Abstracts of Interest
Selected by:
Adila Abdul Halim
Abstract: 2407.18318
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Title:Photon-likeness of hadron showers and impact of Lorentz boosting
View PDF HTML (experimental)Abstract:We examine the probability of proton-induced air showers at $E>10$\,EeV being misidentified as photon-induced due to neutral pions receiving a major part of the primary energy in the first interaction, thereby enhancing the electromagnetic shower component by their $\pi^0 \to \gamma \gamma$ decay. Using CORSIKA simulations, we demonstrate the relevance of this effect at EeV energies. However, the probability for such photon-like events drops down strongly at the highest energies due to the increasing probability of Lorentz boosted $\pi^0$'s suffering hadronic interactions before decay. Different hadronic interaction models suggest that photon-like hadronic events may be observed at current UHECR observatories. A quantitative comparison of the observed number of background events found in recent photon searches published by the Pierre Auger Collaboration allows us conclude that the hypothesis of upwards fluctuations of $\pi^0$-production alone is insufficient to explain the data.
Abstract: 2407.18549
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Title:Accretion properties of soft X-ray transient XTE J1856+053 during its 2023 Outburst
View PDF HTML (experimental)Abstract:Soft X-ray transients are a subclass of the low mass X-ray binaries that occasionally show a sudden rise in their soft X-ray luminosity; otherwise, they remain in an extremely faint state. We investigate the accretion properties of the soft X-ray transient XTE J1856+053 during its 2023 outburst obtained by NICER and NuSTAR data in July. We present detailed results on the timing and spectral analysis of the X-ray emission during the outburst. The power spectral density shows no quasi-periodic oscillation features. The source's spectrum on July 19 can be well-fitted with a multi-color blackbody component, a power-law component, and a reflection component with a broadened iron emission line. NICER spectra can be well-fitted by considering a combination of a blackbody and a power-law. The source exhibits a transition within just five days from a soft state to an intermediate state during the outburst decline phase. The inner accretion disk has a low inclination angle ($\sim18^\circ$). The spectral analysis also suggests a high-spin ($a>0.9$) BH as the central accreting object.
Abstract: 2407.18980
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Title:Time evolution of the synchrotron X-ray emission in Kepler's SNR: the effects of turbulence and shock velocity
View PDF HTML (experimental)Abstract:The maximum energy of electrons in supernova remnant (SNR) shocks is typically limited by radiative losses, where the synchrotron cooling time equals the acceleration time. The low speed of shocks in a dense medium increases the acceleration time, leading to lower maximum electron energies and fainter X-ray emissions. However, in Kepler's SNR, an enhanced electron acceleration, which proceeds close to the Bohm limit, occurs in the north of its shell, where the shock is slowed by a dense circumstellar medium (CSM). To investigate whether this scenario still holds at smaller scales, we analyzed the temporal evolution of the X-ray synchrotron flux in filamentary structures, using the two deepest Chandra/ACIS X-ray observations, performed in 2006 and 2014. We examined spectra from different filaments, we measured their proper motion and calculated the acceleration to synchrotron time-scale ratios. The interaction with the turbulent and dense northern CSM induces competing effects on electron acceleration: on one hand, turbulence reduces the electron mean free path enhancing the acceleration efficiency, on the other hand, lower shock velocities increase the acceleration time-scale. In most filaments, these effects compensate each other, but in one region the acceleration time-scale exceeds the synchrotron time-scale, resulting in a significant decrease in nonthermal X-ray emission from 2006 to 2014, indicating fading synchrotron emission. Our findings provide a coherent understanding of the different regimes of electron acceleration observed in Kepler's SNR through various diagnostics.
