arXiv subjects

Chandreyee Maitra

Publications and source records attributed to Chandreyee Maitra.

At least 37 records · Page 2Linked to original sources

arXiv2023

The High Energy X-ray Probe (HEX-P): Studying Extreme Accretion with Ultraluminous X-ray Sources

Ultraluminous X-ray sources (ULXs) represent an extreme class of accreting compact objects: from the identification of some of the accretors as neutron stars to the detection of powerful winds travelling at 0.1-0.2 c, the increasing evidence points towards ULXs harbouring stellar-mass compact objects undergoing highly super-Eddington accretion. Measuring their intrinsic properties, such as the accretion rate onto the compact object, the outflow rate, the masses of accretor/companion -- hence their progenitors, lifetimes, and future evolution -- is challenging due to ULXs being mostly extragalactic and in crowded fields. Yet ULXs represent our best opportunity to understand super-Eddington accretion physics and the paths through binary evolution to eventual double compact object binaries and gravitational wave sources. Through a combination of end-to-end and single-source simulations, we investigate the ability of HEX-P to study ULXs in the context of their host galaxies and compare it to XMM-Newton and NuSTAR, the current instruments with the most similar capabilities. HEX-P's higher sensitivity, which is driven by its narrow point-spread function and low background, allows it to detect pulsations and broad spectral features from ULXs better than XMM-Newton and NuSTAR. We describe the value of HEX-P in understanding ULXs and their associated key physics, through a combination of broadband sensitivity, timing resolution, and angular resolution, which make the mission ideal for pulsation detection and low-background, broadband spectral studies.

astro-ph.HE↗

arXiv2023

Clumpy wind studies and the non-detection of cyclotron line in OAO 1657-415

Winds of massive stars are suspected to be inhomogeneous (or clumpy), which biases the measures of their mass loss rates. In High Mass X-ray Binaries (HMXBs), the compact object can be used as an orbiting X-ray point source to probe the wind and constrain its clumpiness. We perform spectro-timing analysis of the HMXB OAO 1657-415 with non-simultaneous NuSTAR and NICER observations. We compute the hardness ratio from the energy-resolved light curves, and using an adaptive rebinning technique, we thus select appropriate time segments to search for rapid spectral variations on timescales of a few hundreds to thousands of seconds. Column density and intensity of Iron K$α$ line were strongly correlated, and the recorded spectral variations were consistent with accretion from a clumpy wind. We also illustrate a novel framework to measure clump sizes, masses in HMXBs more accurately based on absorption measurements and orbital parameters of the source. We then discuss the limitations posed by current X-ray spacecrafts in such measurements and present prospects with future X-ray missions. We find that the source pulse profiles show a moderate dependence on energy. We identify a previously undetected dip in the pulse profile visible throughout the NuSTAR observation near spin phase 0.15 possibly caused by intrinsic changes in accretion geometry close to the neutron star. We do not find any evidence for the debated cyclotron line at $\sim$ 36\,keV in the time-averaged or the phase-resolved spectra with NuSTAR.

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arXiv2022

Pulse Phase Variation of Cyclotron Line in HMXB 4U 1907+09 with ASTROSAT LAXPC

We present timing and spectral analysis of data from an observation of the High Mass X-ray Binary pulsar 4U 1907+09 with the LAXPC instrument onboard AstroSat. The light curve consisted of a flare at the beginning of the observation, followed by persistent emission. The pulsar continues to spin down, and the pulse profile is found to be double-peaked up to 16 keV with the peaks separated by a phase of $\sim0.45$. Significant energy dependence of the pulse profile is seen with diminishing amplitude of the secondary peak above 16 keV, and increasing amplitude of the main peak upto 40 keV and a sharp decline after that. We confirm earlier detections of the Cyclotron Resonance Scattering Feature (CRSF) in 4U 1907+09 at $\sim 18.5\pm 0.2$ keV in the phase-averaged spectrum with a high detection significance. An intensity resolved spectral analysis of the initial flare in the light curve shows that the CRSF parameters do not change with a change in luminosity by a factor of 2.6. We also performed pulse phase-resolved spectral analysis with ten independent phase bins. The energy and the strength of the cyclotron line show pulse phase dependence that is in agreement with previous measurements. Two features from the current observation: different energy dependence of the two pulse peaks and a strong CRSF only around the secondary peak, both indicate a deviation from a dipole geometry of the magnetic field of the neutron star, or complex beaming pattern from the two poles.

