arXiv subjects

Aditya S. Mondal

Publications and source records attributed to Aditya S. Mondal.

17 recordsLinked to original sources

arXiv2026

The first comprehensive spectral and timing study of the ultra-compact X-ray binary 4U 1812-12 with NICER and NuSTAR

The source 4U 1812-12 is a persistent, weakly variable low-mass X-ray binary containing a neutron star. The source was observed by NICER between 2019 and 2021 and, more recently, by NuSTAR in 2025. During the NICER and NuSTAR observations, the source was detected in a hard spectral state with a bolometric luminosity of $\sim 1.90\times 10^{36}$ ergs s$^{-1}$. Its $3-70$ keV NuSTAR spectrum is characterized by a soft thermal emission from the disc, a hard Comptonized emission from the corona, and its reflection from the accretion disc. The NuSTAR energy spectrum exhibits the clear presence of disc reflection features, fitted using a self-consistent relativistic reflection model {\tt relxill}. Our reflection modeling indicates a moderately ionized accretion disc (log\:$ξ\sim2.72$) extending close to the neutron star surface ($R_{in}\lesssim 1.72\:R_{ISCO}$), and viewed through a small inclination angle ($i\sim 25$ degrees). Assuming that the magnetic field ($B$) truncates the disc, we found $B\lesssim 2.54\times 10^{8}$ G, comparable to the typical values observed for NS LMXBs. The $1.0-9.5$ keV NICER spectra are also characterized by a soft thermal component and a dominant hard Comptonized component. During NICER observations, the disc temperature exhibits a small variation within $\sim 0.69-0.84$ keV. In contrast, the power law photon index, $Γ$, exhibits a large variation of $\sim 0.8-1.5$, implying a substantial change in the Comptonized emission. Moreover, NICER timing analysis reveals broadband aperiodic variability with significant QPO-like features at $0.379\pm 0.008$ Hz and $0.724\pm 0.025$ Hz, having fractional rms amplitudes of $2.9\pm 0.6\%$ and $4.1\pm 0.5\%$, respectively.

astro-ph.HE↗

arXiv2026

On the effect of gravitational repulsion on geodesic deviation in the Schwarzschild-de Sitter spacetime

Here we discuss the effect of gravitational repulsion on geodesic deviation in the Schwarzschild-de Sitter spacetime. In particular, the geodesic deviation shows different behaviour inside or outside a critical surface, which is the divider between attractive and repulsive regions of gravity. This in turn makes it possible to locate the critical surface and regions of gravitational repulsion and attraction from the behaviour of geodesic deviation.

gr-qc↗

arXiv2026

Probing the accretion geometry of the transient accreting millisecond pulsar SAX J1808.4-3658: transitions to the propeller regime

We analyze three NuSTAR observations and two NICER observations of the transient accreting millisecond pulsar SAX J1808.4-3658 in the hard spectral state during its most recent outbursts in 2022 and 2025. The spectral analysis of the persistent emission shows that the continuum is well described by an absorbed thermal Comptonization model with a high plasma temperature of ~25-90 keV. A prominent iron emission line around 5-8 keV and a Compton hump around 15-30 keV have been detected from all NuSTAR observations, indicating the reflection of the hard X-ray photon from the accretion disk. We employ the relativistic reflection model relxillCP to describe the reflection phenomena. The spectral fit of three NuSTAR observations shows that the inner disk radius moves outward, the Comptonized thermal emission decreases in flux, the mass accretion rate decreases, and the disk becomes less ionized as we proceed from the 2022 to the 2025 observations. Reflection studies also reveal a moderate inclination of the source within ~30-50 degrees. During the 2025 September observation, the inner radius of the disk is significantly truncated (~23R_g), and the corresponding magnetospheric radius is comprehensively larger than the disk's co-rotation radius, suggesting a hint of the transition to the propeller regime. Although the disk is truncated at the larger radius, accreted material is still reaching the surface of the neutron star, which is confirmed through the detection of a Type-I X-ray burst during this NuSTAR observation. The spectral analysis of the burst suggests helium burning at a low ignition depth.

astro-ph.HE↗

arXiv2026

Exploring the spectral characteristics of the periodic burster 4U 1323-62: Type-I X-ray burst and persistent emission

