arXiv subjects

V. Jithesh

Publications and source records attributed to V. Jithesh.

At least 19 recordsLinked to original sources

arXiv2026

Complex Energy-Dependent Behaviour of Quasi-Periodic Oscillation Observed in GRS 1915+105

We present complex energy-resolved properties of quasi-periodic oscillations (QPOs) in the black hole X-ray binary GRS 1915+105 using an observation from the LAXPC instrument onboard AstroSat. Power density spectra (PDSs) are constructed in multiple energy bands and modeled with multi-Lorentzian components to investigate the energy dependence of QPO properties. The QPO frequency shows a modest increase with energy. Dynamic PDS analysis does not reveal clear evidence for time-dependent evolution of the QPO frequency, suggesting that the observed frequency shift is not primarily driven by temporal variability. We perform simultaneous fitting of energy-resolved PDSs and find that a model in which the QPO feature is described by two Lorentzian components provides a better fit. The two components exhibit different evolution in fractional root mean square amplitude as a function of energy. We further examine the phase-lag properties by simultaneously modeling the PDS and the real and imaginary parts of the cross-spectrum and find distinct phase-lag behavior for the two components. Overall, these results indicate that the apparent energy-dependent evolution of the QPO feature may be a result of the presence of more than one variability component.

astro-ph.HE↗

arXiv2026

Transient and Variable Ultra-luminous X-ray Sources in NGC 4552

We searched for transient and variable X-ray sources in the elliptical galaxy NGC 4552 using the {\it Chandra} and {\it XMM-Newton} observations from 2001-2012. We detected 14 transient and variable X-ray sources within the 4$R_{e}$ region of the galaxy, which exhibited peak state and undetected flux behaviour in the {\it Chandra} observations. Among them, two sources (T1 and T14) exceed the X-ray luminosity $10^{39}\rm~erg~s^{-1}$ in the detected observations and have the peak state and undetected luminosity ratio of $\geq 14.3$ and $\geq 8.7$, respectively, which we classified as transient and variable ultra-luminous X-ray sources (ULXs). For the third relatively bright transient source, the flux varies by a factor of 3 across the detected observations, and the hardness ratio analysis suggests a normal X-ray binary system. The remaining sources have luminosities in the $10^{37}-10^{38} \rm~erg~s^{-1}$ range, and the spectral analysis indicates that the sources may belong to neutron star or black hole X-ray binary systems, either in the hard or thermal-dominated state. The identified transient and variable ULXs exhibited high variability and soft spectral characteristics, which are consistent with ULXs accreting at super-Eddington rates. The near-infrared (NIR) counterpart search using {\it JWST} observations identified potential NIR counterparts for three mildly variable sources, and their magnitudes in different NIR bands suggest a red supergiant companion for each source. Deep X-ray monitoring observations can shed further light on the new transient and variable ULXs.

astro-ph.HE↗

arXiv2026

X-Ray Spectral Variability of the TeV HBL Blazar PG 1553+113 with XMM-Newton

We present an extensive X-ray spectral variability study of the TeV photon-emitting high-energy-peaked BL Lacertae object PG 1553+113, using the data from EPIC-PN camera of XMM-Newton, which observed the source during its operational period from Sep 2001 to Nov 2024. X-ray spectra in this energy range, $0.6-7.0$ keV, were fitted with absorbed Power-law (PL) and absorbed Log-Parabola (LP) models. We found with 99$\%$ confidence that 14 of them were fit well by LP models having parameters in the range $α\simeq2.13-2.80$, and $β\simeq0.04-0.18$, one spectrum favours a LP model with $β<0$, while simple PL models with $Γ\simeq2.53-2.69$ were sufficient to describe the X-ray spectra of the remaining 15. Two of these 30 observations showed strong signatures of an additional inverse Compton component, while one showed weaker indications. On fitting joint Optical Monitor and EPIC-PN data with LP models, we found synchrotron peaks in the energy range of $ν_s\simeq4.59-48.61$ eV. This indicates that the spectral evolution is probably caused by variations in particle acceleration or cooling conditions within the jet.

astro-ph.HE↗

arXiv2026

Tracing Fe K X-ray reverberation lag in the energy-resolved spectra of Narrow-line Seyfert 1 galaxy Ton S180

