arXiv subjects

Hemanth Manikantan

Publications and source records attributed to Hemanth Manikantan.

16 recordsLinked to original sources

arXiv2026

EXPO: a quantum leap in fast, wide-band X-ray polarimetry for astrophysics

The Enhanced X-ray Polarimetry Observatory (EXPO) is a mission concept proposed to ESA as an M8 candidate, with a prospective launch in 2041. Building on the scientific success of IXPE, EXPO is designed to overcome its two main limitations, the narrow 2-8 keV energy band and the very slow repointing time, and to enable new scientific capabilities. A wide energy band and fast repointing are essential for investigating the hard X-ray emission of magnetars and black-hole binaries, particle acceleration in supernova remnants and pulsar-wind nebulae, radiative transfer in highly magnetized plasmas, X-ray reflection in accretion flows and active galactic nuclei, and the prompt and afterglow emission of gamma-ray bursts and magnetar flares. EXPO comprises five focusing X-ray telescopes and gas photoelectric polarimeters based on the Timepix ASIC family with InGrid amplification, enabling three-dimensional track imaging and operation in the 2-35 keV band through optimized low- and medium-energy detector configurations. The mirror modules use proven electroformed nickel technology with Au-C coatings and an XMM-like focal length of 7.5 m. The polarimeters are complemented by a coded-mask Wide Field Instrument (WFI), derived from SVOM/ECLAIRs for continuous monitoring of a 2 sr field of view; a Spectral Imaging Camera (SIC), based on stacked CMOS and CdTe detectors for broadband imaging spectroscopy and accurate spectro-polarimetric decomposition; and an Instrument Control Unit (ICU) for payload management, onboard WFI image reconstruction, transient identification, and autonomous spacecraft repointing requests. These capabilities extend X-ray polarimetry into the hard X-ray domain and open a new observational window on fast transients, time-domain astrophysics, and multi-messenger astronomy.

astro-ph.IM↗

arXiv2026

Ion Irradiation of GridPix Detector at Bonn Isochronous Cyclotron

The potential successor of the IXPE --- the EXPO or Enhanced X-ray Polarimetry Observatory will have a new X-ray polarimeter based on GridPix technology. To certify this detector for operation within the expected radiation environment conditions, the irradiation tests were performed at the Bonn Isochronous Cyclotron at HISKPabbreviation\footnote{ stands for Helmholtz-Institut f\"ur Strahlen- und Kernphysik or Helmholz Institute for Radiation and Nuclear Physics}, University of Bonn. Two beam modes were used --- proton beam at 14\,MeV and 100\,MeV $^{14}\mathrm{N}^{5+}$ ions. The aforementioned GridPix detector contains the readout ASIC, Timepix, with $256\times256$ pixel grid with a pitch of \SI{55}{\micro\meter} and covered with a protection layer. The Al grid stands on the SU8 pillars above pixels forming the amplification stage. The alignment of grid holes and silicon pixels enable primary electron detection. This paper focuses on the description and results of these testbeams. The accumulated ion rates exceed by far the expected limits without a significant deterioration in chip performance.

physics.ins-det↗

arXiv2026

A Focal-plane X-ray Polarimeter with Spectral and Timing Capabilities for Future Missions

The NASA-ASI mission Imaging X-ray Polarimeter Explorer (IXPE) firmly established X-ray polarimetry as a core observation pillar of high-energy astrophysics, alongside imaging, timing, and spectroscopy. While IXPE made groundbreaking discoveries in the 2$-$8 keV range on both point and extended X-ray sources, the Gas Pixel Detectors onboard IXPE demand substantial improvements to be ready for the next generation of imaging X-ray polarimeters. Here, we present the development of a detector prototype that can deliver the next generation of sensitive imaging X-ray polarimetry across the energy band of 2--30 keV. We use Ar/DME based gas volume to absorb X-rays by the photoelectric effect and a pixelated CMOS readout ASIC, Timepix3, with excellent timing capabilities to image the resulting photoelectron track in three dimensions. The Timepix3 is integrated with a multiplication stage called InGrid that enables single-primary-electron detection from the gas volume. The photoelectron track reconstructed in three dimensions increases the polarimetric sensitivity towards the lowest operable energies, and the deadtime-free operation of the ASIC facilitates the usage as a focal plane instrument on high-throughput X-ray mirrors. With this prototype, we demonstrate a low-medium-energy X-ray polarimeter with excellent timing and moderate spectral capabilities.