We report on the results obtained by the analysis of persistent and type-I thermonuclear X-ray burst emission observed from the periodic burster 4U 1323-62. These analyses are based on the NuSTAR observation performed on 2024 August 7 for a total exposure of around 90 ks. The persistent emission is well described by an absorbed thermal Comptonization model. An absorption edge is also detected at an energy of approximately 7.42 keV, which indicates the presence of absorbing material in the vicinity of this system. Six bursts have been observed during this observation, wherein we find the burst recurrence time to be approximately 4.52 hr. All the bursts exhibit the characteristics of a sharp rise and exponential decay. We perform the time-resolved spectroscopy of the burst spectra described by a model consisting of thermal emission from the neutron star surface and a varying persistent emission component to study the evolution of burst parameters. The enhancement of the persistent emission during burst exposure is characterized by the scaling parameter f a, which reflects the increasing strength of the burst-disc interaction with burst intensity, likely driven by Poynting-Robertson drag. The spectral analysis of bursts estimate the average apparent blackbody emitting radius of the neutron star to lie within 1.5-3.5 km. The ignition depths computed from the burst parameters indicate short Type-I thermonuclear bursts from a mixed hydrogen-helium fuel layer.

astro-ph.HE↗

arXiv2025

NuSTAR discovers a long type-I X-ray burst from the clocked burster GS 1826-24

The source GS~1826-24 is a neutron star low mass X-ray binary known as the 'clocked burster' because of its extremely regular bursting behavior. We report on the detection of a long type-I X-ray burst from this source. We perform a detailed spectroscopic analysis of the long X-ray burst, lasting for $\sim 600$ s, seen in the NuSTAR observation carried out on 2022 September. The persistent emission is well described by an absorbed thermal Comptonization model (nthcomp), and the source exhibits a soft spectral state during this observation. The observed burst exhibits a rise time of $\sim 25$ s and a decay time of $\sim 282$ s. The time-resolved spectroscopy of the burst shows a significant departure from a pure thermal spectrum and is described with a model consisting of a varying-temperature blackbody plus an evolving persistent emission component. We observe a significant enhancement in the persistent emission during the burst. The enhancement of the pre-burst persistent flux is possibly due to Poynting-Robertson drag or coronal reprocessing. At the peak of the burst, the blackbody temperature and the blackbody emitting radius reached a maximum of $2.10\pm 0.07$ keV and $5.5\pm 2.1$ km, respectively. The peak flux ($F_{peak}$) during the burst is $\approx 2.4\times 10^{-8}$ ergs cm$^{-2}$ s$^{-1}$, which corresponds to a luminosity of $\approx 9.7\times 10^{37}$ ergs s$^{-1}$.

astro-ph.HE↗

arXiv2025

NuSTAR view of the X-ray transients Swift J174805.3-244637 and IGR J17511-3057

We report on the NuSTAR observations of the neutron star low-mass X-ray binary Swift J174805.3-244637 (hereafter Swift~J17480) and the accreting millisecond X-ray pulsar IGR~J17511-3057 performed on March 4, 2023, and April 8, 2015, respectively. We describe the continuum emission of Swift~J17480 with a combination of two soft thermal components and an additional hard X-ray emission described by a power-law. We suggest that the spectral properties of Swift~J17480 are consistent with a soft spectral state. The source IGR~J17511-3057 exhibits a hard spectrum characterized by a Comptonized emission from the corona. The X-ray spectrum of both sources shows evidence of disc reflection. For the first time, we employ the self-consistent reflection models ({\tt relxill} and {\tt relxillNS}) to fit the reflection features in the \nustar{} spectrum. From the best-fit spectral model, we find an inner disc radius ($R_{in}$) is precisely constrained to $(1.99-2.68)\:R_{ISCO}$ and inclination to $30\pm 1\degree$ for Swift~J17480. We determine an inner disc radius of $\lesssim 1.3\;R_{ISCO}$ and inclination of $44\pm 3\degree$ for IGR~J17511-3057. A low inclination angle of the system is required for both sources. For the source IGR~J17511-3057, spinning at $4.1$ ms, the value of co-rotation radius ($R_{co}$) is estimated to be $\sim 42$ km ($3.6\:R_{ISCO})$, consistent with the position of inner disc radius as $R_{in}\lesssim R_{co}$. We further place an upper limit on the magnetic field strength of the sources, considering the disc is truncated at the magnetospheric radius.