We report the Fe K relativistic reverberation feature for the first time in the Narrow-line Seyfert\,1 galaxy Ton\,S180. Using a long observation from {\it XMM-Newton} we find that the Fe K emission lag peaks at $117\pm49$ s in the lag energy spectrum computed for frequencies $(0.3-8.5) \times 10^{-4}$ Hz. The lag amplitude drops to $22.85\pm14.20$ s as the frequency increases to $(8.5-30) \times 10^{-4}$ Hz. The time-averaged spectrum of the source shows a relatively narrow Fe K line at $\sim6.4$ keV, resulting in black hole spin to be low ($a=\rm 0.43_{-0.14}^{+0.10}$) found from the reflection modelling. We perform general relativistic transfer function modelling of the lag energy spectra individually. This provides an independent timing-based measure of the spin at $a=0.30_{-0.17}^{+0.34}$, and black hole mass $M_{\rm BH} = 0.29_{-0.16}^{+0.01}\times10^8M_{\odot}$, comparable to the previous measurement, and height of the corona $h = 2.59_{-0.33}^{+5.17}r_{\rm g}$. Further, we observe that the Fe K lag and the black hole mass fit well in the linear lag-mass relation shown by other Seyfert 1 galaxies.

astro-ph.HE↗

arXiv2025

Probing the Origin of X-ray Flares in the Low-Hard State of GRS 1915+105 Using AstroSat and NuSTAR

We performed a detailed time-resolved spectral study of GRS 1915+105 during its low-flux rebrightening phase using the broadband capabilities of AstroSat and NuSTAR in May-June 2019. The AstroSat light curves revealed erratic X-ray flares with count rates rising by a factor of $\sim$5. Flares with simultaneous LAXPC and SXT coverage were segmented and fitted using two degenerate but physically motivated spectral models: a reflection-dominated model (hereafter Model A) and an absorption-dominated model (hereafter Model B). In Model A, the inner disk radius $(R_{in})$ shows a broken power-law dependence on flux, indicating rapid inward motion of the disk at higher flux levels. In contrast, Model B shows variable column density in the range of $10^{23}$ to $10^{24}$ cm$^{-2}$, displaying a strong anti-correlation with flux. Both models exhibit significant variation in the ionization parameter between low- and high-flux segments. The total unabsorbed luminosity in the 0.7--30~keV energy range ranged from $6.64 \times 10^{36}$ to $6.33 \times 10^{38}$~erg~s$^{-1}$. Across both models, several spectral parameters exhibited step-function-like behavior around flux thresholds of $5$--$10 \times 10^{-9}$ erg cm$^{-2}$ s$^{-1}$, indicating multiple spectral regimes. The disc flux contribution, more evident in Model B, increased with total flux, supporting an intrinsic origin for the variability. These findings point to a complex interplay between intrinsic disk emission, structured winds, and variable local absorption in driving the flare activity.

astro-ph.HE↗

arXiv2025

Thermonuclear X-ray bursts across the eclipse transitions in the LMXBs EXO 0748-676 and XTE J1710-281

The primary radiation from thermonuclear X-ray bursts observed in the neutron star low-mass X-ray binary (LMXB) systems can interact with various parts of the binary system. This interaction gives rise to secondary radiation in different wavelength ranges, known as reprocessed emission. In eclipsing LMXBs, the reprocessed emission from the bursts can be examined during eclipses, as the primary emission is blocked and only the reprocessed emission is visible. We searched for bursts during eclipses in the archival RXTE data of the eclipsing LMXBs and found them in EXO 0748-676 and XTE J1710-281. In EXO 0748-676, seven bursts were found to occur near eclipse egress, with their tails extending beyond the eclipse, and one such burst was found for XTE J1710-281. We estimate the reprocessing fraction at orbital phases near eclipse egress by modeling the peculiar eclipse bursts detected in both systems, which have tails extending beyond the eclipses. We observe an increasing trend in reprocessing fraction as these eclipse bursts occur closer to the eclipse egress. We discuss the possibilities of reprocessing in the ablated wind from the companion star, the accretion disc, and the disc wind in EXO 0748-676 and XTE J1710-281. Additionally, we observe two decay components in the bursts in EXO 0748-676, which could suggest a complex composition of the accreting fuel. From the burst rise timescales, we place an upper limit on the size of the reprocessing regions in both EXO 0748-676 and XTE J1710-281, finding it comparable to the size of the respective X-ray binaries.