Abstract: 2407.19019
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Title:Eight Years of Light from ASASSN-15oi: Towards Understanding the Late-time Evolution of TDEs
View PDF HTML (experimental)Abstract:We present the results from an extensive follow-up campaign of the Tidal Disruption Event (TDE) ASASSN-15oi spanning $\delta t \sim 10 - 3000$ d, offering an unprecedented window into the multiwavelength properties of a TDE during its first $\approx 8$ years of evolution. ASASSN-15oi is one of the few TDEs with strong detections at X-ray, optical/UV, and radio wavelengths and featured two delayed radio flares at $\delta t \sim 180$ d and $\delta t \sim 1400$ d. Our observations at $> 1400$ d reveal an absence of thermal X-rays, a late-time variability in the non-thermal X-ray emission, and sharp declines in the non-thermal X-ray and radio emission at $\delta t \sim 2800$ d and $\sim 3000$ d, respectively. The UV emission shows no significant evolution at $>400$ d and remains above the pre-TDE level. We show that a cooling envelope model can explain the thermal emission consistently across all epochs. We also find that a scenario involving episodic ejection of material due to stream-stream collisions is conducive to explaining the first radio flare. Given the peculiar spectral and temporal evolution of the late-time emission, however, constraining the origins of the second radio flare and the non-thermal X-rays remains challenging. Our study underscores the critical role of long-term, multiwavelength follow-up.
Abstract: 2407.19134
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Title:Redshift constrain of BL Lac PKS 1424+240
View PDF HTML (experimental)Abstract:In the period between 2009 and 2015, several very high-energy (VHE $> 100$ GeV) gamma-ray flaring events from the BL Lac object PKS 1424+240 were observed by the Cerenkov telescopes VERITAS and MAGIC. It had uncertain redshift ($z$) and using spectroscopical measurement, Paiano et al. (2017) found it to be $z=0.604$. Using four different extragalactic background light (EBL) models and the photohadronic model, nine independently observed VHE gamma-ray spectra of PKS 1424+240 are analyzed and a global $\chi^2$ fit is performed on all observations to estimate the best-fit value for the redshift for each EBL model. Confidence levels (CL) intervals for the redshift are also estimated using all the EBL models. This method is tested by comparing our analysis with the observed value. It is shown that the photohadronic scenario provides an excellent description of all the observed spectra. It is found that the EBL model of Domínguez et al. (2011) is the one that provides the most restrictive limits on the redshift of PKS 1424+240, but in our analysis, $z=0.604$ lies within the $3\sigma$ CL interval of the EBL model of Saldana-Lopez et al. (2021).
Abstract: 2407.19957
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Title:Inspecting binary-neutron-star mergers as production sites of high-energy neutrinos
View PDF HTML (experimental)Abstract:The end state of binary-neutron-star (BNS) mergers can manifest conditions to produce high-energy neutrinos. Inspired by the event GW170817, detected in gravitational waves and in optical/infrared emission, we investigate a scenario in which cosmic-ray (CR) particles are accelerated, in a population of BNS mergers, in the energy range from the knee to the ankle. By taking into account the measured thermal and non-thermal energy density of the photon fields in the source environment as a function of the time after the merger, we model the CR interactions and the consequent neutrino production. We propagate the escaped CR and neutrino fluxes through the extragalactic space and compare the expected diffuse fluxes to the experimental data and current limits. Depending on the accelerated CR mass composition and on the contribution of this possible source class to the sub-ankle CR flux, we provide indications on the rate of BNS merger events and the amount of baryons present in the source site.
Abstract: 2407.20186
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Title:On the leptonic contribution to the ultra high-energy diffuse gamma-ray background
View PDF HTML (experimental)Abstract:The ultra high-energy (UHE) diffuse gamma-ray background holds important information on the propagation of cosmic rays in the Galaxy. However, its measurements suffer from a contamination from unresolved sources whose importance remains unclear. In this Letter, we propose a novel data-driven estimate of the contribution of unresolved leptonic sources based on the information present in the ATNF and the LHAASO catalogs. We find that in the inner Galaxy at most $\sim60\%$ of the diffuse flux measured by LHAASO at $10\,\rm{TeV}$ may originate from unresolved leptonic sources, and this fraction drops with energy to less than $20\%$ at $100\,\rm{TeV}$. In the outer Galaxy, the contribution of unresolved leptonic sources is always subdominant. It is less than $\sim 20\%$ at $10\,\rm{TeV}$ and less than $\sim 8\%$ above $\sim25\,\rm{TeV}$. We conclude that the UHE diffuse background should be dominated by photons from a hadronic origin above a few tens of $\rm{TeV}$.