astro-ph.HE↗

arXiv2024

Relativistic X-ray reflection from the accreting millisecond X-ray pulsar IGR J17498-2921

The accreting millisecond X-ray pulsar IGR J17498-2921 went into X-ray outburst on April 13-15, 2023, for the first time since its discovery on August 11, 2011. Here, we report on the first follow-up \nustar{} observation of the source, performed on April 23, 2023, around ten days after the peak of the outburst. The \nustar{} spectrum of the persistent emission ($3-60$ \kev{} band) is well described by an absorbed blackbody with a temperature of $kT_{bb}=1.61\pm 0.04$\kev{}, most likely arising from the NS surface and a Comptonization component with power-law index $Γ=1.79\pm0.02$, arising from a hot corona at $kT_{e}=16\pm 2$ keV. The X-ray spectrum of the source shows robust reflection features which have not been observed before. We use a couple of self-consistent reflection models, {\tt relxill} and {\tt relxillCp}, to fit the reflection features. We find an upper limit to the inner disc radius of $ 6\: R_{ISCO}$ and $ 9\: R_{ISCO}$ from {\tt relxill} and {\tt relxillCp} model, respectively. The inclination of the system is estimated to be $\simeq 40\degr$ from both reflection models. Assuming magnetic truncation of the accretion disc, the upper limit of magnetic field strength at the pole of the NS is found to be $B\lesssim 1.8\times 10^{8}$ G. Furthermore, the \nustar{} observation revealed two type I X-ray bursts and the burst spectroscopy confirms the thermonuclear nature of the burst. The blackbody temperature reaches nearly $2.2$ keV at the peak of the burst.

astro-ph.HE↗

arXiv2023

Relativistic X-ray reflection and highly ionized absorption in the spectrum of NS LMXB 1A 1744-361

We present the results from the spectral and timing analysis of the accreting neutron star 1A~1744-361 from the \nustar{} observation performed in its 2022 outbursts. The unabsorbed bolometric X-ray luminosity during this observation in the energy band $0.1-100\kev{}$ is $3.9\times 10^{37}$ erg~s$^{-1}$, assuming a distance of $9$ kpc. During this observation, the source was in the banana branch of the atoll track. The source spectrum exhibits relativistic disc reflection and clear absorption features when an absorbed blackbody and cut-off power-law model describes the continuum emission. The $3-50\kev{}$ source spectrum is well fitted using a model combination consisting of an absorbed single-temperature blackbody and a reflection model along with the addition of a warm absorber component. The inner-disk radius, $R_{in}$, obtained from the reflection fit is $\sim(1.61-2.86)R_{ISCO}=(8.4-14.9)R_{g}$ ($17.6-31.2$ km for a $1.4\Msun$ NS). This measurement allowed us to further constrain the magnetic field strength to $B\lesssim 0.94\times 10^{9}$G. The strong absorption features $\sim 6.91\kev{}$ and $\sim 7.99\kev{}$ imply the presence of highly ionized absorbing material with a column density $N_{H}$ of $\sim 3\times 10^{22}$ cm$^{-2}$, emanating from the accretion disk in the form of disc wind with an outflow velocity of $v_{out}\simeq 0.021c\simeq 6300$ km s$^{-1}$.

astro-ph.HE↗

arXiv2023

The complex spectral behavior of the newly discovered neutron star X-ray binary Swift J1858.6-0814

We report on the \nustar{} observation of the newly discovered neutron star X-ray binary Swift~J1858.6-0814 taken on 23rd March 2019. The light curve of the source exhibits several large flares during some time intervals of this observation. The source is softer in the high-intensity interval where the large flaring activity mainly occurs. We perform time-resolved spectroscopy on the source by extracting spectra for two different intensity intervals. The source was observed with a $3-79 \kev{}$ luminosity of $\sim 9.68\times 10^{36}$ ergs/s and $\sim 4.78\times 10^{36}$ ergs/s for high and low-intensity interval, respectively assuming a distance of $15$ kpc. We find a large value of the absorbing column density ($\rm{N_{H}}\sim 1.1\times 10^{23}$ cm$^{-2}$), and it appears to be uncorrelated with the observed flux of the source. Each spectrum shows evidence of Fe K$α$ emission in the $5-7$\kev{} energy band, an absorption edge around $\sim 7-8$\kev{}, and a broad Compton hump above $15$\kev{}, indicating the presence of a reflection spectrum. The observed features are well explained by the contribution of a relativistic reflection model and a partially covering absorption model. From the best-fit spectral model, we found an inner disc radius to be $4.87_{-0.96}^{+1.63}\;R_{ISCO}$ (for the high-intensity interval) and $5.68_{-2.78}^{+9.54}\;R_{ISCO}$ (for the low-intensity interval), indicating a significant disc truncation. The disk inclination is found to be relatively low, $i< 33^{0}$. We further place an upper limit on this source's magnetic field strength considering the disc is truncated at the magnetospheric radius.

