arXiv subjects

Sachindra Naik

Publications and source records attributed to Sachindra Naik.

At least 19 recordsLinked to original sources

arXiv2026

Multiwavelength View of Black Hole X-ray Binary Swift J151857.0-572147

We investigate multiwavelength timing and spectral properties of the newly discovered black hole X-ray binary Swift J151857.0-572147 during its 2024 outburst using observations from AstroSat, NuSTAR, NICER, Swift, and MeerKAT. The AstroSat/LAXPC20 power density spectrum reveals a $\sim$8 Hz low-frequency QPO with a quality factor of $\sim2.8$. The broadband X-ray spectrum of the source exhibits prominent reflection features, which are modeled to constrain the geometry and physical properties of the accretion disk. The spectral analysis indicates that the source was in the soft/intermediate spectral state, characterized by a steep power law with a photon index of $\Gamma \approx 2.6$ and a relatively high disk temperature of $\sim0.9$ keV. The reflection modeling suggests a disk inclination of $23-31^{\circ}$ and a highly ionized accretion disk with $\log\xi \sim 3.3$-4.0. The MeerKAT monitoring revealed a strong radio variability that closely tracked the hard X-ray evolution, including a bright radio flare during the hard-to-soft state transition, supporting close coupling between the accretion flow and jet activity. Using the quasi-simultaneous AstroSat, Swift/XRT and MeerKAT observations, we investigated the radio/X-ray correlation and found that Swift J151857.0-572147 follows the established $L_{\rm X}$-$L_{\rm R}$ relation for Galactic black hole X-ray binaries. Its location in the $L_{\rm X}$-$L_{\rm R}$ plane is consistent with that of Galactic black hole X-ray binaries, further supporting its black hole nature.

astro-ph.HE↗

arXiv2026

Detection of possible burst oscillation in the neutron star low-mass X-ray binary 4U 1323-62

Burst oscillations observed during thermonuclear X-ray bursts arise from asymmetric brightness patterns on the neutron star surface and provide a direct probe of the neutron star spin frequency. We present a detailed timing analysis of the neutron star low-mass X-ray binary 4U 1323-62 using 2024 observations with XMM-Newton and NuSTAR observatories. We identify nine thermonuclear X-ray bursts in the XMM-Newton/EPIC-pn data, along with eclipsing dips in the light curve. One of the XMM-Newton bursts exhibits a rare doublet structure. In addition, NuSTAR detects six bursts, four of which occur simultaneously with those observed with XMM-Newton. We identify a possible burst oscillation signal at $\sim$611.5 Hz in the XMM-Newton data. The strongest oscillation, detected during the primary burst of the doublet burst, reaches a maximum $Z_1^{2}$ power of $\sim35$. An analytical estimate accounting for the searched frequency and time intervals gives a significance of $\sim3.0\sigma$, whereas independent Monte Carlo simulations yield a more robust global significance of only $\sim2.4\sigma$. We therefore interpret the signal as a tentative detection of burst oscillation. The folded pulse profile in the 0.5-10 keV band is well described by a sinusoid, with a fractional rms amplitude of $\sim30\pm6$%. The oscillation frequency corresponds to a neutron-star spin period of $\sim$1.635 ms, suggesting that 4U 1323-62 may harbor a rapidly rotating millisecond neutron star.

astro-ph.HE↗

arXiv2026

Temporal Variation of the Coronal Radius Parameter in a Jetted Tidal Disruption Event: Swift J1644+57

Tidal Disruption Events are exotic astrophysical phenomena where matter from a star or the interstellar medium is captured by a supermassive black hole. The process liberates enormous energy, within a few months to a year timescale, enough to detect dormant black holes in near as well as the farthest galaxies. We revisit the long-term spectral variabilities associated with the jetted Tidal Disruption Event \source~by exploring the archival X-ray data obtained with Swift/XRT and XMM-Newton observatories. Our analysis reveals that the spectral indices decrease non-monotonically as \source~evolves with time. We also find that the soft (0.3-1.5 keV) and hard (1.5-10 keV) X-ray photon counts are highly correlated with a maximum correlation coefficient of 0.95 and peak at {\it zero} lag. Moreover, the soft and hard band variabilities obtained from XMM-Newton observations are highly correlated with a Pearson cross-correlation coefficient of 0.96. This indicates that the soft and hard X-ray photons are emitted from the same site, which is most likely a Compton cloud, i.e., the corona. Assuming the hard X-ray photons originate from the corona, we find that the coronal parameter undergoes rapid expansion during the early phases when accompanied by a relativistic jet launching and subsequently evolves toward a state of saturation with minor fluctuations in the latter stages. The temporal variation of the coronal radius parameter ($R_{cor}$) is consistent with a simple theoretical conjecture. We also discuss the application of our analytical outcomes to other jetted and non-jetted tidal disruption events.

