Soft X-ray Telescope Research Articles

Exploring the Broadband Spectral and Timing Characteristics of GRS 1915+105 with AstroSat and NICER Observations

In this study, we undertake a spectral-timing analysis of the black hole X-ray binary source GRS 1915+105 using simultaneous observations carried out by AstroSat (Large Area X-ray Proportional Counter, LAXPC and Soft X-ray Telescope, SXT) and Neutron Star Interior Composition Explorer (NICER) in 2017. The source showed two flux levels (high and low), whose energy spectra can be described by the thermal comptonization of disk photons. The spectral parameters obtained by the joint fitting of SXT/LAXPC and NICER/LAXPC were consistent. The power density spectra from LAXPC and NICER revealed a broad, prominent feature at ∼2 Hz. The energy dependence of the fractional rms and time lag of this feature cannot be explained by only variations of coronal spectral parameters. Instead, a model where the coronal heating rate varies first and induces a change in the disk temperature and inner radius can explain the variation. Our results underline the importance of simultaneous observations by AstroSat and NICER and highlight the need for more sophisticated models to explain the spectral-temporal behavior of black hole systems.

Soft X-ray Spectrum Changes over the 35-Day Cycle in Hercules X-1 Observed with AstroSat SXT

Observations of the X-ray binary system Her X-1 by the AstroSat Soft X-ray Telescope (SXT) were carried out in 2020 through 2023 with the goals of measuring X-ray spectrum changes with the 35-day disk precession phase and measuring eclipses at different 35-day phases. Her X-1 exhibits a regular flux modulation with a period of ≃35 days with different intensity levels at various 35-day phases (called “states”). The four multi-day long observations were scheduled to cover most of these states. Each 35-day phase was determined using monitoring observations with the Swift Burst Alert Telescope (BAT). Nine eclipses were observed in the range of 35-day phases, with at least one eclipse during each observation. Data with dips were separated from data without dips. The variation in X-ray spectral parameters vs. 35-day phase shows the following: eclipse parameters are nearly constant, showing that the scattering corona does not change with 35-day phase; dips show an increase in covering fraction but not column density compared to non-dip data; the1 keV line normalization behaves similarly to the powerlaw normalization, consistent with an origin near the powerlaw emission region, likely the magnetospheric accretion flow from the inner disk onto the neutron star; and the blackbody normalization (area) is large (∼3×105 km2) during the Main High and Short High states, consistent with the inner edge of the accretion disk.

The high energy X-ray probe (HEX-P): bringing the cosmic X-ray background into focus

Since the discovery of the cosmic X-ray background (CXB), astronomers have strived to understand the accreting supermassive black holes (SMBHs) contributing to its peak in the 10–40 keV band. Existing soft X-ray telescopes could study this population up to only 10 keV, and, while NuSTAR (focusing on 3–24 keV) made great progress, it also left significant uncertainties in characterizing the hard X-ray population, crucial for calibrating current population synthesis models. This paper presents an in-depth analysis of simulations of two extragalactic surveys (deep and wide) with the High-Energy X-ray Probe (HEX-P), each observed for 2 Ms. Applying established source detection techniques, we show that HEX-P surveys will reach a flux of ∼10−15 erg s−1 cm−2 in the 10–40 keV band, an order of magnitude fainter than current NuSTAR surveys. With the large sample of new hard X-ray detected sources (∼2000), we showcase HEX-P’s ability to resolve more than 80% of the CXB up to 40 keV into individual sources. The expected precision of HEX-P’s resolved background measurement will allow us to distinguish between population synthesis models of SMBH growth. HEX-P will leverage accurate broadband (0.5–40 keV) spectral analysis and the combination of soft and hard X-ray colors to provide obscuration constraints even for the fainter sources, with the overall objective of measuring the Compton-thick fraction. With unprecedented sensitivity in the 10–40 keV band, HEX-P will explore the hard X-ray emission from AGN to flux limits never reached before, thus expanding the parameter space for serendipitous discoveries. Consequently, it is plausible that new models will be needed to capture the population HEX-P will unveil.