astro-ph.HE↗

arXiv2022

The eROSITA Final Equatorial Depth Survey (eFEDS): X-ray emission around star-forming and quiescent galaxies at $0.05<z<0.3$

We aim at characterizing the hot phase of the Circum-Galactic Medium in a large sample of galaxies. We stack X-ray events from the SRG/eROSITA eFEDS survey around central galaxies in the GAMA 9hr field to construct radially projected soft X-ray luminosity profiles as a function of their stellar mass and specific star formation rate. We consider samples of quiescent (star-forming) galaxies in the stellar mass range $2\times 10^{10}$ -- $10^{12}$ M$_\odot$ ($3\times 10^9$ -- $6\times10^{11}$ M$_\odot$). For quiescent galaxies, the X-ray profiles are clearly extended throughout the available mass range; however, the measured profile is likely biased high due to projection effects, as these galaxies tend to live in dense and hot environments. For the most massive star forming samples ($\geq10^{11}$ M$_\odot$), there is a hint of detection of extended emission. For star-forming galaxies with $< 10^{11}$ M$_\odot$ the X-ray stacked profiles are compatible with unresolved sources and consistent with the expected emission from faint Active Galactic Nuclei and X-ray binaries. We measure for the first time the mean relation between average X-ray luminosity and stellar mass separately for quiescent and star-forming galaxies. High-mass ($\geq 10^{11}$ M$_\odot$) star-forming or quiescent galaxies follow the expected scaling of virialized hot haloes, while lower mass star-forming galaxies show a less prominent luminosity and a weaker dependence on stellar mass, consistent with empirical models of the weak AGN population. When comparing our results with state-of-the art numerical simulations, we find an overall consistency on large ($>80$ kpc) scales at masses $\geq 10^{11}$ M$_\odot$, but disagreement on the small scales, where brighter than observed compact cores are predicted. Simulations also do not predict the clear differentiation that we observe between quiescent and star-forming galaxies.

astro-ph.GA↗

arXiv2022

XMM-Newton and SRG/eROSITA observations of the isolated neutron star candidate 4XMM J022141.5-735632

We report the results of follow-up investigations of a possible new thermally emitting isolated neutron star (INS), 4XMM J022141.5-735632, using observations from XMM-Newton and Spectrum Roentgen Gamma (SRG) eROSITA. The analysis is complemented by Legacy Survey imaging in the optical and near-infrared wavelengths. The X-ray source, the first to be targeted by XMM-Newton in an effort to identify new INS candidates from the fourth generation of the XMM-Newton serendipitous source catalogue Data Release 9 (4XMM-DR9), shows a remarkably soft energy distribution and a lack of catalogued counterparts; the very high X-ray-to-optical flux ratio virtually excludes any other identification than an INS. Within current observational limits, no significant flux variation nor change of spectral state is registered over nearly ten years. Future dedicated observations, particularly to search for pulsations, are crucial to shed further light on the nature of the X-ray source and relations to other Galactic neutron stars.

astro-ph.HE↗

arXiv2022

Timing six energetic rotation-powered X-ray pulsars, including the fast-spinning young PSR J0058-7218 and Big Glitcher PSR J0537-6910

Measuring a pulsar's rotational evolution is crucial to understanding the nature of the pulsar. Here we provide updated timing models for the rotational evolution of six pulsars, five of which are rotation phase-connected using primarily NICER X-ray data. For the newly-discovered fast energetic young pulsar, PSR J0058-7218, we increase the baseline of its timing model from 1.4 days to 8 months and not only measure more precisely its spin-down rate nudot = (-6.2324+/-0.0001)x10^-11 Hz s^-1 but also for the first time the second time derivative of spin rate nuddot = (4.2+/-0.2)x10^-21 Hz s^-2. For the fastest and most energetic young pulsar, PSR J0537-6910 (with 16 ms spin period), we detect 4 more glitches, for a total of 15 glitches over 4.5 years of NICER monitoring, and show that its spin-down behavior continues to set this pulsar apart from all others, including a long-term braking index n = -1.234+/-0.009 and interglitch braking indices that asymptote to <~ 7 for long times after a glitch. For PSR J1101-6101, we measure a much more accurate spin-down rate that agrees with a previous value measured without phase-connection. For PSR J1412+7922 (also known as Calvera), we extend the baseline of its timing model from our previous 1-year model to 4.4 years, and for PSR J1849-0001, we extend the baseline from 1.5 years to 4.7 years. We also present a long-term timing model of the energetic pulsar, PSR J1813-1749, by fitting previous radio and X-ray spin frequencies from 2009-2019 and new ones measured here using 2018 NuSTAR and 2021 Chandra data.

