witnessed in the past decade the observation of a puzzling cosmic-ray excess at energies larger than $10$~GeV. The AMS-02 data published this year has new ingredients such as the bump around $300$~GeV followed by a drop at $800$~GeV, as well as smaller error bars. Adopting the background used by the AMS-02 collaboration in their analysis, one can conclude that previous explanations to the new AMS-02 such as one component annihilating and decaying dark matter as well as pulsars seem to fail at reproducing the data. Here, we show that in the right-handed neutrino portal might reside the answer. We discuss a decaying two-component dark matter scenario where the decay products are right-handed neutrinos that have their decay pattern governed by the type I seesaw mechanism. This setup provides a very good fit to data, for example, for a conservative approach including just statistical uncertainties leads to $\chi^2/d.o.f \sim 2.3$ for $m_{DM_1}=2150$~GeV with $\tau_{1}=3.78 \times 10^{26}$ s and $m_{DM_2}=300$ with $\tau_{2}=5.0 \times 10^{27}$ s for $M_N=10$ GeV, and, in an optimistic case, including systematic uncertainties, we find $\chi^2/d.o.f \sim 1.12$, for $M_N = 10$ GeV, with $m_{DM_1}=2200$ GeV with $\tau_{1}=3.8 \times 10^{26}$ s and $m_{DM_2}=323$ GeV with $\tau_{2}=1.68 \times 10^{27}$ s.
Abstract: 1910.04512
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Full Text: [ PostScript, PDF]
Title:GRB X-Ray Flare Properties among Different GRB Subclasses
(Submitted on 10 Oct 2019)
Abstract: Gamma-ray bursts (GRBs) can be divided into three subclasses: X-ray flash (XRF), X-ray rich (XRR), and classical GRB (C-GRB). An X-ray flare is the rebrightening emission shown in the early X-ray afterglow of some GRBs. In this paper, we comprehensively examine the X-ray flare properties among XRF, XRR, and C-GRB subclasses. We utilize the XRF, XRR, and C-GRB subclass samples obtained from the Swift-BAT3 catalog, and the X-ray flare observational properties are collected from Falcone et al., Chincarini et al., and Yi et al. We find that XRFs and XRRs have more bright X-ray flares than C-GRBs. The ratio of the X-ray flare fluence to the prompt emission fluence has different distributions between XRF and C-GRB subclasses. The linear correlation between the duration and the peak time of the X-ray flares is also different between XRF and C-GRB subclasses. We are inclined to identify the GRBs with the bright X-ray flares as XRFs or XRRs. We discuss some issues that are related to the XRF/XRR/C-GRB classification. We also caution the selection effects and the instrument bias in our investigation. Large samples are required in the future to further confirm our results.
Abstract: 1910.04434
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Title:The Insight-HXMT mission and its recent progresses
(Submitted on 10 Oct 2019)
Abstract: The Hard X-ray Modulation Telescope HXMT or also dubbed as Insight HXMT is China s first astronomical satellite . It was launched on 15 th June 2017 in JiuQuan, China and is currently in service smoothly. It was designed to perform point ing , scan ning and gamma ray burst (GRB) observations and , based on the Direct Demodulation Method (DDM), the image of the scanned sky region can be reconstructed. Here we introduce the mission and its progresses in aspects of payload, core sciences, ground calibration/facility , ground segment, data archive, software, in orbit performance, calibration, background model, observations and preliminary results .
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