Spectral and temporal features of GX 13+1 as revealed by AstroSat

ABSTRACT GX 13+1, a neutron star low-mass X-ray binary that exhibits the properties of both atoll and Z sources, is studied using data from Soft X-ray Telescope and Large Area X-ray Proportional Counter (LAXPC) onboard AstroSat. The source traces a ν shaped track in its hardness-intensity diagram (HID). Spectral modelling of the data in the 0.7–30.0 keV energy range, with the model – $\tt {constant}$ × $\tt {tbabs}$ × $\tt {thcomp}$ × $\tt {bbodyrad}$ + $\tt {relxillNS}$, yields orbital inclination angle (θ) of 77$\substack{+10\\-8}$°. Flux resolved spectral analysis reveals the ν shaped pattern in the plots of spectral parameters kTe, kTbb, and Γ versus Fbol, closely resembling the pattern traced in LAXPC HID. This indicates changes in the spectral properties of the corona and the boundary layer/accretion disc. Assuming that the accretion disc truncates at the Alfvén radius, the upper limit of the magnetic field strength (B) at the poles of neutron star in GX 13+1 is calculated to be 5.10 × 108 G (for kA = 1 and η = 0.1), which is close to that of atoll sources. Furthermore, thickness of the boundary layer is estimated to be 5.70 km, which results in the neutron star radius value of ≲14.50 km. Quasi-periodic oscillations (QPOs) at 56 ± 4 and 54 ± 4 Hz are detected in Regions D and E of HID, respectively. The frequencies of these QPOs are similar to the characteristic frequency of horizontal branch oscillation and these do not exhibit a positive correlation with mass accretion rate.

AstroSat observation of the magnetar SGR J1830−0645 during its first detected X-ray outburst

ABSTRACT We present here timing and spectral analyses of SGR J1830−0645 based on an AstroSat observation carried out on 2020 October 16, about a week after the onset of its first detected X-ray outburst. Using data taken with the Soft X-ray Telescope (SXT) and Large Area X-ray Proportional Counter (LAXPC), we have detected 0.9–10 keV coherent pulsations at a period of ∼10.4 s. The pulse profiles were single-peaked, asymmetric, and consisted of minor peaks attributable to hotspots on the neutron star surface. The pulsed fraction evolved significantly with energy, increasing to energies around 5 keV with a steep drop thereafter. The 0.9–25 keV SXT–LAXPC energy spectrum is best described with two thermal components having temperatures ∼0.46 and ∼1.1 keV (emission radii of ∼2.4 and ∼0.65 km, respectively, assuming a distance of 4 kpc) along with a power-law component having a photon index of ∼0.39. We report the detection of 67 X-ray bursts having an average duration of ∼33 ms. The brightest burst lasted for about 90 ms and had a 3–25 keV fluence of ∼5 × 10−9 erg cm−2.

Design and Development of Cfrp Tubular Structure For Soft X-Ray Telescope Payload of Astrosat

India has launched an astronomical space based observatory ASTROSAT in the year 2015 for unravelling the some of the secrets of our universe. Soft X-ray Telescope (SXT) was one among the payloads onboard for studying the sources of Soft X-rays in deep space. A composite structure was required for this purpose. It had to support the payload elements and maintain their relative locations precisely during launch as well as during its operation, in-orbit. The interface with the spacecraft was at the middle of the SXT structure with one part is inside and remaining part is outside of the spacecraft deck. The structural design was carried out to meet all the structural design requirements such as stability, stiffness and strength with minimum mass under the constraints of spacecraft interface and envelope. This paper presents the details of the structural design, development and qualification of a CFRP tubular SXT structure, which has been functioning satisfactorily in-orbit.

Correlations between QPO frequencies and spectral parameters of GRS 1915+105 using AstroSat observations

ABSTRACT In this work, we study the correlation between quasi-periodic oscillation (QPO) frequency and the spectral parameters during various X-ray states in the black hole binary GRS 1915+105 which matches well with the predicted relativistic dynamic frequency (i.e. the inverse of the sound crossing time) at the truncated radii. We have used broad-band data of Large Area X-ray Proportional Counter and Soft X-ray Telescope instruments onboard AstroSat. Spectral fitting shows that the accretion rate varies from ∼0.1 to ∼5.0 × 1018 gm s−1 and the truncated radius changing from the last stable orbit of an almost maximally spinning black hole, ∼1.2 to ∼19 gravitational radii. For this wide range, the frequencies of the C-type QPO (2–6 Hz) follow the trend predicted by the relativistic dynamical frequency model and interestingly, the high-frequency QPO at ∼70 Hz also follows the same trend, suggesting they originate from the innermost stable circular orbit with the same mechanism as the more commonly observed C-type QPO. While the qualitative trend is as predicted, there are quantitative deviations between the data and the theory, and the possible reasons for these deviations are discussed.