astro-ph.HE↗

arXiv2022

Evidence of hard power-law spectral cutoff and disc reflection features from the X-ray transient XTE J1739-285

We report on the nearly simultaneous \nicer{} and \nustar{} observations of the known X-ray transient XTE~J1739-285. These observations provide the first sensitive hard X-ray spectrum of this neutron star X-ray transient. The source was observed on 2020 February 19 in the hard spectral state with a luminosity of $0.007$ of the Eddington limit. The broadband $1-70 \kev{}$ \nicer{} and \nustar{} observation clearly detects a cutoff of the hard spectral component around $34-40 \kev{}$ when the continuum is fitted by a soft thermal component and a hard power-law component. This feature has been detected for the first time in this source. Moreover, the spectrum shows evidence for disc reflection -- a relativistically broadened Fe K$α$ line around $5-8 \kev{}$ and a Compton hump in the $10-20 \kev{}$ energy band. The accretion disc reflection features have not been identified before from this source. Through accretion disc reflection modeling, we constrain the radius of the inner disc to be $R_{in}=3.1_{-0.5}^{+1.8}\;R_{ISCO}$ for the first time. In addition, we find a low inclination, $i\sim 33^{0}$. Assuming the magnetosphere is responsible for such truncation of the inner accretion disc above the stellar surface, we establish an upper limit of $6.2\times 10^8$ G on the magnetic field at the poles.

astro-ph.HE↗

arXiv2022

NuSTAR and AstroSat observations of thermonuclear X-ray bursts with short-recurrence times in 4U 1636$-$536

We report results from the spectro-timing analysis of the atoll source 4U 1636$-$536 observed with NuSTAR and AstroSat during its hard spectral state. In three observations of 207 ks total exposure, we identify 31 thermonuclear X-ray bursts including five doublets and a triplet. Recurrence time as short as 3.8 minutes is seen in one of the doublets. To the best of our knowledge, this is the shortest recurrence time known for this source. Our time-averaged spectroscopy during the bursts indicate the presence of an additional powerlaw or a blackbody component in a few cases, perhaps due to varying temperatures during bursts or plausible deviation from ideal blackbody behavior, however, it is difficult to probe this using the time-resolved spectroscopy owing to limited statistics. Time-resolved bursts are well fit using an absorbed blackbody model with temperatures varying between 1.7 and 2.2 keV. Burst oscillations around 581 Hz are detected with 3$σ$ confidence during the decay phase in two of the X-ray bursts. One of the burst oscillations is seen at 582 Hz, a frequency observed during the 2001 superburst in this source.

astro-ph.HE↗

arXiv2021

Evidence of disc reflection in the X-ray spectrum of the neutron star low mass X-ray binary 4U 1636-536

We present a broadband spectral analysis of the atoll source 4U~1636-536 observed for $\sim92$ ks with \nustar{}. The source was found to be in a low-luminosity state during this observation with $3-79 \kev{}$ X-ray luminosity of $L_{3-79 keV}=(1.03\pm0.01)\times 10^{37}$ ergs/s, assuming a distance of 6 kpc. We have identified and removed twelve type-I X-ray bursts during this observation to study the persistent emission. The continuum is well described by a thermal Comptonization model {\tt nthcomp} with $Γ\sim1.9$, $kT_{e}\sim28 \kev{}$, and $kT_{s}\sim0.9\kev{}$. The \nustar{} data reveal a clear signature of disc reflection, a significantly broad Fe-K emission line (around $5-8\kev{}$), and the corresponding reflection hump (around $15-30\kev{}$). We have modeled the data with two relativistically blurred reflection models. Both families of reflection models prefer truncated inner disc radii prior to the ISCO. We find that the inner disc is truncated with an inner radius of $R_{in}=(3.2-4.7)\;R_{ISCO}$ ($\simeq16-24\,R_{g}\: \text{or}\: 36-54$ km). This inner disc radius suggests that the neutron star magnetic field strength is $B\leq2.0\times10^{9}$ G.