astro-ph.HE↗

arXiv2026

Probing burst-disc interaction and disc reflection in SAX J1808.4$-$3658 with NICER, XMM-Newton, and NuSTAR

We performed a comprehensive study of thermonuclear bursts from the millisecond X-ray pulsar SAX J1808.4$-$3658 with XMM-Newton and NICER. We report the results from the analysis of an intense burst with NICER using a self-consistent and physically motivated disc reflection modeling approach and investigate the burst-disc interaction. The dynamic evolution of the spectral parameters suggested evidence of photospheric radius expansion (PRE) of the neutron star using the disc reflection modeling approach, which indicates a maximum expansion of the photosphere up to 14.8$\pm$0.7 km. The corresponding blackbody temperature drops to a minimum of 1.9 keV. In addition, an emission line at 1 keV is observed, possibly originating from the Ne or Fe L-band transition as a result of the reprocessing of burst photons by cold gas in the accretion disc. The 1 keV emission line flux is found to be strongly correlated with the flux of the reflection component. We also investigated a thermonuclear burst observed with XMM-Newton EPIC-PN from SAX J1808.4$-$3658 using the variable persistent emission method and the disc reflection modeling approach. The X-ray reflection feature is also investigated in persistent emission using a NuSTAR observation. The best-fitting results provide an inner disc radius of $14_{-5.9}^{+9.7}$ $R_g$ and an inclination of $ 38^\circ-60^\circ$ during the NuSTAR observation. The magnetic field is estimated to be $\simeq$3.7 $\times$10$^8$ G at the poles of the neutron star.

astro-ph.HE↗

arXiv2026

Accretion dynamics and coronal geometry in Mrk 530: Insights from 24 years of X-ray monitoring

We present a long-term broadband study of the Seyfert galaxy Mrk~530 spanning $\sim$24 yr (2001--2024). The source remains largely stable across epochs, except in 2018, when a possible quasi-periodic oscillation is observed simultaneously in the UV and X-ray bands, with characteristic timescales of $\sim$90 and $\sim$60 days, characterized by low coherence. Time-resolved spectral analysis shows that this epoch is characterized by comparable coronal cooling and compressional heating timescales, a condition conducive to oscillatory behavior in the inner accretion flow. Other epochs exhibit a clear mismatch between these timescales, and no such variability is observed. The X-ray spectral properties display significant long-term evolution. The photon index and luminosity vary systematically, while a soft excess is present only in early epochs (2001--2006) and weakens thereafter, consistent with an evolving warm corona. Physically motivated models indicate that changes in the accretion rate regulate both the spectral slope and coronal geometry, with higher disc accretion rates producing enhanced cooling, a more compact corona, and softer spectra, and lower rates yielding an expanded hot flow and harder emission. These results suggest that accretion-driven coupling between the disc and corona governs both the long-term spectral evolution and transient short-timescale variability in Mrk~530.

astro-ph.HE↗

arXiv2026

A multi-wavelength study of nearby starburst galaxy M 82

We present a multi-wavelength study of the nearby starburst galaxy M 82 by combining high-resolution Far-ultraviolet (FUV) imaging from the Ultra-Violet Imaging Telescope (UVIT) onboard AstroSat and archival Chandra X-ray observations. Using FUV flux measurements, we estimate a spatially-resolved star formation rate (SFR) across several star-forming clumps within a radius of $\sim$3.6 kpc, finding a total SFR of 0.022 M$_{\odot}$ yr$^{-1}$. The H$_{\alpha}$ recombination line flux yields an SFR of $\sim$0.010 M$_{\odot}$ yr$^{-1}$, while the infrared-based SFR derived from 24 $\mu\mathrm{m}$ emission is significantly higher at 16 - 18 M$_{\odot}$ yr$^{-1}$, suggesting that a substantial fraction of star formation in M 82 is heavily dust-obscured. Morphological comparison of FUV, H$_{\alpha}$, mid-infrared, and soft X-ray emission reveals a strong spatial correlation, tracing multi-phase outflows along the galaxy's minor axis. X-ray spectral analysis using a three-temperature $\texttt{VAPEC}$ model shows enhanced abundances of Ne, Mg, Si, and S, consistent with enrichment from Type-II supernovae. These results demonstrate the importance of combining UV, optical, IR, and X-ray observations to probe both obscured and unobscured star formation, the metal enrichment, and the outflow-driven evolution of starburst galaxies.

