arXiv subjects

Gulab C. Dewangan

Publications and source records attributed to Gulab C. Dewangan.

At least 19 recordsLinked to original sources

arXiv2026

The first comprehensive spectral and timing study of the ultra-compact X-ray binary 4U 1812-12 with NICER and NuSTAR

The source 4U 1812-12 is a persistent, weakly variable low-mass X-ray binary containing a neutron star. The source was observed by NICER between 2019 and 2021 and, more recently, by NuSTAR in 2025. During the NICER and NuSTAR observations, the source was detected in a hard spectral state with a bolometric luminosity of $\sim 1.90\times 10^{36}$ ergs s$^{-1}$. Its $3-70$ keV NuSTAR spectrum is characterized by a soft thermal emission from the disc, a hard Comptonized emission from the corona, and its reflection from the accretion disc. The NuSTAR energy spectrum exhibits the clear presence of disc reflection features, fitted using a self-consistent relativistic reflection model {\tt relxill}. Our reflection modeling indicates a moderately ionized accretion disc (log\:$ξ\sim2.72$) extending close to the neutron star surface ($R_{in}\lesssim 1.72\:R_{ISCO}$), and viewed through a small inclination angle ($i\sim 25$ degrees). Assuming that the magnetic field ($B$) truncates the disc, we found $B\lesssim 2.54\times 10^{8}$ G, comparable to the typical values observed for NS LMXBs. The $1.0-9.5$ keV NICER spectra are also characterized by a soft thermal component and a dominant hard Comptonized component. During NICER observations, the disc temperature exhibits a small variation within $\sim 0.69-0.84$ keV. In contrast, the power law photon index, $Γ$, exhibits a large variation of $\sim 0.8-1.5$, implying a substantial change in the Comptonized emission. Moreover, NICER timing analysis reveals broadband aperiodic variability with significant QPO-like features at $0.379\pm 0.008$ Hz and $0.724\pm 0.025$ Hz, having fractional rms amplitudes of $2.9\pm 0.6\%$ and $4.1\pm 0.5\%$, respectively.

astro-ph.HE↗

arXiv2026

On the assessment of the disk truncation and detection of type-II bursts from the accreting millisecond X-ray Pulsar IGR J17062-6143

We present a spectral analysis of the NuSTAR and NICER observations of the accreting millisecond X-ray pulsar IGR J17062-6143, performed in 2022. The source remained in the hard spectral state during the observations, with a luminosity of about 0.2-1.3$\%$ of the Eddington luminosity. The continuum emission of the NuSTAR spectrum is entirely dominated by a power-law component or by Comptonized emission of disk photons by a plasma with a high electron temperature ($\gtrsim100$ keV). The NuSTAR spectrum also reveals clear evidence of disk reflection, a broad Fe K line around 6-8 keV, and a Compton hump peaking at 20 keV, irrespective of the choice of the continuum models. Our spectral studies suggest a disk extending close to the neutron star surface ($\sim$7-17 $R_{\rm g}$) at low inclination angles ($\sim$20$^\circ$-40$^\circ$), as revealed by a couple of self-consistent relativistic reflection models, relxill and relxillCP. In addition, we detected type-II bursts for the first time in the NICER observation of this source. Light curve profiles of type-II bursts exhibit different patterns, mostly associated with the so-called mode-0 and mode-1 type-II bursts. The energy spectra of the persistent (pre-burst) and burst emission are well described by an absorbed Comptonization component, scattering diskbb- and blackbody-distributed photons, respectively, by a corona with a temperature of 1-3 keV. Although the origin of the type-II burst is not very clear, it has been substantially linked to magnetospheric gating of the accretion flow.

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arXiv2026

Probing the accretion geometry of the transient accreting millisecond pulsar SAX J1808.4-3658: transitions to the propeller regime

We analyze three NuSTAR observations and two NICER observations of the transient accreting millisecond pulsar SAX J1808.4-3658 in the hard spectral state during its most recent outbursts in 2022 and 2025. The spectral analysis of the persistent emission shows that the continuum is well described by an absorbed thermal Comptonization model with a high plasma temperature of ~25-90 keV. A prominent iron emission line around 5-8 keV and a Compton hump around 15-30 keV have been detected from all NuSTAR observations, indicating the reflection of the hard X-ray photon from the accretion disk. We employ the relativistic reflection model relxillCP to describe the reflection phenomena. The spectral fit of three NuSTAR observations shows that the inner disk radius moves outward, the Comptonized thermal emission decreases in flux, the mass accretion rate decreases, and the disk becomes less ionized as we proceed from the 2022 to the 2025 observations. Reflection studies also reveal a moderate inclination of the source within ~30-50 degrees. During the 2025 September observation, the inner radius of the disk is significantly truncated (~23R_g), and the corresponding magnetospheric radius is comprehensively larger than the disk's co-rotation radius, suggesting a hint of the transition to the propeller regime. Although the disk is truncated at the larger radius, accreted material is still reaching the surface of the neutron star, which is confirmed through the detection of a Type-I X-ray burst during this NuSTAR observation. The spectral analysis of the burst suggests helium burning at a low ignition depth.