Abstract: 2407.20305
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Title:A Short History of the First 50 Years: from the GRB Prompt Emission and Afterglow Discoveries to the Multimessenger Era
View PDFAbstract:More than fifty years have been elapsed from the first discovery of a Gamma Ray Burst (GRB) with the American Vela satellites, and more than twenty-five years from the discovery with the BeppoSAX satellite of the first X-ray afterglow of a GRB. Thanks to the afterglow discovery and to the possibility given to the optical and radio astronomers to discover the GRB optical counterparts, the long-time mystery about the origin of these events was solved. Now we know that GRBs are huge explosions, mainly ultra relativistic jets, in galaxies at cosmological distances. Starting from the first GRB detection with the Vela satellites, I will review the story of these discoveries, those obtained with BeppoSAX, the contribution to GRBs by other satellites and ground experiments, among them Venera, Compton Gamma Ray Observatory, HETE2, Swift, Fermi, AGILE, MAGIC, H.E.S.S., which were, and some of them are still, very important for the study of GRB properties. Then I will review the main results obtained thus far and the still open problems and prospects of the GRB astronomy.
Abstract: 2407.20348
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Title:Late Jets, Early Sparks: Illuminating the Pre-Maximum Bumps in Superluminous Supernovae
View PDF HTML (experimental)Abstract:Superluminous supernovae (SLSNe) radiate $\gtrsim 10-100$ times more energy than ordinary stellar explosions, implicating a novel power source behind these enigmatic events. One frequently discussed source, particularly for hydrogen-poor (Type I) SLSNe, is a central engine such as a millisecond magnetar or accreting black hole. Both black hole and magnetar engines are expected to channel a fraction of their luminosity into a collimated relativistic jet. Using 3D relativistic hydrodynamical simulations, we explore the interaction of a relativistic jet, endowed with a luminosity $L_{\rm j}\approx10^{45}\,{\rm erg\,s^{-1}}$ and duration $t_{\rm eng} \approx$ days compatible with those needed to power SLSNe, launched into the envelope of the exploding star. The jet successfully breaks through the expanding ejecta and its shocked cocoon powers ultraviolet/optical emission lasting several days after the explosion and reaching a peak luminosity $\gtrsim 10^{44}$ erg s$^{-1}$, corresponding to a sizable fraction of $L_{\rm j}$. This high radiative efficiency is the result of the modest adiabatic losses the cocoon experiences due to the low optical depths of the enlarged ejecta at these late times, e.g., compared to the more compact stars in gamma-ray bursts. The luminosity and temperature of the cocoon emission match those of the ``bumps'' in SLSNe light curves observed weeks prior to the optical maximum in many SLSNe. Confirmation of jet breakout signatures by future observations (e.g., days to weeks-long internal X-ray emission from the jet for on-axis observers, spectroscopy confirming large photosphere velocities $v/c \gtrsim 0.1$, or detection of a radio afterglow) would offer strong evidence for central engines powering SLSNe.