astro-ph.GA↗

arXiv2026

Discovery of burst oscillations in the newly discovered millisecond X-ray pulsar SRGA J144459.2$-$604207

Burst oscillations during thermonuclear X-ray bursts are powered by thermonuclear energy on the neutron star (NS) surface and typically occur close to the spin frequency of the NS. We performed a comprehensive timing analysis of all thermonuclear bursts from the newly discovered millisecond X-ray pulsar SRGA J144459.2$-$604207, observed with NICER, XMM-Newton, and NuSTAR during the 2024 outburst. A total of 39 bursts were detected, allowing for a detailed search for burst oscillations, which had not been previously observed from this source. We report the discovery of burst oscillations at 447.7$-$448.0 Hz from SRGA J144459.2$-$604207 using XMM-Newton and NuSTAR data, consistent with the spin frequency of the NS. The strongest burst oscillation in the XMM-Newton data occurred with a single-trial significance of $5.1σ$ and maximum $Z^2$ power of $\sim31$. In the NuSTAR data, the strongest oscillation signal has a significance of $5.2σ$ and maximum $Z^2$ power of $\sim32$. The folded pulse profile corresponding to the strongest signal in the 0.5-10 keV band of the XMM-Newton data shows a sinusoidal shape with a fractional rms amplitude of $\sim8.5\%$, while the measurements of the NuSTAR data (3-40 keV range) yield $\sim21\%$. These results represent the first detection of burst oscillations in SRGA J144459.2$-$604207. Additionally, we report the detection of 447.6 Hz oscillations occurring just before a burst onset observed with XMM-Newton. This marks only the second instance in which burst oscillations have been observed before the burst onset.

astro-ph.HE↗

arXiv2025

Evolution of Accretion Properties in Mrk 1040 using long-term X-ray Observations

We present a comprehensive long-term, multi-epoch spectral and timing study of the Seyfert 1 Active Galactic Nucleus (AGN) Mrk~1040, utilizing X-ray observations spanning from 2009 to 2024 ($\sim$15 years). The source exhibits pronounced spectral and temporal variability, indicative of transitions between different accretion regimes in the vicinity of the central supermassive black hole. The earlier reported soft excess is re-examined within a uniform, physically motivated multi-epoch framework. We confirm the presence of this soft excess in the 2009 observation, where it is well described by a warm, extended Comptonizing corona with $kT_{\rm e,warm} \sim 0.26$~keV and a radial extent of $R_{\rm warm} \sim 30~r_g$. In subsequent epochs, the soft excess is not statistically significant, possibly due to a combination of enhanced ionized absorption, intrinsic weakening of the warm Comptonizing region, or partial truncation of the inner disc. A strong correlation between the soft and hard X-ray fluxes suggests a common physical origin for both components, likely within a multi-layered Comptonizing structure that evolved into a compact and thermally stable corona after 2013. The observed spectral variability, together with changes in the Fe~K$α$ line strength, reflects the evolving coronal geometry and accretion flow dynamics. Variations in the intrinsic column density ($N_H$) further indicate that Mrk~1040 is embedded within a clumpy, dynamically variable absorber responding to changes in the accretion rate. Using the TCAF model, we estimate the black hole mass as $M_{\rm BH} = (4.50 \pm 1.62) \times 10^7~M_\odot$, consistent with previous estimates.

astro-ph.HE↗

arXiv2025

Discovery of Changing-Look behavior in AGN NGC 3822: A long-term multiwavelength study