astro-ph.IM↗

arXiv2026

Instruments for Focal Plane X-Ray Polarimetry in the Next Decade

The successful detection of X-ray polarization from many celestial sources belonging to different classes by the IXPE mission has opened a new window in X-ray astronomy. While an impressive number of scientific topics have already been addressed by IXPE, many of them would benefit from a new class of instrumentation that could be launched on a relatively short time scale. In this contribution, we present the development activities of a focal-plane polarimeter whose goal is to extend the energy range of IXPE up to tens of keV, with better sensitivity and lower background. Our design is based on the use of multilayer mirrors and stacked instrumentation, comprising either a low- or medium-energy imaging photoelectric polarimeter and an active Compton polarimeter. Such an approach relies on hardware with flight heritage and -- although still under development for the specific application in X-ray polarimetry -- it has the potential to answer compelling scientific questions and to soon become competitive from the point of view of feasibility for space applications.

astro-ph.IM↗

arXiv2025

The First X-Ray Polarimetry of an Eclipsing Low-Mass X-Ray Binary: Serendipitous IXPE Observation of AX J1745.6-2901

We present the first X-ray polarimetric measurement of the neutron star low-mass X-ray binary system AX J1745.6-2901 conducted by the Imaging X-ray Polarimetry Explorer (IXPE) satellite. This transient source, located within $ \sim $1.5' of the Galactic center, was observed serendipitously during a MAXI J1744-294 observation with a duration of 150 ks. The complex nature of the region in which AX J1745.6-2901 is located poses a challenge for studying its polarization. By performing a detailed analysis of the contamination from MAXI J1744-294 and the Galactic center diffuse emission, we find the source polarization degree PD = 14.7$\% \pm$ 4.0$\%$ and polarization angle PA = 122$^\circ \pm 8^\circ$. The phase-resolved analysis shows increase in polarization during the eclipse phase, with PD = 34.2$\% \pm$ 8.7$\%$, suggesting that the polarization-inducing mechanisms are of scattering nature, probably originating from disk winds.

astro-ph.HE↗

arXiv2025

XMM-Newton view of pulsating iron fluorescent emission from Centaurus X3

Cen X-3 is a bright, high-mass X-ray binary pulsar. We present a pulse phase-resolved X-ray spectral analysis of data from an archival XMM-Newton observation of Cen X-3 taken during the high state of the source. The observation was entirely in the out-of-eclipse part of the binary orbit. We study the pulse phase variability of the three K${\alpha}$ fluorescent emission lines from near-neutral, Helium-like and Hydrogen-like iron, along with the iron K${\beta}$ emission line. All four lines show clear modulation with the pulse phase of the neutron star, and modulation is found to be higher for the lines from highly ionised iron compared to the neutral lines. Structures within the light travel distance corresponding to the pulse period of the neutron star are likely responsible for the pulse phase modulation of the emission lines. We have also investigated the orbital phase dependence of the pulse phase variability in the iron lines by dividing the data into four segments at different orbital phases of Cen X-3. The pulse phase modulation behaviour of the four lines is quite identical at different orbital phases of Cen X-3, indicating the pulsed iron emission region is persistent in nature and probably phase aligned with respect to the observer. The accretion stream intercepting the line of sight can probably produce the observed phase dependence of the iron fluorescence emission lines.

astro-ph.HE↗

arXiv2025

The legacy of the IXPE instrument and prospects for the next generation of polarimetric photoelectric X-ray detectors

Imaging X-ray polarimetry with IXPE has demonstrated the scientific potential of the technique but also revealed the need for significant detector upgrades, particularly with the read-out ASIC that images photoelectron tracks and possibly the multiplication stage. Building on this experience, we are developing a next generation three-dimensional photoelectron track polarimeter based on the GridPix detector, originally developed for axion and axion-like-particle searches. We report on the current status of prototype development and preparations for the ion-irradiation tests. Preliminary proton beam irradiation runs at the Bonn Isochronous Cyclotron facility of the University of Bonn verified both this generation of ASIC's tolerance to high radiation doses present in space and the capability of the cyclotron facility to operate at sufficiently low rates for controlled tests.

