arXiv subjects

Tanuman Ghosh

Publications and source records attributed to Tanuman Ghosh.

12 recordsLinked to original sources

arXiv2026

X-ray polarization in magnetized neutron stars

X-ray polarimetry has opened a new window into understanding the physics around magnetized compact objects. IXPE detection of linear polarization from such systems has prompted a new spurt of theoretical modeling. Our study is based on the dominant paradigm that the observed polarization arises from the scattering of photons around highly magnetized systems. Our main focus is the dependence of the polarization of the scattered light on properties of the incoming light, i.e., geometry and the polarization state, and the determination of the spectral shape of the polarized light for a wide range of magnetic field strengths. We also analyze the impact of vacuum birefringence on photon polarization. We show that, generically, we expect a higher linear degree of polarization from magnetars as compared to normal pulsars, which is in agreement with IXPE observations. Under some conditions, our study helps to understand the observed degree of polarization from normal pulsars and low-magnetized neutron stars and their spectral dependence. However, we cannot conclusively explain the spectral shape of the observed polarization for magnetars using only a single component emission from scattering in a strong magnetic field. This probably points to the system being more complex, e.g., multi-component, than our study allows for. Upcoming X-ray polarimeters with broader energy coverage could probe some of our other predictions, e.g., the spectral shape of the polarized light close to the resonance frequency.

astro-ph.HE↗

arXiv2026

HESS J1832$-$085: evidence for a new gamma-ray binary candidate

The Galactic plane survey conducted by the High Energy Stereoscopic System (H.E.S.S.) has revealed numerous teraelectronvolt (TeV) sources, many of which remain unidentified. HESS~J1832$-$085 is a point-like TeV source lacking a confirmed multiwavelength (MWL) counterpart. In this paper, we present evidence that HESS~J1832$-$085 is likely a gamma-ray binary. We aim to investigate the nature of HESS~J1832$-$085 using {\it Fermi}-LAT and X-ray data, complemented by broadband radiative modeling, to assess its classification as a potential gamma-ray binary. We analyzed $\sim$17.3~yr of {\it Fermi}-LAT data between 0.1 and 500~GeV to establish the gigaelectronvolt (GeV) counterpart of HESS~J1832$-$085, including performing spectral, spatial, and periodicity analyses. Archival X-ray observations were examined to search for a counterpart and to characterize its spectrum and potential variability. The broadband emission was interpreted using models commonly applied to gamma-ray binaries. We detect a point-like GeV gamma-ray source spatially consistent with HESS~J1832$-$085, with spectral properties compatible with known gamma-ray binaries. No significant GeV periodic modulation is detected. A potential X-ray counterpart is identified in archival X-ray data, exhibiting a hard, absorbed spectrum and moderate variability. The broadband spectral energy distribution is reproduced by the adopted binary radiative model. Our results indicate that HESS~J1832$-$085 is likely a gamma-ray binary candidate, motivating dedicated MWL follow-up observations to confirm the source nature.

astro-ph.HE↗

arXiv2025

Synchrotron radiation from NGC 470 HLX1 - a hidden hyperluminous accreting neutron star?

We present the first broadband spectral analysis of NGC 470 HLX1, a hyperluminous X-ray source that exhibits significant flux variability over different epochs. We investigate the feasibility of synchrotron radiation with varying latitude from a magnetized neutron star to explain the source's spectra. Although the statistical quality of the data does not allow us to rule out simple phenomenological models, it is adequate to constrain the parameter space for this more physically motivated model. We also shed light on the particle acceleration mechanisms and maximum Lorentz factor of electrons within the neutron star magnetospheric plasma under super-Eddington accretion conditions. In our broadband spectral modeling, the detection of a blackbody-like component suggests the presence of a disk near the corotation radius or an outflow ejected from the disk. The viability of synchrotron emission in an HLX system offers new insights about the nature of these sources, motivating further sample studies to assess whether most of these sources are powered by accreting neutron stars.

astro-ph.HE↗

arXiv2025

Probing spectral variability in NGC 4490 ULX-8 over 24 years of XMM-Newton, Chandra and Swift-XRT observations

We present a spectral variability study of the ultraluminous X-ray source NGC 4490 ULX-8 based on 14 Chandra, 6 XMM-Newton and 19 Swift-XRT observations obtained between 2000 and 2024. The X-ray spectra are modelled using absorbed power-law and absorbed multicolour disc blackbody models. The best-fit photon indices span 0.9-2.7, while the inferred inner disc temperatures lie in the range 1.0-1.6 keV. We detect pronounced long-term variability in the unabsorbed X-ray luminosity on multi-year timescales, while variability within individual observations is comparatively modest. A Hardness-Intensity Diagram of the source shows no clear transition between hard and soft states; however, two recent observations taken on 2022 December 1 and 2024 May 4 show a sharp increase in brightness. The spectra across all observations are dominated by smooth, single-component curvature in the 0.3-10 keV band, consistent with the broadened-disc regime of ultraluminous X-ray sources. A correlation analysis reveals a weak positive X-ray luminosity-photon index trend that remains statistically supported after controlling for related degeneracies, indicating that it is not driven solely by fitting covariance. The luminosity-inner disc temperature relation is only weakly constrained, but remains compatible, within uncertainties, with both thin-disc and slim-disc scalings. Using disc parameters derived from higher-quality XMM-Newton spectra, we obtain model-dependent estimates of the characteristic inner disc radius and compact-object mass as functions of inclination and spin. The reported results are consistent with a stellar-mass black hole accretor operating at or near the Eddington limit.