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arXiv2026

Evolution of the Inner Accretion Flow in Swift J1727.8$-$1613 across Intermediate States: Insights from Broadband Spectral and Timing Analysis

We present a comprehensive broadband spectral and variability study of the newly detected black hole X-ray binary Swift~J1727.8--1613 in the intermediate states during its 2023 outburst, using multi-mission observations from NICER, NuSTAR, AstroSat, and Insight-HXMT. Spectral data up to 78 keV in the hard-intermediate state (HIMS) require models with two Comptonizing regions. In contrast, models with a single Comptonizing region adequately describe the soft-intermediate states (SIMS), implying a significant evolution in the disk-corona geometry between the states. The hard X-ray tail above $100$ keV in the HIMS, detected with both AstroSat/CZTI and Insight-HXMT/HE, indicates that the electron population in the corona is not purely thermal but rather hybrid, with a power-law distribution above the thermal cutoff. While both the reflection modeling and disk continuum fitting favor a truncated disk geometry in the HIMS, the disk in the SIMS moves substantially closer to the innermost stable circular orbit, accompanied by a significant rise in disk temperature. This interpretation is further supported by the increase in the QPO frequency from $\sim1.3$ to $\sim6.6$ Hz. From joint modeling of the disk continuum and reflection component and assuming the distance of 3.4 kpc, we estimate a black hole mass of $10.3^{+5.5}_{-2.5}~M_\odot$, spin of $0.79^{+0.07}_{-0.15}$, and the disk inclination angle of $\sim37\degr$--$53\degr$, which match well with the previously reported spectro-polarimetric measurements. We find a weakly variable or stable disk and a highly variable Comptonized component.

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arXiv2026

Exploring the spectral characteristics of the periodic burster 4U 1323-62: Type-I X-ray burst and persistent emission

We report on the results obtained by the analysis of persistent and type-I thermonuclear X-ray burst emission observed from the periodic burster 4U 1323-62. These analyses are based on the NuSTAR observation performed on 2024 August 7 for a total exposure of around 90 ks. The persistent emission is well described by an absorbed thermal Comptonization model. An absorption edge is also detected at an energy of approximately 7.42 keV, which indicates the presence of absorbing material in the vicinity of this system. Six bursts have been observed during this observation, wherein we find the burst recurrence time to be approximately 4.52 hr. All the bursts exhibit the characteristics of a sharp rise and exponential decay. We perform the time-resolved spectroscopy of the burst spectra described by a model consisting of thermal emission from the neutron star surface and a varying persistent emission component to study the evolution of burst parameters. The enhancement of the persistent emission during burst exposure is characterized by the scaling parameter f a, which reflects the increasing strength of the burst-disc interaction with burst intensity, likely driven by Poynting-Robertson drag. The spectral analysis of bursts estimate the average apparent blackbody emitting radius of the neutron star to lie within 1.5-3.5 km. The ignition depths computed from the burst parameters indicate short Type-I thermonuclear bursts from a mixed hydrogen-helium fuel layer.

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arXiv2025

Accretion Geometry of the New Galactic Black Hole Candidate AT2019wey in the Hard State

We perform broadband spectral and timing studies of the Galactic low-mass black hole candidate AT2019wey using quasi-simultaneous NICER, Swift, and NuSTAR observations obtained in 2022. The long-term MAXI light curve, along with the hardness-intensity diagram (HID), indicates that the source remained in the hard state and did not switch to the soft state. Spectral modeling using two different model combinations reveals that the broadband spectrum is best described by two distinct Comptonizing regions, associated reflection components, and thermal emission from the disk. The harder Comptonizing region dominates ($\gtrsim80\%$) the total flux and is primarily responsible for the observed reflection features from the distant part of the disk. We find that the accretion disk is truncated at a radius of $\sim16-56~r_{\rm{g}}$, while the luminosity is $\sim1.9\%$ of the Eddington limit, assuming a black hole mass of $10 ~ M_\odot$ and distance of 8 kpc. Our spectral results also show consistency in the estimated inner disk radius obtained through two independent methods: modeling the disk continuum and the reflection spectrum. The variability studies imply the presence of intrinsic disk variability, likely originating from an instability in the disk. We also detect hard time lags at low frequencies, possibly arising from the inward propagation of mass accretion rate fluctuations from the outer to the inner regions of the accretion disk. Moreover, an observed deviation of the lag-energy spectrum from the log-linear trend at $\lesssim 0.7$ keV is most likely attributed to thermal reverberation, arising from the reprocessing of hard coronal photons in the accretion disk.