astro-ph.HE↗

arXiv2025

Quintessence-Chameleon transitions in anisotropic Kiselev model of neutron stars

We investigate a chameleon scalar field dynamically interacting with a Kiselev-type metric, where the static anisotropic fluid part of the metric is replaced by a density-dependent scalar field non-minimally coupled to curvature. This construction enables a transition from screened behavior in high-density regions-where the scalar acquires an effective mass m_ϕ\proptoρ^1/2-to unscreened quintessence dynamics at large scales, characterized by a critical screening radius r_\rm crit\propto m_ϕ^-1. By solving the modified TOV equations under spherical symmetry, we show that radial scalar gradients \partial_rϕinduce pressure anisotropies Δp\propto r^-1 in neutron star envelopes, while deviations from general relativity are suppressed deep in the core r<r_\rm crit\sim 0.03\,\mathrm{km} without destabilizing it. We further demonstrate that increasing the scalar coupling βenhances scalar energy density, which counteracts anisotropic pressure support and slightly reduces the maximum mass. The resulting (stable) configurations yield maximum mass M_max\approx 1.75\pm0.280\,M_\odot and radii R\approx11.35\pm1.11\,\mathrm{km}, consistent with conservative upper and lower bounds from multimessenger observations. Scalar contributions induce a modest suppression in the dimensionless tidal deformability Λ_\rm STΛ_\rm GR\sim0.137 at 1.4\,M_\odot i.e, 86% suppression compared to typical GR star, falling well within the LIGO/Virgo range 70\lesssimΛ_1.4\lesssim 580. These results demonstrate that environmentally screened scalar fields can dynamically generate anisotropies and modify neutron star structure without violating current astrophysical bounds. In particular, percent-level deviations in compactness and tidal response provide falsifiable signatures of short-range scalar forces, offering a novel target for next-generation GW and multimessenger surveys.

gr-qc↗

arXiv2025

Spectral evolution of GX 17+2 using AstroSat and NICER observations

We study the spectral evolution of the Z-track source GX 17+2 using AstroSat and NICER observations taken between 2016 and 2020. The AstroSat observations cover the period when the source is in the normal branch (NB) and the flaring branch (FB), while for the NICER ones the variability can be associated with the FB branch. The source spectra at different regions of the branches are well described by accretion disk emission, blackbody surface emission and a thermal Comptonization component. In the NB, the total bolometric unabsorbed flux remains constant and the variation is due to changes in the Comptonization, disk fluxes. In particular, the inferred luminosity ($L_{\rm T}$) and accretion rate ($\dot M$) remain constant, while there is significant variation in the inner disk radii and fraction of disk photons entering the corona, indicating changes in the geometry of the system. On the other hand, in the FB, there is significant variation in luminosity from $\sim 4.0$ to $\sim 7.0 \times 10^{38}$ ergs s$^{-1}$. Despite this significant variation in luminosity and in the inner disk radii, the accretion efficiency defined as $η= L_{\rm T}/{\dot M} c^2$, remains nearly constant at $\sim 0.20$ throughout the evolution of the source, as expected for a neutron star system.

astro-ph.HE↗

arXiv2025

X-Ray Spectral Variability of Thirteen TeV High Energy Peaked Blazars with XMM-Newton

We present a comprehensive study of the X-ray spectral variability observed in 13 TeV photon emitting high energy peaked BL Lacs(HBLs). These data come from 54 XMM-Newton EPIC-PN pointed observations made during its operational period from June 2001 through July 2023. We performed spectral studies in the energy range 0.6-10 keV by fitting X-ray spectra of the pointed observations with power law (PL) and log parabolic (LP) models. We found at 99% confidence level that 31 of these X-ray spectra were best fitted with a range of LP models with local photon indices (at 1.0 keV), alpha in the range of 1.75-2.66, and convex curvature parameter beta in the range of 0.02-0.25. PL models with photon index Gamma in the range of 1.78-2.68 best described the spectra of fourteen-pointed observations. Nine PN spectra resulted in negative curvature parameters in fitting an LP model, and eight among them were significant (beta is at least twice beta_err). We fitted broken power law (BPL) models to these eight X-ray spectra and found spectral hardening (Delta_Gamma) in the range of 0.06-0.54 for these observations. EPIC-MOS spectra were also studied for those eight observations to search for similar trends, and we were able to find them in only two, one observation each of PKS 0548-322 and Mrk 501. This indicates the possibility of the co-existence of an inverse Compton component along with the dominant synchrotron component for these two observations. We also performed correlation studies between various log-parabolic spectral parameters and briefly discuss their possible implications.

