arXiv subjects

Amar Deo Chandra

Publications and source records attributed to Amar Deo Chandra.

10 recordsLinked to original sources

arXiv2026

Spectral study of the outburst decay of the accreting millisecond X-ray pulsar SRGA J144459.2-604207

We study the spectra of the accreting X-ray millisecond pulsar SRGA J144459.2-604207 during the 2024 outburst using the Neutron star Interior Composition Explorer (NICER) and Swift observations. The spectra during the outburst decay, reflares, and quiescent state are explored using the absorbed Comptonized model. We find that the spectra during the quiescent state can also be explained using the absorbed power-law and absorbed blackbody model. The spectral evolution of the source is explored as the outburst decays into quiescence. We study the long-term quiescent X-ray activity of the source spanning roughly 45 years and find that the long-term quiescent luminosity may be explained using the deep crustal heating model. We also find that the coronal activity of the companion star alone cannot power the quiescent X-ray luminosity of the source. We place the source on the radio-X-ray luminosity plane and compare its position with other sources. We estimate the propeller luminosity of the source and find that it is smaller than the estimated luminosity during reflares and the quiescent state during the 2024 outburst. Several reflares are detected during the outburst decay, one of which is near-simultaneous with the detection of an ultrafast outflow and radio emission. We explore plausible mechanisms that may power outflow, radio emission and associated jet formation in this accreting binary.

astro-ph.HE↗

arXiv2026

A candidate cyclotron line at 1.89 keV in the ultraluminous X-ray source NGC 4861 X-2

In this Letter, we report the detection of an absorption-like feature at ~1.89 keV in Chandra/ACIS spectra of the ultraluminous X-ray source NGC 4861 X-2, based on the deepest observation (ObsID 20992; ~58 ks). The feature is consistently recovered across independent continuum models and significantly improves the fit statistics. Monte Carlo simulations yield a detection significance of ~3.5-4.1 sigma, depending on the adopted continuum, and a blind line scan reveals a single, localized peak at the same energy. The observed properties are consistent with a proton cyclotron resonant scattering feature (CRSF), implying a magnetic field strength of B ~(3-4) x 10^14 G. The spectrum is well described by a multicolor disk blackbody (diskbb) with kTin ~0.8 keV or a strongly curved continuum with a low cutoff energy (cutoffpl; Ecut ~1.3 keV). The source shows variability confined to the soft X-ray band in the two Chandra observations where the absorption-like feature is detected. In these observations, a candidate periodic signal at P ~7.4 s is also detected, with a global significance of ~2.5 sigma.

astro-ph.HE↗

arXiv2026

Exploring the magnetic field of the ultraluminous X-ray pulsar NGC 4631 X-8

NGC 4631 X-8 is an ultraluminous X-ray pulsar (ULXP) having a spin period of about 9.7 s, discovered using XMM-Newton observations in 2025. The pulsar is known to show one of the largest spin-up rates ($\sim -9.6 \times 10^{-8}$ s s$^{-1}$) among the ULXP population. We explore the surface magnetic field of the neutron star in this source using different models, and find that the inferred magnetic field lies in the range of about $0.3-7 \times 10^{14}$G. We study the long-term magnetic field and spin period evolution of the pulsar assuming steady accretion using prevalent theoretical mechanisms and find that the pulsar will likely evolve to become a millisecond pulsar having decayed magnetic field of about $\sim 10^{9}$G in about a million years. The scenario of the formation of a millisecond pulsar is also probed using an estimate of the super-Eddington duty cycle of about 14% from the literature, which suggests that the neutron star would accrete sufficient matter to likely become a recycled millisecond pulsar. Exploring the magnetic field as well as the spin period evolution properties of ULXPs may enable us to understand the poorly understood evolutionary features of ULXPs, shed light on one of the pathways of millisecond pulsar formation and also help us to understand the possibility of transient super-Eddington accretion phases in newborn magnetars, which are believed to power energetic events such as long gamma-ray bursts and Type I superluminous supernovae.