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arXiv2025

NuSTAR discovers a long type-I X-ray burst from the clocked burster GS 1826-24

The source GS~1826-24 is a neutron star low mass X-ray binary known as the 'clocked burster' because of its extremely regular bursting behavior. We report on the detection of a long type-I X-ray burst from this source. We perform a detailed spectroscopic analysis of the long X-ray burst, lasting for $\sim 600$ s, seen in the NuSTAR observation carried out on 2022 September. The persistent emission is well described by an absorbed thermal Comptonization model (nthcomp), and the source exhibits a soft spectral state during this observation. The observed burst exhibits a rise time of $\sim 25$ s and a decay time of $\sim 282$ s. The time-resolved spectroscopy of the burst shows a significant departure from a pure thermal spectrum and is described with a model consisting of a varying-temperature blackbody plus an evolving persistent emission component. We observe a significant enhancement in the persistent emission during the burst. The enhancement of the pre-burst persistent flux is possibly due to Poynting-Robertson drag or coronal reprocessing. At the peak of the burst, the blackbody temperature and the blackbody emitting radius reached a maximum of $2.10\pm 0.07$ keV and $5.5\pm 2.1$ km, respectively. The peak flux ($F_{peak}$) during the burst is $\approx 2.4\times 10^{-8}$ ergs cm$^{-2}$ s$^{-1}$, which corresponds to a luminosity of $\approx 9.7\times 10^{37}$ ergs s$^{-1}$.

astro-ph.HE↗

arXiv2025

Ultraluminous X-ray sources in the group-centric elliptical galaxy NGC 5813

The number of Ultraluminous X-ray Sources (ULXs) is observed to be correlated with the current star formation rate in late-type galaxies and with the stellar mass in early-type galaxies (ETGs). Since there is very little gas, dust or star formation in ETGs, it has been suggested that most of the ULXs associated with them could be high luminosity Low Mass X-ray Binaries (LMXBs) or foreground/background sources. It has been reported that NGC 5813, the central dominant (cD) galaxy in the NGC 5846 group of galaxies, which shows signs of a possible recent merger event, has an unusually high number of ULXs. We have undertaken a multi-epoch spectral study of the persistent ULXs in the galaxy using Chandra and XMM-Newton observations. Of the eight ULXs reported elsewhere, four have been re-identified, two are not consistently detected across all nine Chandra observations, and two are found to be foreground sources. One new persistent ULX has been identified. We present a spectral analysis of the five ULXs with luminosity consistently greater than $10^{39}$ erg/s in nine Chandra-ACIS observations, and assess their variability, adding data from XMM-Newton. The association of these ULXs with globular clusters was examined: we find one ULX lying within the field of an HST observation within 0.1$^\prime$ of the centre of a globular cluster. Optical and UV counterparts are found for another ULX. One of the ULXs is found to be variable over the time scale of days, but there is no unambiguous evidence of longer-term variability for the remaining ULXs.

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arXiv2025

Investigating Accretion Disk-Corona in Seyfert 1 galaxies: A UV/X-ray Spectral Study of Mrk 813 and RBS 688

We present a broadband UV/X-ray spectral study of two Seyfert 1 galaxies, Mrk 813 and RBS 688, primarily based on AstroSat observations. These active galactic nuclei host relatively large super-massive black holes ($M_{BH} \sim 10^8 - 10^9M_{\odot}$), suffer negligible internal extinction/absorption, and are well suited for probing the inner regions of their accretion disks using far UV and soft X-ray spectra. In the case of Mrk 813, the AstroSat and HST far UV spectra are steeper than those expected from a standard accretion disk; the deficit of emission at shorter wavelengths suggests a truncated accretion disk with an inner radius $r_{in} \sim 70r_g$. Joint UV/X-ray broadband spectral modelling with FAGNSED and RELAGN models suggests that the apparent truncation in Mrk 813 is most likely due to the presence of a warm Comptonising disk in the inner regions that is responsible for the observed soft X-ray excess emission. RBS 688 lacks the soft X-ray excess emission, and the UV data are entirely consistent with a standard disk that appears to extend very close to the innermost stable circular orbit. Our study suggests the formation of the warm, optically-thick Comptonising corona in the innermost disk regions at higher Eddington fraction.