astro-ph.IM↗

arXiv2025

Unveiling the intensity-dependent wake structure of Vela X-1 using MAXI/GSC

Context. Vela X-1 is among the earliest discovered high-mass X-ray binary (HMXB) pulsars. In such systems, the companion's stellar wind is strongly affected by ionisation from X-rays emitted by the compact object. A smooth, isotropic stellar wind model cannot account for the observed orbital variation in absorption column density. A stream-like photoionisation wake trailing the neutron star has been proposed to explain this variation. Aims. We investigated the variability of the circumbinary environment at different intensity levels of the Vela X-1 and used a model similar to the above-mentioned stream-like photoionisation wake to explain the asymmetric absorption column density present in the source. Methods. The 2.0-20.0 keV MAXI/GSC spectrum was well modelled with a Comptonised continuum absorbed by local and interstellar material. Nearly 13 years of MAXI/GSC data was used to constrain the variations in absorption column density in Vela X-1 from orbital-phase and intensity-and-orbital-phase resolved spectroscopy. Results. The long-term light curve of Vela X-1 shows orbit-to-orbit intensity level variations without any apparent super-orbital periodicity. The orbital-phase resolved spectroscopy in multiple intensity levels reveals asymmetric variation in absorption column density changes across the intensity levels. Conclusions. We confirm that the orbital variation in absorption column density in Vela X-1 cannot be explained by a smooth stellar wind alone using long-term MAXI/GSC data. An additional component, such as a photoionisation or accretion wake, is required. The wake structure is present across intensity levels, with geometry varying by intensity. The long-term MAXI/GSC data enabled us to vary wake parameters and derive best-fit stellar wind parameters for the time-averaged intensity, reproducing observed absorption column density across intensity levels. (Abbreviated.)

astro-ph.HE↗

arXiv2024

Studying the X-ray absorption characteristics of Centaurus X-3 using nearly 14 years of MAXI/GSC data

Centaurus X-3 is a persistent high-mass X-ray binary with the long-term light curve from the source exhibiting orbit-to-orbit intensity variations with no apparent superorbital periodicity. We used $\sim$13.5 years of MAXI/GSC data to study the long-term behaviour of X-ray absorption caused by the stellar wind from the companion star and any absorbing structures present in the binary. We used orbital-phase-resolved spectroscopy to study the variation in the photoelectric absorption along the line of sight of the source for both the intensity-averaged data and intensity-resolved data after dividing all the data binned with orbital period into three intensity levels. We find an asymmetric variation in the photoelectric absorption along the line of sight across an orbit of the source. The orbital-phase-resolved spectra show a clear increase in photoelectric absorption after $\phi_\text{orb}\sim$ 0.5, which deviates from a spherically symmetric stellar wind model. The flux of Cen X-3 shows significant variation between consecutive orbits. An intensity-resolved spectral analysis of the source was performed, followed by an intensity-resolved and orbital-phase-resolved spectral analysis, which showed that at the medium and high intensity levels, the orbital-phase-resolved photoelectric absorption is slightly asymmetric with respect to mid-phase ($\phi_\text{orb}=$ 0.5). The asymmetry is very pronounced at the lowest intensity level and cannot be explained by a spherically symmetric wind from the companion star. The differences in the orbital phase-dependence of absorption for different intensity levels suggest that the presence of an accretion wake, photoionization wake, or tidal stream is more prominent at a lower intensity level for Centaurus X-3 than at a higher intensity level.

astro-ph.HE↗

arXiv2024

Luminosity dependence of the multiple cyclotron lines in 4U 0115$+$63

The Be X-ray binary 4U 0115$+$63 underwent a giant outburst in 2023 with the X-ray luminosity of the source reaching 10$^{38}$ erg s$^{-1}$. During the outburst, two target of opportunity observations were made with \textit{NuSTAR}. The main goal of this work is to model the multiple cyclotron scattering features (CRSFs) present in 4U 0115$+$63 and study their dependence on the luminosity of the source. The 3$-$79 keV broadband X-ray coverage of \textit{NuSTAR} allowed us to properly model the continuum and investigate the nature of the multiple cyclotron resonance scattering features present in the source spectrum. We used the epoch-folding technique to find the pulsation from the source and also studied the changes in the cyclotron line energy with an order of magnitude variation in the source luminosity. We detected five cyclotron lines during the 2023 outburst near 12, 16, 24, 34, and 47 keV. The $\sim$16 keV cyclotron line cannot be harmonically related to the other cyclotron lines at $\sim$12 keV and $\sim$24 keV. This indicates the presence of two fundamental lines in the spectrum of 4U 0115$+$63 at 12 keV and 16 keV. With the inclusion of the two latest \textit{NuSTAR} observations, we have expanded the data set of the CRSF line center to encompass a broad range of luminosity. This enables us to comprehensively investigate the relationship between the centroid energy of the cyclotron lines and luminosity. The CRSF line center shows no anticorrelation with luminosity, unlike previously reported. Instead, a weak positive correlation is found in four out of the five detected cyclotron lines of 4U 0115$+$63. The luminosity variation of the two fundamental CRSFs could be well explained with the prediction from the collisionless shock model.