Abstract: 2407.20507
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Title:Neutrino Oscillation Effects on the Luminosity of Neutrino-Dominated Accretion Flows Around Black Holes
View PDF HTML (experimental)Abstract:The stellar-mass black hole surrounded by the neutrino-dominated accretion flow (NDAF) is proposed to be the central engine of the gamma-ray burst (GRB). In this work, we investigate the neutrino/anti-neutrino luminosity and annihilation luminosity from the NDAF and pair-annihilation model, taking into account the neutrino oscillation above the accretion disk. The disk hydrodynamical properties are modelled using the empirical solution previously derived with boundary conditions including the effect of electron degeneracy and neutrino trapping. Our key parameters are the mass accretion rate and the black hole spin given in the range of $\dot{M} = 0.1$ - $10$ $M_\odot$ s$^{-1}$ and $ 0 \le a < 1$, respectively. Without neutrino oscillation, the obtained neutrino/anti-neutrino luminosity is $\sim 10^{51}$ - $10^{53}$ $\textrm{erg s}^{-1}$, while the neutrino annihilation luminosity is found to be $\sim 10^{46}$ - $10^{51}$ $\textrm{erg s}^{-1}$. In the presence of neutrino oscillation in the vacuum limit, the electron neutrino annihilation luminosity decreases by $\lesssim 22\%$ through the flavor transformation, while the muon- and tau-neutrino luminosity can increase up to $\sim 45\%$ and $ 60\%$, respectively. As a result, the total annihilation luminosity can be reduced up to $\sim 19\%$ due to the oscillation process above the disk. Finally, we also investigate the case whereby the CP-violating phase is changed from $\delta \textrm{CP} =$ $0^{\circ}$ to $245^{\circ}$. However, our results reveal that the CP-violating phase has minimal impact on neutrino annihilation luminosity.
Abstract: 2407.21277
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Title:Two-Component gamma-ray Emission Spectrum and X-Ray Polarization of the Radio Galaxy Pictor A
View PDF HTML (experimental)Abstract:Pictor A is a $\gamma$-ray emitting radio galaxy and has a bright hotspot called WHS, located $\sim$4 arcmin away from the nucleus. In this letter, we present an analysis of its 16-year Fermi-LAT data and report the first Imaging X-ray Polarimetry Explorer (IXPE) observation for this source. Our analysis of the Fermi-LAT observations reveals evidence of two components in the average $\gamma$-ray spectrum of Pictor A, exhibiting a statistically significant hardening from $\Gamma^1_{\gamma}=3.25\pm0.15$ to $\Gamma^2_{\gamma}=1.81\pm0.07$ at a break energy of $2.46\pm0.09$ GeV. The evident variability of $\gamma$-rays is observed in Pictor A. Interestingly, the variability is dominated by the component below the break energy, and the component above the break energy shows no variability. Furthermore, we find that a power-law function can adequately fit the spectrum during high-flux states, whereas a broken power-law is still required to explain the spectrum during low-flux state. We suggest that the low-energy component originates from the nucleus, while the high-energy component primarily stems from WHS. The broadband spectral energy distributions of both nucleus and WHS can be well represented by a simple leptonic model, with both $\gamma$-ray components attributed to the synchrotron-self-Compton (SSC) process. The analysis of IXPE data on the nucleus yields an upper limit to the polarization degree $\Pi_{\rm X}<$8.9\% in the 2--8 keV band, agreeing with its X-ray emission originating from SSC. However, $\Pi_{\rm X}=23.5\%\pm5.6\%$ is observed at a confidence level of $>99\%$ in the 5--7 keV band, and the possible physical origin of this narrow-energy-band polarization signal is discussed.
Abstract: 2407.21371
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Title:Einstein Probe discovery of a super-soft outburst from CXOU J005245.0-722844: a rare BeWD binary in the Small Magellanic Cloud
View PDF HTML (experimental)Abstract:On May 27 2024, the Wide-field X-ray Telescope onboard the Einstein Probe (EP) mission detected enhanced X-ray emission from a new transient source in the Small Magellanic Cloud (SMC) during its commissioning phase. Prompt follow-up with the EP Follow-up X-ray Telescope, the Swift X-ray Telescope and Nicer have revealed a very soft, thermally emitting source (kT$\sim$0.1 keV at the outburst peak) with an X-ray luminosity of L$\sim$4$\times$10$^{38}$ erg s$^{-1}$, coincident with CXOU J005245.0-722844. This super-soft outburst faded very quickly in a week time. Several emission lines and absorption edges were present in the X-ray spectrum, such as the Oxygen (0.57 keV) and Neon (0.92 keV) He-like emission lines, and deep Nitrogen (0.67 keV) and Oxygen (0.87 keV) absorption edges. The X-ray emission resembles typical nova outbursts from an accreting white dwarf (WD) in a binary system, despite the X-ray source being historically associated with an O9-B0e massive star exhibiting a 17.55 days periodicity in the optical band. The discovery of this super-soft outburst nails down CXOU J005245.0-722844 as a BeWD X-ray binary: an elusive evolutionary stage where two main-sequence massive stars have undergone a common envelope phase and experienced at least two episodes of mass transfer. In addition, the very short duration of the outburst and the presence of Ne features hint at a rather massive, i.e., close to the Chandrasekhar limit, Ne-O WD in the system.