astro-ph.HE↗

arXiv2022

The VMC Survey -- XLIX. Discovery of a population of quasars dominated by nuclear dust emission behind the Magellanic Clouds

Following the discovery of SAGE0536AGN ($z \sim$ 0.14), with the strongest 10-$μ$m silicate emission ever observed for an Active Galactic Nucleus (AGN), we discovered SAGE0534AGN ($z \sim$ 1.01), a similar AGN but with less extreme silicate emission. Both were originally mistaken as evolved stars in the Magellanic Clouds. Lack of far-infrared emission, and therefore star-formation, implies we are seeing the central engine of the AGN without contribution from the host galaxy. They could be a key link in galaxy evolution. We used a dimensionality reduction algorithm, t-SNE (t-distributed Stochastic Neighbourhood Embedding) with multi-wavelength data from Gaia EDR3, VISTA survey of the Magellanic Clouds, AllWISE and the Australian SKA Pathfinder to find these two unusual AGN are grouped with 16 other objects separated from the rest, suggesting a rare class. Our spectroscopy at SAAO/SALT and literature data confirm at least 14 of these objects are extragalactic ($0.13 < z < 1.23$), all hosting AGN. Using spectral energy distribution fitter CIGALE we find that the majority of dust emission ($> 70 \%$) in these sources is due to the AGN. Host galaxies appear to be either in or transitioning into the green valley. There is a trend of a thinning torus, increasing X-ray luminosity and decreasing Eddington ratio as the AGN transition through the green valley, implying that as the accretion supply depletes, the torus depletes and the column density reduces. Also, the near-infrared variability amplitude of these sources correlates with attenuation by the torus, implying the torus plays a role in the variability.

astro-ph.GA↗

arXiv2022

The eROSITA extragalactic CalPV serendipitous catalog

The eROSITA X-ray telescope on board the Spectrum-Roentgen-Gamma (SRG) observatory performed calibration and performance verification (CalPV) observations between September 2019 and December 2019, ahead of the planned four-year all-sky surveys. Most of them were deep, pointing-mode observations. We present here the X-ray catalog detected from the set of extra-galactic CalPV observations released to the public by the German eROSITA consortium, and the multiband counterparts of these X-ray sources. We developed a source detection method optimized for point-like X-ray sources by including extended X-ray emission in the background measurement. The multiband counterparts were identified using a Bayesian method from the CatWISE catalog. Combining 11 CalPV fields, we present a catalog containing 9515 X-ray sources, whose X-ray fluxes were measured through spectral fitting. CatWISE counterparts are presented for 77% of the sources. Significant variabilities are found in 99 of the sources, which are also presented with this paper. Most of these fields show similar number counts of point sources as typical extragalactic fields, and a few harbor particular stellar populations.

astro-ph.HE↗

arXiv2021

Discovery of four super-soft X-ray sources in XMM-Newton observations of the Large Magellanic Cloud

Super-soft X-ray sources were established as a heterogeneous class of objects from observations of the Large Magellanic Cloud (LMC). We have searched for new sources of this class in the X-ray images obtained from the XMM-Newton survey of the LMC and additional archival observations. We first selected candidates by visual inspection of the image, and screened out artefacts which can mimic super-soft X-ray sources as well as bright foreground stars which create optical loading on the CCD image. We finally obtained 4 new super-soft X-ray sources for which we performed detailed X-ray timing and spectral analysis and searched for possible optical counterparts to identify their nature. XMMU J050452.0-683909 is identified as the central star of the planetary nebula SMP LMC21 in the LMC. We suggest XMMU J051854.8-695601 and XMMU J050815.1-691832 as new soft intermediate polars from the nature of their X-ray spectrum. Their estimated absorption-corrected luminosities and the blackbody radii indicate that they are located in our Galaxy, rather than the LMC. We discovered coherent pulsations of 497 s from XMMU J044626.6-692011 which indicates a magnetic cataclysmic variable nature of the source. The location of XMMU J044626.6-692011 in the LMC or our Galaxy is less clear. It could either be a white dwarf in the LMC with nuclear burning on its surface near the Eddington limit, or another soft intermediate polar in our Galaxy. The discovery of new super-soft X-ray sources makes a significant contribution to the known population in our own Galaxy. An observed higher density of sources in the direction of the Magellanic Clouds can likely be explained by the relatively low Galactic column density in their direction as well as a large number of existing observations sensitive at low X-ray energies.