astro-ph.HE↗

arXiv2024

Serendipitous detection of an intense X-ray flare in the weak-line T Tauri star KM Ori with SRG/eROSITA

Weak-line T Tauri stars (WTTS) exhibit X-ray flares, likely resulting from magnetic reconnection that heats the stellar plasma to very high temperatures. These flares are difficult to identify through targeted observations. Here, we report the serendipitous detection of the brightest X-ray flaring state of KM Ori in the eROSITA DR1 survey. Observations from SRG/eROSITA, Chandra X-ray Observatory, and XMM-Newton are analysed to assess the X-ray properties of KM Ori, thereby establishing its flaring state at the eROSITA epoch. The long-term (1999-2020) X-ray light curve generated for the Chandra observations confirmed that eROSITA captured the source at its highest X-ray flaring state recorded to date. Multi-instrument observations support the X-ray flaring state of the source, with time-averaged X-ray luminosity ($L_{0.2-5keV}$) reaching $\sim 1.9\times10^{32}{erg~s^{-1}}$ at the eROSITA epoch, marking it the brightest and possibly the longest flare observed to date. Such intense X-ray flares have been detected only in a few WTTS. The X-ray spectral analysis unveils the presence of multiple thermal plasma components at all epochs. The notably high luminosity ($L_{0.5-8~keV}\sim10^{32}{erg~s}^{-1}$), energy ($E_{0.5-8~keV}\sim10^{37}$erg), and the elevated emission measures of the thermal components in the eROSITA epoch indicate a superflare/megaflare state of KM Ori. Additionally, the H$α$ line equivalent width of $\sim$-5Åfrom our optical spectral analysis, combined with the lack of infrared excess in the spectral energy distribution, were used to re-confirm the WTTS (thin disk/disk-less) classification of the source. The long-duration flare of KM Ori observed by eROSITA indicates the possibility of a slow-rise top-flat flare. The detection demonstrates the potential of eROSITA to uncover such rare, transient events, thereby providing new insights into the X-ray activity of WTTS.

astro-ph.HE↗

arXiv2024

Chandra X-ray Analysis of Herbig Ae/Be Stars

Herbig Ae/Be (HAeBe) stars are intermediate-mass pre-main sequence stars, characterized by infrared excess and emission lines. They are observed to emit X-rays, whose origin is a matter of discussion and not settled yet. X-ray emission is not expected in HAeBe stars, as they lack the sub-surface convective zone. In this study, we retrieved observations from the Chandra archive for 62 HAeBe stars, among which 44 sources (detection fraction $\sim$71%) were detected in X-rays, with 7 being new detections. We use this sample as a test bed to conduct a comparative analysis of the X-ray properties of HAeBe stars and their low-mass counterparts, T Tauri Stars (TTSs). Further, we compare the X-ray properties of HAeBe stars and TTSs with optical and IR properties to constrain the X-ray emission mechanism in HAeBe stars. We found no correlation between X-ray emission and disk properties of HAeBe stars, confirming that X-rays are not related to accretion shocks. About 56% of HAeBe stars without any known sub-arcsec companions have lower plasma temperatures (kT $\leq$ 2 keV). We observe flaring/variability in HAeBe stars with confirmed low-mass companions. These stars show plasma temperatures > 2 keV, similar to TTSs. Guided by this information we discuss the role of a T Tauri companion for X-ray emission seen in our sample of HAeBe stars. From the results obtained in this paper, we suggest that X-ray emission from HAeBe stars may not be related to accretion shocks or hidden TTS, but rather can be due to magnetically driven coronal emission.

astro-ph.SR↗

arXiv2024

Flares during Eclipses of High Mass X-ray Binary Systems Vela X-1, 4U 1700-37, and LMC X-4