astro-ph.HE↗

arXiv2020

On the disc reflection spectroscopy of NS LMXB Serpens~X-1: analysis of a recent NuSTAR observation

We present \nustar{} observation of the atoll type neutron star (NS) low-mass X-ray binary (LMXB) Serpens~X-1 (Ser~X-1) performed on 17 February 2018. We observed Ser~X-1 in a soft X-ray spectral state with 3-79 keV luminosity of $L_\text{X}\sim0.4\times 10^{38}$ erg s$^{-1}$ ($\sim 23\%$ of the Eddington luminosity), assuming a distance of 7.7 kpc. A positive correlation between intensity and hardness ratio suggests that the source was in the banana branch during this observation. The broadband 3-30 keV NuSTAR energy spectrum can be well described either by a three-component continuum model consisting of a disk blackbody, a single temperature blackbody and a power-law or by a two-component continuum model consisting of a disk blackbody and a Comptonization component. A broad iron line $\sim 5-8$ keV and the Compton back-scattering hump peaking at $\sim$10-20 keV band are clearly detected in the X-ray spectrum. These features are best interpreted by a self-consistent relativistic reflection model. Fits with relativistically blurred disc reflection model suggests that the inner disc radius $R_{in}$ is truncated prior to the ISCO at $(1.9-2.5)\;R_{ISCO}$ ($\simeq11.4-15\,R_{g}\: \text{or}\: 26-34$ km) and the accretion disc is viewed at an low inclination of $i\simeq16^\circ-20^\circ$. The disc is likely to be truncated either by a boundary layer or by the magnetosphere. Based on the measured flux and the mass accretion rate, the maximum radial extension for the boundary layer is estimated to be $\sim6.4\:R_{g}$ from the NS surface. The truncated inner disc in association with pressure from a magnetic field sets an upper limit of $B\leq1.9\times10^{9}$ G.

astro-ph.HE↗

arXiv2019

Study of the reflection spectrum of the bright atoll source GX 3+1 with NuSTAR

We report on the \nustar{} observation of the atoll type neutron star (NS) low-mass X-ray binary GX~3+1 performed on 17 October 2017. The source was found in a soft X-ray spectral state with $3-70$keV luminosity of $L_\text{X}\sim3\times 10^{37}$ ergs s$^{-1}$ ($\sim 16\%$ of the Eddington luminosity), assuming a distance of 6 kpc. A positive correlation between intensity and hardness ratio suggests that the source was in the banana branch during this observation. The broadband $3-70$keV \nustar{} spectral data can be described by a two-component continuum model consisting of a disk blackbody ($kT_\text{disc}\sim1.8$keV) and a single temperature blackbody model ($kT_\text{bb}\sim2.7$keV). The spectrum shows a clear and robust indication of relativistic reflection from the inner disc which is modelled with a self-consistent relativistic reflection model. The accretion disc is viewed at an inclination of $i\simeq22^\circ-26^\circ$ and extended close to the NS, down to $R_\text{in}=(1.2-1.8) R_\text{ISCO}\:(\simeq6.1-9.1\,R_{g}\: \text{or}\: 14-20.5$ km) which allows an upper limit on the NS radius ($\leq13.5$ km). Based on the measured flux and the mass accretion rate, the maximum radial extension for the boundary layer is estimated to be $\sim6.3\:R_{g}$ from the NS surface. However, if the disc is not truncated by the boundary layer but by the magnetosphere, an estimated upper limit on the polar magnetic field would be of $B\leq6\times10^{8}$ G.