Contrasting X-ray/UV time-lags in Seyfert 1 galaxies NGC 4593 and NGC 7469 usingAstroSatobservations

ABSTRACTWe study accretion disc–corona connection in Seyfert 1 galaxies using simultaneous UV/X-ray observations of NGC 4593 (2016 July 14–18) and NGC 7469 (2017 October 15–19) performed with AstroSat. We use the X-ray (0.5–7.0 keV) data acquired with the Soft X-ray Telescope (SXT) and the UV (FUV: 130–180 nm, NUV: 200–300 nm) data obtained with the Ultra-Violet Imaging Telescope (UVIT). We also use the contemporaneous Swift observations of NGC 4593 and demonstrate AstroSat’s capability for X-ray/UV correlation studies. We performed UV/X-ray cross-correlation analysis using the interpolated and the discrete cross-correlation functions and found similar results. In the case of NGC 4593, we found that the variations in the X-rays lead to those in the FUV and NUV bands by ∼38 ks and ∼44 ks, respectively. These UV lags favour the disc-reprocessing model; they are consistent with the previous results within uncertainties. In contrast, we found an opposite trend in NGC 7469 where the soft X-ray variations lag those in the FUV and NUV bands by ∼41 ks and ∼49 ks, respectively. The hard lags in NGC 7469 favour the thermal Comptonization model. Our results may provide direct observational evidence for the variable intrinsic UV emission from the accretion disk, which acts as the seed for thermal Comptonization in a hot corona in a lamp-post-like geometry. The non-detection of disk reverberation photons in NGC 7469, using AstroSat data, is most likely due to a high accretion rate resulting in a hot accretion disc and large intrinsic emission.

Spectro-temporal and type I X-ray burst analysis of GX 3+1 using AstroSat observations

ABSTRACT GX 3+1, an atoll type neutron star low-mass X-ray binary, was observed four times by Soft X-ray Telescope and The Large Area X-ray Proportional Counter on-board AstroSat between 2017 October 5 and 2018 August 9. The hardness-intensity-diagram of the source showed it to be in the soft spectral state during all the four observations. The spectra of the source could be adequately fit with a model consisting of blackbody ($\tt {bbody}$) and power--law ($\tt {powerlaw}$) components. This yielded the blackbody radius and mass accretion rate to be ∼8 km and ∼2 × 10−9 M⊙ y−1, respectively. In one of the observations, a type I X-ray burst having a rise and e-folding time of 0.6 and 5.6 s, respectively, was detected. Time-resolved spectral analysis of the burst showed that the source underwent a photospheric radius expansion. The radius of the emitting blackbody in GX 3+1 and its distance were estimated to be 9.19 $\substack{+0.97\\ -0.82}$ km and 10.17 $\substack{+0.07\\ -0.18}$ kpc, respectively. Temporal analysis of the burst yielded upper limits of the fractional root mean square amplitude of 7${{\ \rm per\ cent}}$, 5${{\ \rm per\ cent}}$, and 6${{\ \rm per\ cent}}$ during burst start, burst maximum, and right after the radius expansion phase, respectively.

Multiwavelength study of NGC 1365: The obscured active nucleus and off-nuclear compact X-ray sources

ABSTRACTWe present a multiwavelength study of the active nucleus and the off-nuclear X-ray sources in the nearby spiral galaxy, NGC 1365 using three simultaneous UV/X-ray observations by AstroSat over a two months period and archival IR observations performed with Spitzer and Herschel. Utilizing the data from the Soft X-ray Telescope (SXT) onboard AstroSat, we find spectral variability mainly caused by the variation in the X-ray column density, (NH ∼ 1022–1023 cm−2). With the accurate spatial resolution of the UVIT onboard AstroSat, we separate the intrinsic AGN flux from the host galaxy emission and then correct for the Galactic and the internal reddening. We detect no significant variation in the NUV emission over the observation period. The AGN in FUV band is undetectable due to heavy intrinsic extinction. Further, the multiwavelength IR/UV/X-ray AGN SED reveals that the AGN is in a low-luminosity phase with accretion rate ∼0.01 LEdd. The steady UV emission and strong X-ray absorption variability suggest that the obscuring clouds are likely compact and affect the compact X-ray source only and do not possibly cover the extended UV emitting region. In addition, the UVIT is able to resolve two bright spots at a radius of 7 arcsec (∼6.3 Kpc) from the central nucleus in the South-West (SW) direction. In the UVIT image of the entire galaxy, we identify UV counterparts to four Chandra identified bright X-ray sources. One well-known ultra-luminous X-ray source (ULX) NGC 1365 X2 is identified with its UV counterpart at 86 arcsec from the nucleus in the North-East (NE) direction from the active nucleus.