In eclipsing X-ray binary systems, the direct X-ray emission is blocked by the companion star during the eclipse. We observe only reprocessed emission that contains clues about the environment of the compact object and its chemical composition, ionization levels, etc. We have found flares in some X-ray binaries during their eclipses. The study of eclipse flares provides additional clues regarding the size of the reprocessing region and helps distinguish between different components of the X-ray spectrum observed during the eclipse. In the archival data, we searched for flares during eclipses of high-mass X-ray binaries and found flares in three sources: Vela X-1, LMC X-4, and 4U 1700-37. Comparing spectral properties of the eclipse flare and non-flare data, we found changes in the power-law photon index in all three sources and multiple emission lines in Vela X-1 and 4U 1700-37. The fluxes of prominent emission lines showed a similar increase as the overall X-ray flux during the eclipse flare, suggesting the lines originate in the binary environment and not in the interstellar medium. We also observed a soft excess in 4U 1700-37 that remains unchanged during both eclipse flare and non-flare states. Our analysis suggests that this emission originates from the extremely thin shell of the stellar wind surrounding the photosphere of its companion star. The detection of short (100-200 seconds) count-rate doubling timescale in 4U 1700-37 and LMC X-4 indicates that the eclipse reprocessing occurs in a region larger than, but comparable to the size of the companion star.

astro-ph.HE↗

arXiv2024

Time resolved spectroscopy of a GRS 1915+105 flare during its unusual low state using AstroSat

Since its discovery in 1992, GRS 1915+105 has been among the brightest sources in the X-ray sky. However, in early 2018, it dimmed significantly and has stayed in this faint state ever since. We report on AstroSat and NuSTAR observation of GRS 1915+105 in its unusual low/hard state during 2019 May. We performed time-resolved spectroscopy of the X-ray flares observed in this state and found that the spectra can be fitted well using highly ionised absorption models. We further show that the spectra can also be fitted using a highly relativistic reflection dominated model, where for the lamp post geometry, the X-ray emitting source is always very close to the central black hole. For both interpretations, the flare can be attributed to a change in the intrinsic flux, rather than dramatic variation in the absorption or geometry. These reflection dominated spectra are very similar to the reflection dominated spectra reported for Active Galactic Nuclei in their low flux states.

astro-ph.HE↗

arXiv2023

Extreme photometric and polarimetric variability of blazar S4 0954+65 at its maximum optical and $γ$-ray brightness levels

In 2022 the BL Lac object S4 0954+65 underwent a major variability phase, reaching its historical maximum brightness in the optical and $γ$-ray bands. We present optical photometric and polarimetric data acquired by the Whole Earth Blazar Telescope (WEBT) Collaboration from 2022 April 6 to July 6. Many episodes of unprecedented fast variability were detected, implying an upper limit to the size of the emitting region as low as $10^{-4}$ parsec. The WEBT data show rapid variability in both the degree and angle of polarization. We analyse different models to explain the polarization behaviour in the framework of a twisting jet model, which assumes that the long-term trend of the flux is produced by variations in the emitting region viewing angle. All the models can reproduce the average trend of the polarization degree, and can account for its general anticorrelation with the flux, but the dispersion of the data requires the presence of intrinsic mechanisms, such as turbulence, shocks, or magnetic reconnection. The WEBT optical data are compared to $γ$-ray data from the Fermi satellite. These are analysed with both fixed and adaptive binning procedures. We show that the strong correlation between optical and $γ$-ray data without measurable delay assumes different slopes in faint and high brightness states, and this is compatible with a scenario where in faint states we mainly see the imprint of the geometrical effects, while in bright states the synchrotron self-Compton process dominates.

astro-ph.HE↗

arXiv2023

A Search for X-ray/UV Correlation in the Reflection-Dominated Seyfert 1 Galaxy Mrk 1044

Correlated variability between coronal X-rays and disc optical/UV photons provides a very useful diagnostic of the interplay between the different regions around an active galactic nucleus (AGN) and how they interact. AGN that reveal strong X-ray reflection in their spectra should normally exhibit optical/UV to X-ray correlation consistent with reprocessing -- where the optical/UV emission lag behind the X-rays. While such correlated delay has been seen in some sources, it has been absent in others. \rm{Mrk~1044} is one such source that has been known to reveal strong X-ray reflection in its spectra. In our analysis of three long \textit{XMM-Newton} and several \textit{Swift} observations of the source, we found no strong evidence for correlation between its UV and X-ray lightcurves both on short and long time scales. Among other plausible causes for the non-detection, we posit that higher X-ray variability than UV and strong general relativistic effects close to the black hole may also be responsible. We also present results from the spectral analysis based on \textit{XMM-Newton} and \textit{NuSTAR} observations, which show the strong soft X-ray excess and iron K$α$ line in the 0.3--50 keV spectrum that can be described by relativistic reflection.