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arXiv2025

AstroSat/UVIT far and near UV deep field around IC 4329A

We present high-resolution near-ultraviolet (NUV) and far-ultraviolet (FUV) deep imaging of the field around the Seyfert galaxy IC~4329A based on five observations performed with the Ultra-Violet Imaging Telescope (UVIT), onboard AstroSat. The long exposures of 82.9 ks in NUV (N245M; $λ_{mean}=2447$Å ; $Δλ= 270$Å) and 92.2~ks in FUV (F154W; $λ_{mean} = 1541$Å; $Δλ=380$Å) bands constitute the deepest observations with $5σ$ detection limits of AB magnitudes $m_{NUV}= 26.2$ and $m_{FUV} = 25.7$. Leveraging UVIT's excellent angular resolution (FWHM $\sim 1.2-1.8^{\prime \prime}$), we performed a detailed analysis of the IC~4329A field and detected (above 5$σ$ significance level) a total of 4437 and 456 sources in the NUV and FUV bands, respectively. A large number of these detected sources were unknown previously. We performed astrometry and photometry on all detected sources. By cross-matching our catalogue with Gaia-DR3 and XMM-Newton DR12 catalogues, we found 651 optical and 97 X-ray counterparts of our sources. Additionally, we explored UV variability of point sources, identifying 28 NUV sources as variable with a significance above the $2.5σ$ level. Of these, only three sources exhibited variability in the FUV band. Utilising the NUV and Gaia fluxes, we determined that two previously catalogued white dwarf candidates are misclassified. Furthermore, we highlight galaxies with atypical morphology, including ring-like structures, multiple compact central sources, bifurcating spiral arms, etc. Follow-up optical spectroscopy and multi-wavelength observations are imperative to further investigate the nature of the sources within this field.

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arXiv2025

Ultraviolet spectroscopy of the black hole X-ray binary MAXI J1820+070 across a state transition

We present ultraviolet (UV) spectroscopic observations covering three distinct accretion states of the low-mass X-ray binary (LMXB) MAXI J1820+070: the luminous hard state, a hard-intermediate state and the soft state. Our observations were obtained during the 2018 eruption of MAXI J1820+070 with the Hubble Space Telescope (HST) and AstroSat observatory. The extinction towards the source turns out to be low - $\rm E_{B-V} = 0.2 \pm 0.05$ - making it one of the best UV accretion laboratories among LMXBs. Remarkably, we observe only moderate differences between all three states, with all spectra displaying similar continuum shapes and emission lines. Moreover, the continua are not well-described by physically plausible irradiated disc models. All of this challenges the standard reprocessing picture for UV emission from erupting LMXBs. The UV emission lines are double-peaked, with high-ionization lines displaying higher peak-to-peak velocities. None of the lines display obvious outflow signatures, even though blue-shifted absorption features have been seen in optical and near-infrared lines during the hard state. The emission line ratios are consistent with normal abundances, suggesting that the donor mass at birth was low enough to avoid CNO processing ($\rm M_{2,i} \lesssim 1.0 - 1.5 {\mathrm M_{\odot}}$). Finally, we study the evolution of UV variability in our time-resolved HST observations (hard and hard-intermediate states). All UV power spectra can be modelled with a broken power-law, superposed on which we tentatively detect the $\simeq 18$s quasi-periodic oscillation (QPO) that has been seen in other spectral bands.

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arXiv2025

NuSTAR view of the X-ray transients Swift J174805.3-244637 and IGR J17511-3057

We report on the NuSTAR observations of the neutron star low-mass X-ray binary Swift J174805.3-244637 (hereafter Swift~J17480) and the accreting millisecond X-ray pulsar IGR~J17511-3057 performed on March 4, 2023, and April 8, 2015, respectively. We describe the continuum emission of Swift~J17480 with a combination of two soft thermal components and an additional hard X-ray emission described by a power-law. We suggest that the spectral properties of Swift~J17480 are consistent with a soft spectral state. The source IGR~J17511-3057 exhibits a hard spectrum characterized by a Comptonized emission from the corona. The X-ray spectrum of both sources shows evidence of disc reflection. For the first time, we employ the self-consistent reflection models ({\tt relxill} and {\tt relxillNS}) to fit the reflection features in the \nustar{} spectrum. From the best-fit spectral model, we find an inner disc radius ($R_{in}$) is precisely constrained to $(1.99-2.68)\:R_{ISCO}$ and inclination to $30\pm 1\degree$ for Swift~J17480. We determine an inner disc radius of $\lesssim 1.3\;R_{ISCO}$ and inclination of $44\pm 3\degree$ for IGR~J17511-3057. A low inclination angle of the system is required for both sources. For the source IGR~J17511-3057, spinning at $4.1$ ms, the value of co-rotation radius ($R_{co}$) is estimated to be $\sim 42$ km ($3.6\:R_{ISCO})$, consistent with the position of inner disc radius as $R_{in}\lesssim R_{co}$. We further place an upper limit on the magnetic field strength of the sources, considering the disc is truncated at the magnetospheric radius.