astro-ph.HE↗

arXiv2017

NuSTAR view of the Z-type neutron star low-mass X-ray binary Cygnus~X--2

We report on the \nustar{} observation of the Z-type neutron star low-mass X-ray binary Cygnus X--2 performed on 7 January 2015. During this observation, the source exhibited sudden decrease in count rate (dips) and stronger variability in $3-79\kev$ X-ray lightcurve. The hardness-intensity diagram shows that the source remained in the so-called \quotes{normal branch} of the Z-track, although an extended \quotes{flaring branch} is observed during the dips. The source was in a soft spectral state with the $3-45\kev$ luminosity of $L\simeq(0.5-1.1)\times 10^{38}$ erg s$^{-1}$, assuming a distance of 8 kpc. Both the non-dip and dip X-ray spectra are well represented by models in which the soft band is dominated by the emission from the disc, while the hard X-ray band is dominated by the Comptonized emission from the boundary layer/corona and its reflected emission from the disc. The X-ray spectrum also revealed a broad Fe K$α$ emission line which is nearly symmetric at the higher flux and asymmetric when the flux is reduced by a factor of $\sim 2$. The relativistic reflection model predicts the inner radius of the accretion disc as $R_{in}\simeq2.5-6.0\:R_{ISCO}\:(\simeq30-73$ Km) for the non-dip state and $R_{in}\simeq2.0-2.6\:R_{ISCO}\:(\simeq24-32$ Km) for the dip state. If the inner disc is truncated due to the pressure arising from a magnetic field, this implies an upper limit of the magnetic field strength of $\leq7.6\times10^{9}$ G at the magnetic poles which is consistent with other estimates.

astro-ph.HE↗

arXiv2016

Broadband X-ray emission and the reality of the broad iron line from the Neutron Star - White Dwarf X-ray binary 4U 1820-30

Broad relativistic iron lines from neutron star X-ray binaries are important probes of the inner accretion disk. The X-ray reflection features can be weakened due to strong magnetic fields or very low iron abundances such as is possible in X-ray binaries with low mass, first generation stars as companions. Here we investigate the reality of the broad iron line detected earlier from the neutron star low mass X-ray binary 4U~1820--30 with a degenerate helium dwarf companion. We perform a comprehensive, systematic broadband spectral study of the atoll source using \suzaku{} and simultaneous \nustar{} \& \swift{} observations. We have used different continuum models involving accretion disk emission, thermal blackbody and thermal Comptonization of either disk or blackbody photons. The \suzaku{} data show positive and negative residuals in the region of iron K band. These features are well described by two absorption edges at $7.67\pm0.14\kev$ and $6.93\pm0.07\kev$ or partial covering photoionized absorption or by blurred reflection. Though, the simultaneous \swift{} and \nustar{} data do not clearly reveal the emission or absorption features, the data are consistent with the presence of either absorption or emission features. Thus, the absorption based models provide an alternative to the broad iron line or reflection model. The absorption features may arise in winds from the inner accretion disk. The broadband spectra appear to disfavour continuum models in which the blackbody emission from the neutron star surface provides the seed photons for thermal Comptonization. Our results suggest emission from a thin accretion disk ($kT_{disk} \sim 1\kev$), Comptonization of disk photons in a boundary layer most likely covering a large fraction of the neutron star surface and innermost parts of the accretion disk, and blackbody emission ($kT_{bb} \sim 2\kev$) from the polar regions.

astro-ph.HE↗

arXiv2015

XMM-Newton view of a hard X-ray transient IGR J17497-2821

We present spectral and energy dependent timing characteristics of the hard X-ray transient IGR J17497-2821 based on XMM-Newton observations performed five and nine days after its outburst on 2006 September 17. We find that the source spectra can be well described by a hard (Gamma ~ 1.50) powerlaw and a weak multicolour disk blackbody with inner disk temperature kT_{in} ~ 0.2 KeV. A broad iron K - alpha line with FWHM ~ 27000 Km/s, consistent with that arising from an accretion disk truncated at large radius, was also detected. The power density spectra of IGR J17497 - 2821, derived from the high resolution (30 micro second) timing mode XMM-Newton observations, are characterised by broadband noise components that are well modelled by three Lorentzians. The shallow power law slope, low disk luminosity and the shape of the broadband power density spectrum indicate that the source was in the hard state. The rms variability in the softer energy bands (0.3-2 KeV) found to be ~ 1.3 times that in 2-5 and 5-10 KeV energy bands. We also present the energy dependent timing analysis of the RXTE/PCA data, where we find that at higher energies, the rms variability increases with energy.

astro-ph.HE↗