astro-ph.HE↗

arXiv2021

First studies of the diffuse X-ray emission in the Large Magellanic Cloud with eROSITA

In the first months after the launch in July 2019, eROSITA onboard Spektr-RG (SRG) performed long-exposure observations in the regions around SN 1987A and SNR N132D in the Large Magellanic Cloud (LMC). We analyse the distribution and the spectrum of the diffuse X-ray emission in the observed fields to determine the physical properties of the hot phase of the interstellar medium (ISM). The eROSITA data are complemented by newly derived column density maps for the Milky Way and the LMC, 888 MHz radio continuum map from the Australian Square Kilometer Array Pathfinder (ASKAP), and optical images of the Magellanic Cloud Emission Line Survey (MCELS). We detect significant emission from thermal plasma with kT=0.2 keV in all the regions. There is also an additional higher-temperature emission component from a plasma with kT = 0.7 keV. In addition, non-thermal X-ray emission is significantly detected in the superbubble 30 Dor C. The absorbing column density NH in the LMC derived from the analysis of the X-ray spectra taken with eROSITA is consistent with the NH obtained from the emission of the cold medium over the entire area. Neon abundance is enhanced in the regions in and around 30 Dor and SN 1987A, indicating that the ISM has been chemically enriched by the young stellar population. Emission from the stellar cluster RMC 136 and the Wolf-Rayet stars RMC 139 and RMC 140 is best modelled with a high-temperature (kT>1 keV) non-equilibrium ionisation plasma emission and a non-thermal component with a photon index of Γ =1.3. In addition, the optical SNR candidate J0529-7004 is also detected with eROSITA and we thus confirm the source as an SNR.

astro-ph.HE↗

arXiv2021

Phase-resolved X-ray spectroscopy of PSR B0656+14 with SRG/eROSITA and XMM-Newton

(abridged version) We present a detailed spectroscopic and timing analysis of X-ray observations of the bright radio-to-gamma-ray emitting pulsar PSR B0656+14, which were obtained simultaneously with eROSITA and XMM-Newton during the Calibration and Performance Verification phase of the Spektrum-Roentgen-Gamma mission (SRG) for 100 ks. Using XMM-Newton and NICER we firstly established an X-ray ephemeris for the time interval 2015 to 2020, which connects all X-ray observations in this period without cycle count alias and phase shifts. The mean eROSITA spectrum clearly reveals an absorption feature originating from the star at 570 eV with a Gaussian sigma of about 70 eV, tentatively identified earlier in a long XMM-Newton observation (Arumugasamy et al. 2018). A second absorption feature, described here as an absorption edge, occurs at 260-265 eV. It could be of atmospheric or of instrumental origin. These absorption features are superposed on various emission components, phenomenologically described as the sum of hot (120 eV) and cold (65 eV) blackbody components, both of photospheric origin, and a power-law with photon index Gamma=2. The phase-resolved spectroscopy reveals that the Gaussian absorption line at 570 eV is clearly present throughout ~60% of the spin cycle. The visibility of the line strength coincides in phase with the maximum flux of the hot blackbody. We also present three families of model atmospheres: a magnetised atmosphere, a condensed surface, and a mixed model, which were applied to the mean observed spectrum and whose continuum fit the observed data well. The atmosphere model, however, predicts too short distances. For the mixed model, the Gaussian absorption may be interpreted as proton cyclotron absorption in a field as high as 10^14 G, which is significantly higher than that derived from the moderate observed spin-down.

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arXiv2021

Identification of SRGt 062340.2-265715 as a bright, strongly variable, novalike cataclysmic variable

We report the identification and follow-up of the transient SRG 062340.2-265715 detected with both instruments on board the Spektrum-Roentgen-Gamma mission. Optical spectroscopy of the G=12.5 counterpart firmly classifies the object as a novalike cataclysmic variable (CV) at a distance of 495 pc. A highly significant TESS period of 3.941 hours, tentatively identified with the orbital period of the binary, could not be found when the object was reobserved with TESS two years later. The newer high-cadence TESS data revealed quasi-periodic oscillations around 25 min, while ground-based photometry indicated periodic variability at 32 min. Located in very sparsely populated regions of color-magnitude diagrams involving X-ray and optical magnitudes and colors, the new object could be an X-ray underluminous magnetic CV, an intermediate polar, or an overluminous nonmagnetic CV. The lack of uniquely identified spin and orbital periods prevents a final classification. The site of X-ray production in the system, L(X, bol) = 4.8 x 10^{32} erg/s, remains to be understood given its high variability on long and short timescales.