We present a comprehensive long-term multi-wavelength study of the active galactic nucleus (AGN) NGC 3822, based on 17 years (2008 to 2025) of X-ray, ultraviolet (UV), and optical observations.The dataset includes observations from Swift, XMM-Newton, and NuSTAR, the Very Large Telescope, and the Himalayan Chandra Telescope. Our multiwavelength light curve analysis reveals flux variations across X-ray to optical/UV bands, with an increased variability amplitude at shorter wavelengths. X-ray spectral analysis indicates the presence of intrinsic absorption during the 2016 and 2022 observations; however, this absorption disappeared before and after these epochs. The presence and absence of the absorber are attributed to clouds moving in and out of the line of sight. During the long-term monitoring period, the bolometric luminosity of the source varies between ($1.32-17)\times10^{43}$ erg s$^{-1}$. Optical spectroscopic monitoring reveals changing-look (CL) behaviour in NGC~3822, characterized by the appearance and disappearance of broad emission lines (BELs). These CL transitions are associated with changes in the Eddington ratio rather than changes in the obscuration. The BELs appear only when the Eddington ratio is relatively high ($\sim 3.8\times10^{-3}$) and disappear when it drops to a lower value ($\sim 0.9\times10^{-3}$).

astro-ph.HE↗

arXiv2025

Photospheric radius expansion thermonuclear burst and X-ray reflection from the neutron star X-ray binary 4U 1702-429

We perform a comprehensive study of thermonuclear bursts from the neutron star low-mass X-ray binary 4U 1702-429 detected with NICER and XMM-Newton. The thermonuclear burst detected with NICER shows clear evidence of a photospheric radius expansion (PRE) event and a distinct feature in the burst profile. The burst profiles demonstrate significant energy dependence, with the hardness ratio varying notably during the PRE phase. The radius of the neutron star photosphere expanded to a maximum of $23.1_{-3.2}^{+3.8}$ km while its temperature reached a minimum of 1.4 keV. The time-resolved burst spectra can be modeled using variable persistent emission method, indicating that the soft excess may arise from enhanced mass accretion onto the neutron star, potentially due to the Poynting-Robertson drag. Alternatively, the disk reflection model can be used to explain the soft excess emission during a burst. The time-resolved spectral study is performed for three thermonuclear bursts detected with XMM-Newton. The XMM-Newton time-resolved burst spectra can be modeled using an absorbed blackbody model, without any signatures of the PRE. We conduct a detailed spectral analysis of the 2025 NuSTAR observation of 4U 1702-429, revealing a broad iron line at 6.4 keV and a Compton hump around 20 keV, indicating X-ray reflection features. The disk reflection model relxill provides an inner disk radius of 12 $R_g$ and an inclination angle of $\sim39^{\circ}$. The magnetic field strength at the pole of the neutron star is estimated to be 5.1 $\times10^8$ G, assuming that the accretion disk is truncated at magnetosphere boundary.

astro-ph.HE↗

arXiv2025

Relativistic X-ray reflection and thermonuclear burst from accreting millisecond X-ray pulsar SRGA J144459.2-604207

We present the results obtained from the spectral and temporal study of thermonuclear bursts from the millisecond X-ray pulsar SRGA J144459.2-604207 detected with NICER. The dynamic evolution of the spectral parameters in a broad energy range is also investigated during a simultaneously detected burst with XMM-Newton and NuSTAR. The burst profiles exhibit a strong energy dependence, as observed with XMM-Newton, NICER, and NuSTAR. We investigated the reflection feature during these bursts using the disk reflection model. As observed during the peak of the NICER bursts, the reflection model can contribute 30 per cent of the overall emission. During the NICER bursts, a correlation is observed between the flux of the blackbody and the reflection components. The measurements of the mass accretion rate indicate that the bursts may be powered by a mixed H/He fuel. Moreover, the broadband NICER and NuSTAR spectra are also used to probe the reflection signature in the burst-free persistent region using the relativistic reflection model. Based on the variability of the count rate during the NuSTAR observation, we also investigate the evolution of spectral parameters during two different flux levels of the NuSTAR observation. The inner disk radius (Rin) and the angle of inclination are found to be nearly 11 Rg and 50 degrees, respectively. The magnetic field strength at the poles of the neutron star is estimated to be 6 x 10^8 G, assuming that the inner disk is truncated at the magnetospheric boundary.

astro-ph.HE↗

arXiv2025

A long-term study of Mrk 50 : Appearance and disappearance of soft excess

We present an extensive temporal and spectral study of the Seyfert 1 AGN Mrk 50 using 15 years (2007-2022) of multiwavelength observations from XMM-Newton, Swift, and NuSTAR for the first time. From the timing analysis, we found that the source exhibited variability of $\sim$20 % during the 2007 observation, which reduced to below 10 % in the subsequent observations and became non-variable in the observations from 2010 onward. From the spectral study, we found that the spectra are nearly featureless. Non-detection of absorption in the low-energy domain during the 15 years of observation infers the absence of obscuration around the central engine, rendering the nucleus a `bare' type. A prominent soft X-ray excess below 2 keV was detected in the source spectrum during the observations between 2007 and 2010, which vanished during the later observations. To describe the nature of the soft excess, we use two physical models, such as warm Comptonization and blurred reflection from the ionized accretion disk. Both the physical models explain the nature and origin of the soft excess in this source. Our analysis found that Mrk~50 accretes at sub-Eddington accretion rate ($λ_{Edd}=0.13-0.02$) during all the observations used in this work.