astro-ph.HE↗

arXiv2025

Study on the magnetic field of the ultraluminous X-ray pulsar RX J0209.6-7427

RX J0209.6-7427 is an ultraluminous X-ray pulsar (ULXP) having spin period of about 9.3 s. To date, no cyclotron resonance scattering features have been detected in this source, which can enable direct measurement of the magnetic field of the pulsar. We estimate the surface magnetic field of the neutron star in this source using different models and find that the inferred magnetic field lies in the range of $2.4-4 \times 10^{13}$G. We study the magnetic field and spin period evolution of the source using existing models and find that the magnetic field will decay to about $\sim 10^{9}$G assuming steady accretion and the source will become a millisecond pulsar at the end of the accretion phase of the accreting binary. Comparison between the magnetic field and the spin period of other ULXPs with those of magnetars suggests that some ULXPs may have magnetar-like strong dipolar magnetic fields. Studying the magnetic and spin period evolution of ULXPs may be helpful for understanding magnetar evolution and the millisecond pulsar formation.

astro-ph.HE↗

arXiv2025

Long-term spin-down and low luminosity regime in the Be/X-ray binary pulsar GX 304-1

We carry out timing and spectral studies of the Be/X-ray binary pulsar GX 304-1 using NuStar and XMM-Newton observations. We construct the long-term spin period evolution of the pulsar which changes from a long-term spin-up ($\sim 1.3 \times 10^{-13} $Hz s$^{-1}$) to a long-term spin-down ($\sim -3.4 \times 10^{-14} $Hz s$^{-1}$) trend during a low luminosity state ($\sim 10^{34-35} $erg s$^{-1}$). A prolonged low luminosity regime ($L_X \sim 10^{34-35} $erg s$^{-1}$) was detected during 2005-2010 and spanning nearly five years since 2018 December. The XMM-Newton and NuStar spectra can be described with a power law plus blackbody model having an estimated luminosity of $\sim 2.5 \times 10^{33} $erg s$^{-1}$ and $\sim 3.6 \times 10^{33} $erg s$^{-1}$ respectively. The inferred radius of the blackbody emission is about 100-110 m which suggests a polar-cap origin of this component. From long-term ultraviolet observations of the companion star, an increase in the ultraviolet signatures is detected preceding the X-ray outbursts. The spectral energy distribution of the companion star is constructed which provides a clue of possible UV excess when X-ray outbursts were detected from the neutron star compared to the quiescent phase. We explore plausible mechanisms to explain the long-term spin-down and extended low luminosity manifestation in this pulsar. We find that sustained accretion from a cold disc may explain the prolonged low luminosity state of the pulsar since December 2018 but the pulsar was undergoing normal accretion during the low luminosity period spanning 2005-2010.

astro-ph.HE↗

arXiv2023

AstroSat observations of the Be/X-ray binary XTE J1946+274 during 2018 and 2021 outbursts

We present the timing and spectral studies of the Be/X-ray binary XTE J1946+274 during its 2018 and 2021 giant outbursts using observations with the SXT and the LAXPC instruments on the AstroSat satellite. Unlike the 1998 and 2010 outbursts, where a giant outburst was followed by several low intensity periodic outbursts, the 2018 and 2021 outbursts were single outbursts. The X-ray pulsations are detected over a broad energy band covering 0.5-80 keV from the compact object. We construct the spin evolution history of the pulsar over two decades and find that the pulsar spins-up during the outbursts but switches to spin-down state in the quiescent periods between the outbursts. Energy resolved pulse profiles generated in several bands in 0.5-80 keV show that the pulse shape varies with the energy. The energy spectrum of the pulsar is determined for the 2018 and 2021 outbursts. The best fit spectral models require presence of Cyclotron Resonant Scattering Feature (CRSF) at about 43 keV in the energy spectra of both the outbursts. We find indication of possible reversal in the correlation between the cyclotron line energy and luminosity which needs to be ascertained from future observations. Using the best fit spectra the X-ray luminosity of XTE J1946+274 is inferred to be $2.7 \times 10^{37}$ erg s$^{-1}$ for the 2018 observations and $2.3 \times 10^{37}$ erg s$^{-1}$ for the 2021 observations. We discuss possible mechanisms which can drive outbursts in this transient Be X-ray binary.

astro-ph.HE↗

arXiv2022

Adaptive hyperspectral imaging using structured illumination in a spatial light modulator-based interferometer