Broad-band mHz QPOs and spectral study of LMC X-4 with AstroSat

ABSTRACT We report the results of broad-band timing and spectral analysis of data from an AstroSat observation of the high-mass X-ray binary LMC X-4. The Large Area X-ray Proportional Counter (LAXPC) and Soft X-ray Telescope (SXT) instruments onboard the AstroSatobserved the source in 2016 August. A complete X-ray eclipse was detected with the LAXPC. The 3–40 keV power density spectrum showed the presence of coherent pulsations along with a ∼26 mHz quasi-periodic oscillation feature. The spectral properties of LMC X-4 were derived from a joint analysis of the SXT and LAXPC spectral data. The 0.5–25 keV persistent spectrum comprised of an absorbed high-energy cut-off power law with photon index of Γ ∼ 0.8 and cut-off at ∼16 keV, a soft thermal component with kTBB ∼ 0.14 keV, and Gaussian components corresponding to Fe Kα, Ne ix, and Ne x emission lines. Assuming a source distance of 50 kpc, we determined 0.5–25 keV luminosity to be ∼2 × 1038 erg s−1.

AstroSat observation of rapid type-I thermonuclear burst from low-mass X-ray binary GX 3$$+$$1

We report the results of an observation of low-mass X-ray binary GX 3 $$+$$ 1 with AstroSat’s large area X-ray proportional counter (LAXPC) and soft X-ray telescope (SXT) instruments on-board for the first time. We have detected type-I thermonuclear burst ( $$\sim $$ 15 s) present in the LAXPC 20 light curve with a double peak feature at higher energies and our study of the hardness–intensity diagram reveals that the source was in a soft banana state. The pre-burst emission could be described well by a thermally Comptonized model component. The burst spectra is modeled adopting a time-resolved spectroscopic method using a single color blackbody model added to the pre-burst model, to monitor the parametric changes as the burst decays. Based on our time-resolved spectroscopy, we claim that the detected burst is a photospheric radius expansion (PRE) burst. During the PRE phase, the blackbody flux is found to be approximately constant at an averaged value of $$\sim $$ 2.56 in $$10^{-8}$$ ergs s $$^{-1}$$ cm $$^{-2}$$ units. On the basis of literature survey, we infer that AstroSat/LAXPC 20 has detected a burst from GX 3 $$+$$ 1 after more than a decade, which is also a PRE one. Utilizing the obtained burst parameters, we provide a new estimation to the source distance, which is $$\sim $$ $$9.3 \pm 0.4$$ kpc, calculated for an isotropic burst emission. Finally, we discuss and compare our findings with the published literature reports.

Time-scale-dependent X-ray to UV time lags of NGC 4593 using high-intensity XMM–Newton observations with Swift and AstroSat

ABSTRACT We present a 140 ks observation of NGC 4593 with XMM–Newton providing simultaneous and continuous pn X-ray and OM UV (UVW1 2910 Å) light curves, which sample short-time-scale variations better than previous observations. These observations were simultaneous with 22 d of Swift X-ray and UV/optical monitoring reported previously, and 4 d of AstroSat Soft X-ray Telescope (SXT), far (FUV 1541 Å) and near (NUV 2632 Å) UV allowing lag measurements between them and the highly-sampled XMM. From the XMM, we find that UVW1 lags behind the X-rays by 29.5 ± 1.3 ks, ∼half the lag previously determined from the Swift monitoring. Re-examination of the Swift data reveals a bimodal lag distribution, with evidence for both the long and short lags. However, if we detrend the Swift light curves by LOWESS filtering with a 5 d width, only the shorter lag (23.8 ± 21.2 ks) remains. The NUV observations, compared to pn and SXT, confirm the ∼30 ks lag found by XMM, and after 4 d filtering is applied to remove the long-time-scale component, the FUV shows a lag of ∼23 ks. The resultant new UVW1, FUV, and NUV lag spectrum extends to the X-ray band without requiring additional X-ray to UV lag offset, which if the UV arises from reprocessing of X-rays implies direct illumination of the reprocessor. By referencing previous Swift and HST lag measurements, we obtain an X-ray to optical lag spectrum, which agrees with a model using the KYNreverb disc-reprocessing code, assuming the accepted mass of 7.63 × 106 M⊙ and a spin approaching maximum. Previously noted lag contribution from the BLR in the Balmer and Paschen continua are still prominent.