astro-ph.HE↗

arXiv2025

Broadband study of the SMC pulsar RX J0032.9-7348 during its X-ray brightening in 2024

We present the results of the broadband timing and spectral analysis of the poorly understood SMC pulsar RX J0032.9-7348 (= SXP 7.02) using NuSTAR and NICER observations during its X-ray brightening in 2024. Our timing analysis revealed a pulsation period of approximately 7.02 s in the X-ray light curve. The pulse profile obtained in the broad energy range is double-peaked and asymmetric in nature and shows moderate variation with the energy. An absorbed power-law model describes the 0.5-8 keV NICER spectra well. The 3-50 keV NuSTAR spectrum is best described with an absorbed power-law modified with a high-energy cutoff model. We find no evidence of iron or cyclotron line features in the energy spectrum. During our observation period, the 0.5-50 keV luminosity varies in the range of $\sim 8\times10^{36} - 4\times10^{37}$ erg s$^{-1}$. We also discuss the dependence of spectral parameters on the rotational phase of the pulsar through phase-resolved spectroscopy.

astro-ph.HE↗

arXiv2025

Probing thermonuclear bursts and X-ray reflection features in Aql X-1 during 2024 outburst

We report the broadband timing and spectral properties of the neutron star low-mass X-ray binary Aql X-1 during the 2024 outburst with NICER, NuSTAR, and Swift observatories. We detected six thermonuclear X-ray bursts during the NICER and NuSTAR observations, with the observed X-ray burst profiles exhibiting a strong energy dependence. The time-resolved burst spectra indicate the presence of soft excess during the burst, which can be modeled by using a variable persistent emission method ($f_a$ method), or the relxillNS reflection model. We found that the reflection model can contribute $\sim$20% of total emission as observed during the NICER burst. The reflection and blackbody component fluxes are strongly correlated as observed during a burst. The excess emission is possible due to the enhanced mass accretion rate to the neutron star due to the Poynting-Rodertson drag and a fraction of burst emission may be reflected from the disk. The bursts did not show photospheric radius expansion during the peak. Moreover, we examined the burst-free accretion emission in the broadband range with NuSTAR, NICER, and Swift at two epochs of the outburst. The persistent emission showed X-ray reflection feature, which can be well modeled with the relativistic reflection model relxillCp. The inner disk radius (R$_{in}$) is found to be nearly 22 and 10 times $\rm R_{g}$ for two observations, respectively. Assuming that the inner disk is truncated at the magnetospheric radius, the magnetic field strength at the poles of the neutron star is estimated to be $(0.6-1.9) \times 10^9$ G.

astro-ph.HE↗

arXiv2024

Optical and X-ray Studies of the Be/X-ray Binary IGR J06074+2205

We present the results obtained from X-ray and optical analysis of the Be/X-ray binary IGR~J06074+2205, focusing on before, during, and after the X-ray outbursts in October and December 2023. The properties of the neutron star in the binary are investigated using NICER and NuSTAR observations during the X-ray outbursts. The pulse profiles across a broad energy range, are found to be strongly dependent on luminosity and energy, revealing the complex nature of the emitting region. An absorbed power-law can describe each NICER spectrum in the 1-7 keV band. The 3-79 keV NuSTAR spectrum can be well-described by a negative and positive power-law with an exponential cut-off model. Utilizing the MAXI/GSC long-term light curve, we estimate the probable orbital period to be 80 or 80/n (n=2,3,4) days. We investigate the evolution of the circumstellar disc around the Be star by using optical spectroscopic observations of the system between 2022 and 2024. We observe variable H$α$ and FeII emission lines with an increase in equivalent width, indicating the presence of a dynamic circumstellar disc. A distinct variation in the V/R value for H$α$ and FeII lines is also observed. The appearance of additional emission lines, such as HeI (5875.72 Å), HeI (6678 Å), and HeI (7065 Å), during the post-outburst observation in February 2024 suggests the growing of a larger or denser circumstellar disc. The disc continues to grow without any noticeable mass loss, even during the 2023 X-ray outbursts, which may lead to a future giant X-ray outburst.