We develop a novel hyperspectral imaging system using structured illumination in an SLM-based Michelson interferometer. In our design, we use a reflective SLM as a mirror in one of the arms of a Michelson interferometer, and scan the interferometer by varying the phase across the SLM display. For achieving the latter, we apply a checkerboard phase mask on the SLM display where the gray value varies between 0-255, thereby imparting a dynamic phase of up to 262° to the incident light beam. We couple a supercontinuum source into the interferometer in order to mimic an astronomical object such as the Sun, and choose a central wavelength of 637.4 nm akin to the strong emission line of Fe X present in the solar spectrum. We use a bandwidth of 30 nm, and extract fringes corresponding to a spectral resolution of 3.8 nm which is limited by the reflectivity of the SLM. We also demonstrate a maximum wavelength tunability of ~8 nm by varying the phase over the phase mask with a spectral sampling of around 0.03 nm between intermediate fringes. The checkerboard phase mask can be adapted close to real time on time-scales of a few tens of milliseconds to obtain spectral information for other near-contiguous wavelengths. The compactness, potential low cost, low power requirements, real-time tunability and lack of moving mechanical parts in the setup implies that it can have very useful applications in settings which require near real-time, multi-wavelength spectroscopic applications, and is especially relevant in space astronomy.

physics.optics↗

arXiv2021

Detection of nearly periodic spin period reversals in Vela X-1 on long time-scales: inkling of solar-like cycle in the donor star?

We explore the long-term evolution of the spin period of the High Mass X-ray Binary (HMXB) pulsar Vela X-1 over a period of 46 yr. Our analysis indicates nearly periodic variations in the spin period of the pulsar on time-scales of about 5.9 yr. There is suggestion of an overall spin-down behaviour of the pulsar though it is noticed that the source appears to stay near its equilibrium period 283.4 s since MJD 51000, with rather erratic spin-up/spin-down episodes around this value. Our study suggests nearly cyclic turnover in the spin behaviour of the pulsar from spin-up to spin-down regimes on time-scales of about 17-19 yr. To our knowledge this is the first report of periodic variation in the spin behaviour of a wind-fed accreting pulsar. We also observe erratic episodes of spin-up and spin-down behaviour on relatively shorter time-scales which is a well known archetype of this wind-fed X-ray pulsar. We investigate whether nearly periodic long-term spin period changes in the pulsar can be explained by using known mechanisms of torque reversals in the accretion powered neutron stars. It appears that changes in the accretion environment of the pulsar using current ideas can probably lead to long-term spin period changes in this X-ray pulsar.

astro-ph.HE↗

arXiv2020

Study of Recent outburst in the Be/X-ray binary RX J0209.6-7427 with AstroSat: A new ultraluminous X-ray pulsar in the Magellanic Bridge?

We present the timing and spectral studies of RX J0209.6-7427 during its rare 2019 outburst using observations with the Soft X-ray Telescope (SXT) and Large Area X-ray Proportional Counter (LAXPC) instruments on the AstroSat satellite. Pulsations having a periodicity of 9.29 s were detected for the first time by the NICER mission in the 0.2-10 keV energy band and, as reported here, by AstroSat over a broad energy band covering 0.3-80 keV. The pulsar exhibits a rapid spin-up during the outburst. Energy resolved folded pulse profiles are generated in several energy bands in 3-80 keV. To the best of our knowledge this is the first report of the timing and spectral characteristics of this Be binary pulsar in hard X-rays. There is suggestion of evolution of the pulse profile with energy. The energy spectrum of the pulsar is determined and from the best fit spectral values, the X-ray luminosity of RX J0209.6-7427 is inferred to be ${1.6}\times 10^{39}$ ergs s$^{-1}$. Our timing and spectral studies suggest that this source has features of an ultraluminous X-ray pulsar in the Magellanic Bridge. Details of the results are presented and discussed in terms of the current ideas.

astro-ph.HE↗

arXiv2018

Rapid phase calibration and increased efficiency of a spatial light modulator using novel phase masks and an iterative algorithm

We develop an improved phase calibration method of a reflective spatial light modulator (SLM) using interferometry by employing novel phase masks. We generate the optimised phase masks by using Iterative Fourier Transform Algorithm (IFTA) and demonstrate that they perform with 18$\%$ better efficiency over global linear corrections in the look-up table (LUT). In addition, we also implement global linear corrections in the LUT and correspondingly modify the phase encoding of blazed gratings to improve the efficiency of our phase-limited SLM. In the process, we definitively determine the actual maximum phase throw of our SLM which provides a recipe for users to verify supplier specifications.Besides obtaining array of 1D/2D spots having high uniformity (~90$\%$) using IFTA, our result exemplifies the use of iterative algorithms for improving efficiency of phase limited SLMs. Finally, our method enables threefold faster phase measurements, and to the best of our knowledge, is the first endeavour directed towards enabling rapid phase characterisation of an SLM using interferometric measurements and can have very useful applications in settings which often require faster phase calibrations.

physics.optics↗