astro-ph.SR↗

arXiv2021

The ASKAP-EMU Early Science Project: 888 MHz Radio Continuum Survey of the Large Magellanic Cloud

We present an analysis of a new 120 deg$^{2}$ radio continuum image of the Large Magellanic Cloud (LMC) at 888 MHz with a bandwidth of 288 MHz and beam size of $13\rlap{.}^{\prime\prime}9\times12\rlap{.}^{\prime\prime}1$, from the Australian Square Kilometre Array Pathfinder (ASKAP) processed as part of the Evolutionary Map of the Universe (EMU) survey. The median Root Mean Squared noise is 58 $μ$Jy beam$^{-1}$. We present a catalogue of 54,612 sources, divided over a GOLD list (30,866 sources) complete down to 0.5 mJy uniformly across the field, a SILVER list (22,080 sources) reaching down to $<$ 0.2 mJy and a BRONZE list (1,666 sources) of visually inspected sources in areas of high noise and/or near bright complex emission. We discuss detections of planetary nebulae and their radio luminosity function, young stellar objects showing a correlation between radio luminosity and gas temperature, novae and X-ray binaries in the LMC, and active stars in the Galactic foreground that may become a significant population below this flux level. We present examples of diffuse emission in the LMC (H II regions, supernova remnants, bubbles) and distant galaxies showcasing spectacular interaction between jets and intracluster medium. Among 14,333 infrared counterparts of the predominantly background radio source population we find that star-forming galaxies become more prominent below 3 mJy compared to active galactic nuclei. We combine the new 888 MHz data with archival Australia Telescope Compact Array data at 1.4 GHz to determine spectral indices; the vast majority display synchrotron emission but flatter spectra occur too. We argue that the most extreme spectral index values are due to variability.

astro-ph.GA↗

arXiv2021

Comprehensive broadband study of accreting neutron stars with Suzaku: Is there a bi-modality in the X-ray spectrum?

We present a broadband spectral analysis of accreting neutron stars using data from XIS and PIN onboard \emph{Suzaku}. From spectral fits of these sources with a single continuum model including a powerlaw and high energy cut-off, cyclotron lines (where required), we studied the correlation between various spectral parameters. Among 39 sources we studied, 16 are those where the existence of a cyclotron line is known in literature, and 29 need a cutoff energy. Among these 29 sources, 18 have cutoff energy bunched in a range of 3-10 keV while for 11 sources, it spreads over 12-25 keV. This bi-modal behaviour is not based on the specific nature of the systems being a Be XRB or supergiant HMXB, nor on different beaming patterns characterizing their X-ray emission (as inferred from simultaneous study of their pulse profiles). The broadband coverage of \emph{Suzaku} also shows that the cutoff energies saturate for higher values of cyclotron line energies - consistent with previous works in literature - for both the groups and the width of the cyclotron line show a weak correlation with the cyclotron line energy. We also find an anticorrelation with luminosity for both spectral index and folding energy, respectively. Unlike previous works, we did not detect any anticorrelation between X-ray luminosity and EW of K$α$ lines. Finally, we show that the EW and flux of the iron K$α$ line are smaller in SFXTs than classical NS-HMXBs. We discuss these findings in terms of different properties of stellar winds and accretion mechanisms.

astro-ph.HE↗

arXiv2020

New Optically Identified Supernova Remnants in the Large Magellanic Cloud

We present a new optical sample of three Supernova Remnants and 16 Supernova Remnant (SNR) candidates in the Large Magellanic Cloud(LMC). These objects were originally selected using deep H$α$, [SII] and [OIII] narrow-band imaging. Most of the newly found objects are located in less dense regions, near or around the edges of the LMC's main body. Together with previously suggested MCSNR J0541-6659, we confirm the SNR nature for two additional new objects: MCSNR J0522-6740 and MCSNRJ0542-7104. Spectroscopic follow-up observations for 12 of the LMC objects confirm high [SII]/H$α$ a emission-line ratios ranging from 0.5 to 1.1. We consider the candidate J0509-6402 to be a special example of the remnant of a possible Type Ia Supernova which is situated some 2$^\circ$ ($\sim 1.75$kpc) north from the main body of the LMC. We also find that the SNR candidates in our sample are significantly larger in size than the currently known LMC SNRs by a factor of $\sim 2$. This could potentially imply that we are discovering a previously unknown but predicted, older class of large LMC SNRs that are only visible optically. Finally, we suggest that most of these LMC SNRs are residing in a very rarefied environment towards the end of their evolutionary span where they become less visible to radio and X-ray telescopes.