astro-ph.HE↗

arXiv2024

Optical spectroscopy of 1A 0535+262 before, during, and after the 2020 giant X-ray outburst

We present the findings from our study of the Be/X-ray binary 1A 0535+262/HD 245770 during the giant X-ray outburst in October 2020. We utilized the 1.2-m telescope at Mount Abu Infrared observatory for optical observations of the Be companion star. The outburst reached a peak X-ray flux of approximately 11 Crab in the 15-50 keV range, marking the highest ever recorded X-ray outburst from the pulsar. We conducted optical observations in the 6000-7200 angstrom range before, during, and after the X-ray outburst, aiming to examine the evolution of the circumstellar disc of the Be star from February 2020 to February 2022. Our optical spectra displayed prominent emission lines at 6563 angstrom (H I), 6678 angstrom (He I), and 7065 angstrom (He I). Notably, the H$α$ line exhibited significant variability in the spectra. Prior to and during the outburst, the line profiles appeared single-peaked, and asymmetric with broad red and blue wings, respectively. However, post-outburst observations revealed a double-peaked profile with asymmetry in the blue wing. Our pre-outburst observations confirmed a larger Be circumstellar disc that diminished in size as the outburst progressed. Additionally, the observed variations in the H$α$ line profile and parameters indicate the presence of a highly misaligned, precessing, and warped Be disc.

astro-ph.HE↗

arXiv2024

Long-term X-ray temporal and spectral study of a Seyfert galaxy Mrk 6

We present a long-term X-ray study of a nearby Active Galactic Nucleus Mrk 6, utilizing observations from XMM-Newton, Suzaku, Swift and NuSTAR observatories, spanning 22 years from 2001 to 2022. From timing analysis, we estimated variance, normalized variance, and fractional rms amplitude in different energy bands.The temporal study shows fractional rms amplitude ($F_{\rm var}$) below $10\%$ for the shorter timescale $(\sim60 ~\rm ks)$ and above $20\%$ for the longer timescale ($\sim \rm weeks$ ). A complex correlation is observed between the soft $(0.5-3.0$ keV) and hard $(3.0-10.0$ keV) X-ray bands of different epochs of observations. This result prompts a detailed investigation through spectral analysis, employing various phenomenological and physical models on the X-ray spectra. Our analysis reveals a heterogeneous structure of the obscuring material surrounding Mrk 6. A partially ionized absorber exhibits a rapid change in location and extends up to the narrow line regions or torus. In contrast, another component, located far from the central engine, remained relatively stable. During the observation period, the source luminosity in the 3.0--10.0 keV range varies between $(3-15) \times10^{42}$ erg $\rm sec^{-1}$.

astro-ph.HE↗

arXiv2024

Long-term study of the first Galactic ultraluminous X-ray source Swift J0243.6+6124 using NICER

We present the results obtained from detailed X-ray timing and spectral studies of X-ray pulsar Swift J0243.6+6124 during its giant and normal X-ray outbursts between 2017 and 2023 observed by the Neutron star Interior Composition Explorer (NICER). We focused on the timing analysis of the normal outbursts. A distinct break is found in the power density spectra of the source. The corresponding break frequency and slope of power-laws around the break vary with luminosity, indicating the change in accretion dynamics with mass accretion rate. Interestingly, we detected quasi-periodic oscillations within a specific luminosity range, providing further insights into the underlying physical processes. We also studied the neutron star spin period evolution and a luminosity variation in pulse profile during the recent 2023 outburst. The spectral analysis was conducted comprehensively for the giant and all other normal outbursts. We identified a double transition at luminosities of $\approx$7.5$\times$10$^{37}$ and 2.1$\times$10$^{38}$ erg s$^{-1}$ in the evolution of continuum parameters like photon index and cutoff energy with luminosity. This indicates three distinct accretion modes experienced by the source mainly during the giant X-ray outburst. A soft blackbody component with a temperature of 0.08-0.7 keV is also detected in spectra. The observed temperature undergoes a discontinuous transition when the pulsar evolves from a sub- to super-Eddington state. Notably, in addition to an evolving 6-7 keV iron line complex, a 1 keV emission line was observed during the super-Eddington state of the source, implying the X-ray reflection from the accretion disc or outflow material.

astro-ph.HE↗