astro-ph.HE↗

arXiv2025

A Study of Spectral Variability between flaring and non-flaring state in M74 X-1

We conducted an extensive long-term spectral and timing study of the ultraluminous X-ray source (ULX) M74 X-1, using data taken between 2001 and 2021 by Chandra and XMM-Newton X-ray observatories. Our analysis shows that flares are present in some observations, whereas they are absent in others. Flaring state exhibits two-component spectra at a lower average flux level, whereas the non-flaring state displays single-component spectra at a higher average flux level. The M74 X-1 spectra are best described by the combination of accretion disk and Comptonization components, a dual thermal disk blackbody model, and a modified multi-temperature disk blackbody model. Using the dual thermal disk blackbody model, we obtain cool and hot temperatures of $T_{in}$ (cool) = $0.38^{+0.08}_{-0.06}$ keV and $T_{in}$ (hot) = $1.67^{+0.18}_{-0.13}$ keV, respectively, suggesting two temperature emitting regions and indicating possible presence of outflowing wind along with the accretion disk. We found a Gaussian feature at $E_{line}$ = $0.96^{+0.05}_{-0.11}$ keV with $\sigma$ = $0.11^{+0.13}_{-0.06}$ keV in the spectra of the flaring state which can be interpreted as the unresolved wind feature in the system when compared to similar feature seen in other ULXs. Plotting the hardness luminosity diagram, we get a trend of increasing hardness with luminosity, suggesting the presence of geometrical beaming in a low-inclination system. Additionally, using the hot disk blackbody component from the dual thermal disk blackbody model, we estimate the mass of the compact object to be M = $7.1^{+1.4}_{-1.3}$ M$_\odot$, classifying it as a stellar-mass black hole and confirming super-Eddington accretion in the system.

astro-ph.HE↗

arXiv2023

Constraint on the accretion of NGC 6946 X-1 using broadband X-ray data

We analyze broadband X-ray data of NGC 6946 X-1 and probe plausible accretion scenarios in this ULX. NGC 6946 X-1 is a persistent soft source with broadband continuum spectra described by two thermal disk components. The cool accretion disk temperature $\rm T_{cool} \sim 0.2$ keV and the presence of $\sim 0.9$ keV emission/absorption broad feature suggests the evidence of optically thick wind due to super-critical accretion. The hot geometrically modified accretion disk has an inner temperature of $\rm T_{hot} \sim 2$ keV with a radial dependent profile $\rm T(r) \propto r^{-0.5}$, expected in a slim disk scenario. Further, the measurement based on a realistic inclination angle of the disk indicates that the mass of the host compact object is comparable to $\rm \sim 6-10 ~M_{\odot}$ non-rotating black hole or the system hosts a moderately magnetized neutron star with $\rm B \lesssim 2 \times 10^{11}$ G magnetic field. Overall, the detected spectral curvature, high luminosity, flux contribution from two thermal disk components, and estimated accretion rate imprint the super-Eddington accretion scenario.

astro-ph.HE↗

arXiv2023

Synchrotron cutoff in Ultraluminous X-ray sources

The origin of spectral curvature at energies $E\simeq 10$ keV in ultraluminous X-ray sources is not well understood. In this paper, we propose a novel mechanism based on synchrotron radiation to explain this cutoff. We show that relativistic plasma can give rise to observed spectral curvature for neutron star magnetic fields due to the variation in the latitude of synchrotron radiation. We analyze the NuSTAR data of two bright pulsar ULXs, NGC 5907 ULX1 and NGC 7793 P13, and provide estimates of the physical parameters of these sources. We fit the data for synchrotron emission at various latitudes and show that the spectral cutoff in these cases can be explained for a large range of acceptable physical parameters, e.g., a semi-relativistic plasma with $γ\simeq 20$ for high latitudes or a highly relativistic plasma ($γ\simeq 10^5$) for emission close to the electron's orbital plane in a typical magnetic field of $B\simeq 10^{12} \, \rm G$. We also discuss how such an emission mechanism can be distinguished from other proposed models. A corollary to our study is that most ULXs might be neutron stars as they display such a spectral cutoff.