Abstract: 2408.00406
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Title:Anti-correlation between Flux and Photon Index of Hard X-ray Emission from The Crab
View PDF HTML (experimental)Abstract:Using \textit{Swift} Burst Alert Telescope (BAT) event-mode data during Gamma Ray Burst (GRB) occurrences, we conducted spectral analysis for the Crab system. From 38 good observations, which spans over a period of 18 years from 2006 to 2023, we found that the Crab's X-ray flux does not only flicker, but also significantly anti-correlates to its spectral power-law photon index. Since emission contribution of the Crab pulsar in this energy range is small, this anti-correlation is mainly about the emission of the Crab nebula. We suggest that this anti-correlation is an observational supporting evidence for the long-standing notion that the nebula emission is due to synchrotron radiation of shocked pulsar winds in the nebula.
Abstract: 2408.00608
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Title:Discovery of Polarized X-Ray Emission from the Accreting Millisecond Pulsar SRGA J144459.2-604207
View PDF HTML (experimental)Abstract:We report on the discovery of polarized X-ray emission from an accreting millisecond pulsar. During a 10-day-long coverage of the February 2024 outburst of SRGA J144459.2-604207, the Imaging X-ray Polarimetry Explorer (IXPE) detected an average polarization degree of the 2-8 keV emission of 2.3% +/- 0.4% at an angle of 59° +/- 6° (East of North; uncertainties quoted at the 1${\sigma}$ confidence level). The polarized signal shows a significant energy dependence with a degree of 4.0% +/- 0.5% between 3 and 6 keV and < 2% (90% c.l.) in the 2-3 keV range. We used NICER, XMM-Newton, and NuSTAR observations to obtain an accurate pulse timing solution and perform a phase-resolved polarimetric analysis of IXPE data. We did not detect any significant variability of the Stokes parameters Q and U with the spin and the orbital phases. We used the relativistic rotating vector model to show that a moderately fan-beam emission from two point-like spots at a small magnetic obliquity ($\simeq$ 10°) is compatible with the observed pulse profile and polarization properties. IXPE also detected 52 type-I X-ray bursts, with a recurrence time $\Delta t_{rec}$ increasing from 2 to 8 h as a function of the observed count rate $C$ as as $\Delta t_{rec} \simeq C^{-0.8}$ We stacked the emission observed during all the bursts and obtained an upper limit on the polarization degree of 8.5% (90% c.l.).
Abstract: 2408.00614
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Title:The large-scale anisotropy and flux (de-)magnification of ultra-high-energy cosmic rays in the Galactic magnetic field
View PDF HTML (experimental)Abstract:We calculate the arrival direction distribution of ultra-high-energy cosmic rays (UHECRs) with a new suite of models of the Galactic magnetic field (GMF), assuming sources follow the large-scale structure of the Universe. We find a significantly reduced dipole amplitude compared to previous GMF models, and trace the change to the accidental position of the peak of the extragalactic UHECR flux, which falls at the boundary of the strong flux de-magnification due to the GMF toward the central region of the Galaxy. This serendipitous sensitivity of UHECR anisotropies to the GMF model will be a powerful probe of the source distribution as well as Galactic and extragalactic magnetic fields. Demagnification by the GMF also impacts visibility of some popular source candidates.
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