astro-ph.HE↗

arXiv2024

Energy dependence of Quasi-periodic oscillations in accreting X-ray pulsars

We present the results from an investigation of the energy dependence of Quasi-Periodic Oscillations (QPOs) exhibited by accreting X-ray pulsars using data from archival \textit{XMM-Newton}, \textit{NuSTAR}, \textit{RXTE}, and \textit{NICER} observations. In a search for the presence of QPOs in 99 \textit{XMM-Newton} and \textit{NuSTAR} observations, we detected QPOs in eleven observations of five sources, viz., 4U 1626--67 (48 mHz), IGR J19294+1816 (30 mHz), V 0332+53 (2, 18 and 40 mHz), Cen X--3 (30 mHz), and XTE J1858+034 (180 mHz). A positive correlation of the QPO rms amplitude with energy is exhibited by 4U 1626--67, IGR J19294+1816, Cen X--3 and XTE J1858+034, while no energy dependence is observed in V 0332+53. We also analysed the energy spectrum to decouple thermal (soft-excess) from non-thermal emission and determine if the soft-excess has different QPO properties. We found no evidence for different QPO characteristics of the soft excess. The \textit{NuSTAR} observations of V 0332+53 during the Type-I outburst in 2016 show the presence of twin QPOs at 2.5 mHz and 18 mHz, while the \textit{XMM-Newton} and \textit{NuSTAR} observations during the Type-II outburst in 2015 show a QPO at 40 mHz. We review the observed QPO properties in the context of QPOs found in other types of accreting sources and the models usually used to explain the QPOs in accreting X-ray pulsars.

astro-ph.HE↗

arXiv2024

Detection of a cyclotron line in the Be X-ray pulsar IGR J06074$+$2205

IGR J0607.4+2205 is a transient Be X-ray binary discovered two decades ago. IGR J0607.4+2205 underwent an outburst in 2023 during which it was observed twice with \textit{NuSTAR}. The main goal of this work is to model the broadband X-ray spectrum of IGR J0607.4+2205 during the outburst and to study the variations of the spectral and timing features at different intensities. We extracted the light curve and spectrum of the source from the two \textit{NuSTAR} observations carried out during the recent outburst in the energy range of 3$-$78 keV. We used the epoch folding technique to find pulsation from the source and to study the changes in emission characteristics from the source with energy across an order of magnitude variation in source luminosity. IGR J0607.4+2205 shows pulsations with a period of $\sim$347.6 s during both the observations, with a pulse fraction of $\geq$50\%. The broadband spectrum of the source was modelled using a power-law continuum with a high-energy cutoff. During the first observation, a cyclotron absorption line at $\sim$51 keV was also present in the source with an optical depth of $\sim$1.3. However, no cyclotron line feature was detected in the second observation when the source was an order of magnitude fainter. Additionally, soft excess was detected in the second observation, which was modelled with a black body component emerging from close to the neutron star (NS). We report the first ever detection of a cyclotron line in the broadband spectrum of IGR J0607.4+2205 centred at 51$\pm$1 keV. The magnetic field strength of the NS is estimated to be $\sim$4$\times$$10^{12}$ G from the centroid energy of the absorption line. A significant change is observed in the pulse profile with luminosity during the decay of the outburst, indicating an associated change in the beaming pattern.

astro-ph.HE↗

arXiv2023

Studying the variability of fluorescence emission and the presence of clumpy wind in HMXB GX 301$-$2 using XMM-Newton

We present the results from an analysis of data from an \textit{XMM-Newton} observation of the accreting high mass X-ray binary pulsar GX 301$-$2. Spectral analysis in the non-flaring segment of the observation revealed that the equivalent width of the iron fluorescence emission is correlated with the observed absorption column density and the ratio of the iron K$β$ and K$α$ line strength varied with the flux of the source. Coherent pulsations were detected with the spin period of the pulsar of 687.9$\pm$0.1 s, and a secondary pulsation was also detected with a period of 671.8$\pm$0.2 s, most prominent in the energy band of the iron line. At the spin period of the neutron star, the pulsation of the iron line has a low amplitude and the profile is different from the continuum. Pulse phase-resolved spectroscopy also revealed pulsations of the iron emission line during the non-flaring segment of the light curve. At the secondary period, both the iron line and the continuum have nearly identical pulse fraction and pulse profile. The additional periodicity can be attributed to the beat frequency between the spin of the neutron star and the Keplerian frequency of a stellar wind clump in retrograde motion around the neutron star. Reprocessed X-ray emissions originating from the clump can produce the observed secondary pulsations both in the continuum and the iron fluorescence line. The clump rotating around the neutron star is estimated to be approximately five lt-s away from the neutron star.