Anticorrelated lags in a neutron star Z source GX 5-1: AstroSat’s View

ABSTRACT We report the cross-correlation function studies of a neutron star low-mass X-ray binary, a Z source GX 5-1, using Soft X-ray Telescope (SXT) and Large Area X-ray Proportional Counter (LAXPC) energy bands onboard AstroSat. For the first time, we report the lag between soft (0.8–2.0 keV, SXT) and hard X-ray energy bands (10–20 and 16–40 keV, LAXPC) in GX 5-1 and detected lags of the order of a few tens to hundreds of seconds in the horizontal branch. We interpreted them as the readjustment time-scale of the inner region of the accretion disc. We used various two components and three-component spectral models to unfold the spectra and observed the changes in soft and hard component fluxes that were exhibiting horizontal branch oscillation variations. It was observed that the bbody component assumed to be originating from the boundary layer over the NS and was also found to vary along with the HBO variation where lags were detected. We constrained the size of the comptonizing region of the order 15–55 km, assuming that lags were due to variation in the size of the corona. We noticed a similar size of the comptonizing region after employing other models and suggest that the overall size of corona must be of the order of a few tens of km to explain the lags, HBO variation, and respective spectral variations. In a case study, it was noted that the BL size increases as GX 5-1 vary from the top of the HB to the upper vertex.

The cusp plasma imaging detector (CuPID) cubesat observatory: Instrumentation.

The Cusp Plasma Imaging Detector (CuPID) CubeSat observatory is a 6U CubeSat designed to observe solar wind charge exchange in magnetospheric cusps to test competing theories of magnetic reconnection at the Earth's magnetopause. The CuPID is equipped with three instruments, namely, a wide field-of-view (4.6° × 4.6°) soft x-ray telescope, a micro-dosimeter suite, and an engineering magnetometer optimized for the science operation. The instrument suite has been tested and calibrated in relevant environments, demonstrating successful design. The testing and calibration of these instruments produced metrics and coefficients that will be used to create the CuPID mission's data product.

Probing spectral and temporal evolution of the neutron star low-mass X-ray binary 4U 1724–30 with AstroSat

ABSTRACT We report the broad-band spectro-temporal study of the poorly studied accreting neutron star (NS) low mass X-ray binary (LMXB) 4U 1724–30 using data from Soft X-ray Telescope (SXT) and Large Area X-ray Proportional Counters (LAXPC) instruments on board AstroSat. The dim persistent LMXB source was observed with AstroSat over 4 epochs in 2017, all of which corresponded to a low-luminosity non-thermal emission dominated (hard/island) emission state with modest spectral evolution. All the X-ray broad-band spectra can be modelled by a combination of thermal emission from the NS boundary layer (BL) or NS surface and a non-thermal emission component possibly originating from the inverse Comptonization of the disc seed photons. We investigate the presence of frequency and energy-dependent variabilities to probe the origin of the disc/coronal fluctuations. We also report the detection of a Type-I X-ray burst displaying a photospheric radius expansion (PRE). During the burst, a hard X-ray shortage in the 30–80 keV energy band and the enhancement of the persistent emission reveal the burst feedback on the overall accretion process. Using the touch-down burst flux ∼4.25 × 10−8 erg s−1 cm−2, the distance of the source is estimated as ∼8.4 kpc.

Detection of low-frequency breaks in power density spectrum of GX 339−4 in faint low/hard state observations using AstroSat data

ABSTRACT We present the spectro-timing analysis of the black hole binary system GX 339−4 using AstroSat data extracted at the beginning of outbursts in 2017 and 2019. The joint spectral fitting of Large Area X-ray Proportional Counter (LAXPC) and Soft X-ray Telescope (SXT) data revealed that the source was in a faint low/hard state for both 2017 and 2019 observations, with nearly equal photon indexes of ∼1.57 and ∼1.58 and Eddington ratios, L/LEdd, of 0.0011 and 0.0046, respectively. The addition of a reflection component into the spectral modelling improved the fit (Δχ2 ≈ 6 for 2017 and Δχ2 ≈ 7 for 2019), pointing towards the presence of weak reflection features arising due to irradiation of the accretion disc. The power density spectrum consisted of strong band-limited noise with a break at low frequencies, described by a combination of a few zero-centred Lorentzians. The fitting revealed a low-frequency break at ∼6 mHz for 2017 and ∼11 mHz for 2019 observations, whose detection is validated by results from independent detectors (LAXPCs and SXT). The break frequency is roughly consistent with results obtained from earlier observations that showed an evolution of the frequency with flux, which is in accordance with the truncated disc model. Associating the break frequency with the viscous time-scale of the accretion disc, we estimated a truncation radius of ∼93 and ∼61 gravitational radius for 2017 and 2019 observations, respectively.