astro-ph.HE↗

arXiv2022

Spectral variability in NGC 1042 ULX1

We report X-ray spectral variability in an ultraluminous X-ray source NGC 1042 ULX1, using archival XMM-Newton and recent NuSTAR observations. In long-term evolution, the source has shown a trend of variation in spectral hardness. The variability in different XMM-Newton observations is prominent above $\sim 1$ keV. Cool thermal disk component with a characteristic temperature of $\sim 0.2$ keV manifests that the spectral state of NGC 1042 ULX1 in all epochs is similar to that of the ultraluminous state sources. An apparent anti-correlation between luminosity and powerlaw index demonstrates that the source becomes spectrally harder when it is in a brighter state. That is conceivably related to variation in accretion rate, strength of comptonization, wind/outflow in the system or a manifestation of varying disk occultation. Typical hard ultraluminous type spectra indicate that NGC 1042 ULX1 is a low inclination system in general. Spectral properties suggest that, like many other ULXs which show spectral curvature around $\sim 6-10$ keV, NGC 1042 ULX1 could be another stellar-mass super-Eddington accretor.

astro-ph.HE↗

arXiv2022

Hard X-ray flares and spectral variability in NGC 4395 ULX1

We report the detection of flaring events in NGC 4395 ULX1, a nearby ultraluminous X-ray source (ULX), for the first time, using recent XMM-NEWTON observations. The flaring episodes are spectrally harder than the steady emission intervals, resulting in higher fractional variability in the high energy regime. A thin Keplerian and a slim accretion disk provide the best-fit continuum for XMM-NEWTON spectra. All observations show a broad hump-like feature around $\sim 0.9$ keV, which can be associated with a collection of blended emission lines, and suggests the presence of a wind/outflow in this ULX through comparison with other ULXs that show a similar feature. The flaring spectra correspond to higher slim disk temperatures due to higher mass accretion rate under an advection-dominated accretion scenario. The luminosity-temperature (L-T) values in different flux states show a positive trend. When characterized with a powerlaw relation, the L-T profile is broadly consistent with both $L\propto T^2$ and $L\propto T^4$ relations for the analysed data. The empirical predictions for a slim accretion disk in the case of super-Eddington accretion onto a stellar-mass compact object is $L \propto T^2$ which is a possible scenario in ULX1. The origin of the flaring events is understood as an intrinsic change of accretion rate or presence of variable clumpy wind in the inner region of the accretion disk.

astro-ph.HE↗

arXiv2021

Gravitational geometric phase

We show that spinors propagating in curved gravitational background acquire an interaction with spacetime curvature, which leads to a quantum mechanical geometric effect. This is similar to what happens in the case of magnetic fields, known as Pancharatnam-Berry phase. As the magnetic and gravitational fields have certain similar properties, e.g. both contribute to curvature, this result is not difficult to understand. Interestingly, while spacetime around a rotating black hole offers Aharonov-Bohm and Pancharatnam-Berry both kinds of geometric effect, a static spacetime offers only the latter. In the bath of primordial black holes, such gravity induced effects could easily be measured due to their smaller radius.

gr-qc↗

arXiv2021

Geometric phase for Dirac Hamiltonian under gravitational fields in the non-relativistic regime

We show the appearance of geometric phase in a Dirac particle traversing in non-relativistic limit in a time-independent gravitational field. This turns out to be similar to the one originally described as a geometric phase in magnetic fields. We explore the geometric phase in the Kerr and Schwarzschild geometries, which have significant astrophysical implications. Nevertheless, the work can be extended to any spacetime background including that of time-dependent. In the Kerr background, i.e. around a rotating black hole, geometric phase reveals both the Aharonov-Bohm effect and Pancharatnam-Berry phase. However, in a Schwarzschild geometry, i.e. around a nonrotating black hole, only the latter emerges. We expect that our assertions can be validated in both the strong gravity scenarios, like the spacetime around black holes, and weak gravity environment around Earth.

gr-qc↗

arXiv2021

Super-Eddington accretion onto a stellar mass ultraluminous X-ray source NGC 4190 ULX1

We present the results of high-quality XMM-NEWTON observations of a ULX in the galaxy NGC 4190. The detection of spectral cutoff in NGC 4190 ULX1 spectra rules out the interpretation of the ULX to be in a standard low/hard canonical accretion state. We report that the high quality EPIC spectra can be better described by broad thermal component, such as a slim disk. In addition we found long term spectral and flux variability in the source using several XMM-NEWTON and Swift data. A clear anti-correlation between flux and power-law photon index is found which further confirms the unusual spectral state evolution of the ULX. Spectral properties of the ULX suggest that the source is in a broadened disk state with luminosities ($\approx (3-10) \times 10^{39}$ ergs s$^{-1}$) falling in the ultraluminous regime. The positive Luminosity-temperature relation further suggests that the multi color disk model follows the $L \propto T^4$ relation which is expected for a black body disk emission from a constant area and the slim disk model seems to favour $L \propto T^2$ relation consistent with an advection dominated disk emission . From the broadened disk like spectral feature at such luminosity, we estimated the upper limit of the mass of the central compact object from the inner disk radius and found that the ULX hosts a stellar mass black hole.

astro-ph.HE↗