astro-ph.HE↗

arXiv2020

Evolution of timing and spectral characteristics of 4U 1901+03 during its 2019 outburst using the Swift and NuSTAR observatories

We report the results from a detailed timing and spectral study of transient X-ray pulsars, 4U 1901+03 during its 2019 outburst. We performed broadband spectroscopy in the 1-70 keV energy band using four observations made with Swift and NuSTAR at different intensity levels. Our timing results reveal the presence of highly variable pulse profiles dependent on both luminosity and energy. Our spectroscopy results showed the presence of a cyclotron resonance scattering feature (CRSF) at ~30 keV. This feature at 30 keV is highly luminosity and pulse-phase dependent. Phase-averaged spectra during the last two observations, made close to the declining phase of the outburst showed the presence of this feature at around 30 keV. The existence of CRSF at 30 keV during these observations is well supported by an abrupt change in the shape of pulse profiles found close to this energy. We also found that 30 keV feature was significantly detected in the pulse-phase resolved spectra of observations made at relatively high luminosities. Moreover, all spectral fit parameters showed a strong pulse phase dependence. In line with the previous findings, an absorption feature at around 10 keV is significantly observed in the phase-averaged X-ray spectra of all observations and also showed a strong pulse phase dependence.

astro-ph.HE↗

arXiv2020

AstroSat Observations of the first Galactic ULX Pulsar Swift J0243.6+6124

SwiftJ0243.6+6124, the first Galactic ultra-luminous X-ray pulsar, was observed during its 2017-2018 outburst with \emph{AstroSat} at both sub- and super-Eddington levels of accretionwith X-ray luminosities of $L_{X}{\sim}7{\times}10^{37}$ and $6{\times}10^{38}$$ergs^{-1}$, respectively.Our broadband timing and spectral observations show that X-ray pulsations at ${\sim}9.85\rm{s}$ have been detected up to 150keV when the source was accreting at the super-Eddington level.The pulse profiles are a strong function of both energy and source luminosity,showing a double-peaked profile with pulse fraction increasing from $\sim$$10{\%}$ at $1.65\rm{keV}$ to 40--80$\%$ at $70\rm{keV}$.The continuum X-ray spectra are well-modeled with a high energy cut-off power law($Γ$${\sim}$0.6-0.7) and one or two blackbody components with temperatures of $\sim$0.35$\rm{keV}$ and $1.2\rm{keV}$, depending on the accretion level.No iron line emission is observed at sub-Eddington level, while a broad emission feature at around 6.9keV is observed at the super-Eddington level, along with a blackbody radius($121-142\rm{km}$) that indicates the presence of optically thick outflows.

astro-ph.HE↗

arXiv2020

Is superorbital modulation in SMC X-1 caused by absorption in warped precessing accretion disc?

We present a broadband spectral-timing analysis of SMC X-1 at different intensity states of its super-orbital variation using 10 Suzaku and 6 NuSTAR observations. The spectrum in all the states can be described by an absorbed powerlaw with a high energy cutoff and a black-body component along with an iron emission line. Compared to other supergiant HMXBs, the Fe K$α$ line equivalent width is low in SMC X-1 - from less than 10 eV in high state to up to $\sim$ 270 eV in the low states. The spectral shape is dependent on flux with the hard X-ray spectrum steepening with increasing flux. We also report a highly variable normalisation of the power-law component across these 16 super-orbital states. Pulsations in the hard X-rays for both the instruments were detected in all but two observations. The pulse profiles are near sinusoidal with two peaks and the relative intensity of the second peak decreasing with decreasing luminosity. These findings suggest that the super-orbital modulation in SMC X-1 is not caused by absorption in precessing warped accretion disc alone and there are intrinsic changes in X-rays emanating from the neutron star at different super-orbital states. We also note a putative cyclotron line at $\sim$ 50 keV in the \nustar~spectra of three bright states indicating a possible magnetic field of $\sim$ 4.2 $\times$ 10$^{12}$ G. Finally, with the new pulse period measurements reported here, the time base for the secular spin-up of SMC X-1 is increased by thirteen years and the complete pulse period history shows a sudden change in the spin-up trend around 1995.

astro-ph.HE↗