astro-ph.HE↗

arXiv2023

Spectral and Temporal Studies of Swift\,J1658.2--4242 using {\it AstroSat} Observations with {\tt JeTCAF} Model

We present the X-ray spectral and temporal analysis of the black hole X-ray transient Swift J1658.2--4242 observed by {\it AstroSat}. Three epochs of data have been analysed using the JeTCAF model to estimate the mass accretion rates and to understand the geometry of the flow. The best-fit disc mass accretion rate ($\dot m_d$) varies between $0.90^{+0.02}_{-0.01}$ to $1.09^{+0.04}_{-0.03}$ $\dot M_{\rm Edd}$ in these observations, while the halo mass accretion rate changes from $0.15^{+0.01}_{-0.01}$ to $0.25^{+0.02}_{-0.01}$ $\dot M_{\rm Edd}$. We estimate the size of the dynamic corona, that varies substantially from $64.9^{+3.9}_{-3.1}$ to $34.5^{+2.0}_{-1.5}$ $r_g$ and a moderately high jet/outflow collimation factor stipulates isotropic outflow. The inferred high disc mass accretion rate and bigger corona size indicate that the source might be in the intermediate to soft spectral state of black hole X-ray binaries. The mass of the black hole estimated from different model combinations is $\sim 14 M_\odot$. In addition, we compute the quasi-periodic oscillation (QPO) frequencies from the model-fitted parameters, which match the observed QPOs. We further calculate the binary parameters of the system from the decay profile of the light curve and the spectral parameters. The estimated orbital period of the system is $4.0\pm0.4$ hr by assuming the companion as a mid or late K-type star. Our analysis using the JeTCAF model sheds light on the physical origin of the spectro-temporal behaviour of the source, and the observed properties are mainly due to the change in both the mass accretion rates and absorbing column density.

astro-ph.HE↗

arXiv2022

Correlated variability of the reflection fraction with the X-ray flux and spectral index for Mkn 478

The X-ray spectrum of Mkn 478 is known to be dominated by a strong soft excess which can be described using relativistic blurred reflection. Using observations from {\it XMM-Newton}, {\it AstroSat} and {\it Swift}, we show that for the long-term ($\sim$ years) and intermediate-term (days to months) variability, the reflection fraction is anti-correlated with the flux and spectral index, which implies that the variability is due to the hard X-ray producing corona moving closer to and further from the black hole. Using flux-resolved spectroscopy of the {\it XMM-Newton} data, we show that the reflection fraction has the same behaviour with flux and index on short time-scales of hours. The results indicate that both the long-term and short-term variability of the source is determined by the same physical mechanism of strong gravitational light bending causing enhanced reflection and low flux as the corona moves closer to the black hole.

astro-ph.HE↗

arXiv2022

X-ray Intraday Variability of HBL Blazars with XMM-Newton

We present an extensive study on the X-ray intraday variability of ten TeV-emitting high synchrotron peaked blazars (HBLs): 1ES 0229+200, 1ES 0414+009, PKS 0548-322, 1ES 1101-232, 1H 1219+301, H 1426+428, Mrk 501, 1ES 1959+650, PKS 2005-489, and 1ES 2344+514 made with twenty-five XMM-Newton pointed observations during its operational period. Intraday variability has been estimated in three energy bands: soft (0.3--2 keV), hard (2--10 keV) and total (0.3--10 keV). Although seven out of these ten TeV HBLs exhibited some intraday variability at three-sigma levels no major variations exceeding six percent were detected. We explored the spectral properties of the sample by extracting the hardness ratio from the soft and hard bands; no significant variations in the hardness ratio were observed in any source. We performed power spectral density analyses on the variable light-curves by fitting power-laws, yielding slopes lying in the range from 1.11 to 2.93 for different HBLs. We briefly discuss possible emission mechanisms and carry out rough estimates for magnetic fields, electron Lorentz factors and emission region sizes for seven of these HBLs.

astro-ph.HE↗