astro-ph.HE↗

arXiv2023

Investigating the orbital evolution of the eccentric HMXB GX 301$-$2 using long-term X-ray lightcurves

We report the orbital decay rate of the high mass X-ray binary GX 301$-$2 from an analysis of its long-term X-ray light curves and pulsed flux histories from CGRO/BATSE, RXTE/ASM, Swift/BAT, Fermi/GBM and MAXI by timing the pre-periastron flares over a span of almost 30 years. The time of arrival of the pre-periastron flares exhibits an energy dependence (hard lag) and the orbital period decay was estimated after correcting for it. This method of orbital decay estimation is unaffected by the fluctuations in the spin rate of the X-ray pulsar associated with variations in the mass accretion rate. The resulting $\dot P_\textrm{orb}$ $=-(1.98\pm0.28)\times10^{-6}$ s s$^{-1}$ indicates a rapid evolution timescale of $|P_\textrm{orb}/\dot P_\textrm{orb}|\sim 0.6\times10^{5}$ yr, making it the high mass X-ray binary with the fastest orbital decay. Our estimate of $\dot P_\mathrm{orb}$ is off by a factor of $\sim2$ from the previously reported value of $-(3.7\pm0.5)\times10^{-6}$ s s$^{-1}$ estimated from pulsar TOA analysis. We discuss various possible mechanisms that could drive this rapid orbital decay and also suggest that GX 301$-$2 is a prospective Thorne-Żytkow candidate.

astro-ph.HE↗

arXiv2023

An investigation of the "10 keV feature" in the spectra of Accretion Powered X-ray Pulsars with NuSTAR

Some of the accreting X-ray pulsars are reported to exhibit a peculiar spectral feature at $\sim$10 keV, known as the "10 keV feature". The feature has been characterized as either an emission line or an absorption line, and its origin is unknown. It has been found in multiple observations of the same source by different observatories, but not all the observations of any particular source consistently showed the presence of it. In this work, we have carried out a systematic investigation for the presence of the "10 keV feature" using data from NuSTAR, a low background spectroscopic observatory having uninterrupted wide band coverage on either side of 10 keV. We performed a systematic spectral analysis on 58 archival NuSTAR observations of 30 bright X-ray pulsars. The 3$-$79 keV spectral continua of these selected sources were fitted with a model chosen on the basis of its fitting quality in 3$-$15 keV and model simplicity, and then inspected for the presence of the "10 keV feature". Our analysis indicates the presence of such a feature in 16 out of 58 the NuSTAR observations of 11 different sources and is fitted with a Gaussian absorption model centered around 10 keV. Our analysis also suggests that such a feature could be wrongly detected if flare data is not analyzed separately from persistent emission.

astro-ph.HE↗

arXiv2023

Changes in the distribution of circum-binary material around the HMXB GX 301-2 during a rapid spin-up episode of the neutron star

Some accretion powered X-ray pulsars with supergiant companion stars undergo occasional rapid spin-up episodes that last for weeks to a few months. We explore the changes in the accretion environment of the pulsar GX 301-2 during its latest 80 days long spin-up episode in 2019 when the spin frequency of the pulsar increased by ~2% over two orbits of the binary. By performing time-resolved spectroscopy with the MAXI/GSC spectra of the source, we estimated the equivalent hydrogen column density and equivalent width of the iron fluorescence line during the spin-up episode, and compared them with the long-term average values estimated by orbital-phase resolved spectroscopy. The measured absorption column density during the spin-up episode is about twice that of an average orbit, while the equivalent width of the iron line is less than half of an average orbit. Though the spin-up episode started immediately after a pre-periastron flare and lasted for the two consecutive orbits of the binary, the associated enhancement in luminosity started a few days after the pre-periastron flare and lasted only during the first orbit, and some enhancement was seen again during the pre-periastron passage of the second orbit. The absorption column density and iron line equivalent width vary throughout the spin-up episode and are distinct from an average orbit. These observations indicate a significant change in the accretion and reprocessing environment in GX 301-2 during the spin-up episode and may hold important clues for the phenomenon in this source and several other sources with supergiant companions.

astro-ph.HE↗