Quest for the Upcoming Periastron Passage of an Episodic Dust Maker and Particle-accelerating Colliding-wind Binary: WR 125

Abstract We have carried out a long-term infrared and X-ray investigation of the colliding-wind binary WR 125 (WC7 + O9III). The source was monitored using AstroSat Soft X-ray Telescope and the Tata Institute of Fundamental Research Near Infrared Imaging Camera-II mounted at the back of 3.6 m Devasthal Optical Telescope. WR 125 appeared brighter in the near-infrared K-band during the years 2017–2021 which is attributed to another episode of dust formation similar to the one reported during the likely periastron passage at the beginning of the 1990s. This is further supported by enhanced emission observed in the W1 and W2 bands of Wide-field Infrared Survey Explorer from 2018–2019. By combining archival X-ray data sets with our new measurements, long-term variations have been noticed. The source reaches a lower emission state in 2020 June (close to the recent infrared maximum) which could be due to enhanced absorption of X-rays produced in the colliding-wind region by the WC stellar wind close to the periastron in an eccentric orbit. The time interval between the previous and latest X-ray low states may indicate an orbital period of 28–29 years, in fair agreement with the recurrence time of episodic dust production. We also discuss published radio measurements in the context of a common picture based on a long-period binary scenario. These results allow us to draw relevant guidelines for future multiwavelength observations of WR 125.

AstroSat observations of long-duration X-ray superflares on active M-dwarf binary EQ Peg

ABSTRACT We present a comprehensive study of three large long-duration flares detected on an active M-dwarf binary EQ Peg using the Soft X-Ray Telescope of the AstroSat observatory. The peak X-ray luminosities of the flares in the 0.3–7-keV band are found to be within ∼5–10 × 1030$\rm {erg}~\rm {s}^{-1}$. The e-folding rise- and decay-times of the flares are derived to be in the range of 3.4–11 and 1.6–24 ks, respectively. Spectral analysis indicates the presence of three temperature corona with the first two plasma temperatures remain constant during all the flares and the post-flare observation at ∼3 and ∼9 MK. The flare temperature peaked at 26, 16, and 17 MK, which are 2, 1.3, and 1.4 times more than the minimum value, respectively. The peak emission measures are found to be 3.9–7.1 × 1053 cm−3, whereas the abundances peaked at 0.16–0.26 times the solar abundances. Using quasi-static loop modelling, we derive loop lengths for all the flares as 2.5 ± 0.5 × 1011, 2.0 ± 0.5 × 1011, and 2.5 ± 0.9 × 1011 cm, respectively. The density of the flaring plasma is estimated to be 4.2 ± 0.8 × 1010, 3.0 ± 0.7 × 1010, 2.2 ± 0.8 × 1010 cm−3 for flares F1, F2, and F3, respectively. Whereas the magnetic field for all three flares is estimated to be <100 G. The estimated energies of all three flares are ≳ 1034–1035erg, putting them in a category of superflare. All three superflares are also found to be the longest duration flares ever observed on EQ Peg.

Broad-band time-resolved spectroscopy of thermonuclear X-ray bursts from 4U 1636−536 using AstroSat

ABSTRACT Broad-band spectral studies of type I X-ray bursts can put strong constraints on the physics of burst spectra as well as their interaction with the environment. We present the results obtained from the broad-band time-resolved spectroscopy of 15 thermonuclear bursts detected simultaneously from the neutron star atoll source 4U 1636−536 using the Large Area X-ray Proportional Counter (LAXPC) and Soft X-ray Telescope (SXT) onboard AstroSat. During the observations with AstroSat, the low-mass X-ray binary 4U 1636−536 is observed to show a modest spectral evolution within the island state. The broad-band burst spectra are observed to show an excess in addition to the thermal emission from the neutron star surface near the peak of the bursts. We investigate the interpretation of the excess observed near the peak of the burst as re-emission/reprocessing of the photons by the accretion disc/corona or scattering of the photons in the neutron star atmosphere or the enhanced persistent emission due to the Poynting–Robertson drag. This is the first reported broad-band simultaneous study of type I bursts using LAXPC and SXT onboard AstroSat. This kind of study may provide a better understanding of the burst–accretion interaction and how the bursts influence the overall accretion process contributed by the accretion disc as well as the corona.