astro-ph.HE↗

arXiv2024

Relativistic X-ray reflection from the accreting millisecond X-ray pulsar IGR J17498-2921

The accreting millisecond X-ray pulsar IGR J17498-2921 went into X-ray outburst on April 13-15, 2023, for the first time since its discovery on August 11, 2011. Here, we report on the first follow-up \nustar{} observation of the source, performed on April 23, 2023, around ten days after the peak of the outburst. The \nustar{} spectrum of the persistent emission ($3-60$ \kev{} band) is well described by an absorbed blackbody with a temperature of $kT_{bb}=1.61\pm 0.04$\kev{}, most likely arising from the NS surface and a Comptonization component with power-law index $Γ=1.79\pm0.02$, arising from a hot corona at $kT_{e}=16\pm 2$ keV. The X-ray spectrum of the source shows robust reflection features which have not been observed before. We use a couple of self-consistent reflection models, {\tt relxill} and {\tt relxillCp}, to fit the reflection features. We find an upper limit to the inner disc radius of $ 6\: R_{ISCO}$ and $ 9\: R_{ISCO}$ from {\tt relxill} and {\tt relxillCp} model, respectively. The inclination of the system is estimated to be $\simeq 40\degr$ from both reflection models. Assuming magnetic truncation of the accretion disc, the upper limit of magnetic field strength at the pole of the NS is found to be $B\lesssim 1.8\times 10^{8}$ G. Furthermore, the \nustar{} observation revealed two type I X-ray bursts and the burst spectroscopy confirms the thermonuclear nature of the burst. The blackbody temperature reaches nearly $2.2$ keV at the peak of the burst.

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arXiv2024

Numerical $\texttt{AXP4}$ Simulations of Pulse Profiles for Binary Accreting X-ray Pulsars $-$ II: A Case Study of Centaurus X-3

The pulse shapes simulated in the accompanying paper Part $-$ I are compared with observations of a model binary accreting X-ray pulsar, Centaurus X-3. With known Cen X-3 inclination angles provided as input to the $\texttt{AXP4}$ code, the generated pulse profile is suitably compared with the corresponding observed energy-resolved $\textit{AstroSat}$/LAXPC pulse profile. The pulsed fraction is proposed as a robust, quantitative measure for estimating the size of the emission region of Centaurus X-3 by extending the simulations to include spherical caps of varying fractional surface coverage of the neutron star $-$ over the full range of $0-100\%$, up to very large caps (with polar half angle $> 30^{\circ}$). The hotspot radius thus derived drops by an order of magnitude from $12.27$ km to $1.36^{+0.29}_{-0.26}$ km, within the ballpark of the standard model value of $\sim$$1$ km, after including the effect of gravitational light bending, lending further weight to its routine emphasis in the literature. The energy- and luminosity-dependence of the composite gravitationally bent and slab-integrated pulse profiles is further studied. As the pulse profile is sensitive to luminosity variations, the correlation of the size of a finite polar cap and its dependence on X-ray luminosity $-$ through the rate and subsequently, the geometry of accretion $-$ is discussed. Although a single, model pulsar was chosen for this work to exhibit the depth of the physical and astrophysical prospects of such a probe, this exercise can be extended to a wide range of existing X-ray pulse profiles of known binary accreting pulsars available in galactic catalogs, especially, with the possible inclusion of accretion columns (with cylindrical co-ordinate transformation) in the future.

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arXiv2024

Accretion Geometry of GX 339-4 in the Hard State: AstroSat View

We perform broadband ($0.7-100$ keV) spectral analysis of five hard state observations of the low-mass back hole X-ray binary GX~339--4 taken by AstroSat during the rising phase of three outbursts from $2019$ to $2022$. We find that the outburst in 2021 was the only successful/full outburst, while the source was unable to make transition to the soft state during the other two outbursts in 2019 and 2022. Our spectral analysis employs two different model combinations, requiring two separate Comptonizing regions and their associated reflection components, and soft X-ray excess emission. The harder Comptonizing component dominates the overall bolometric luminosity, while the softer one remains relatively weak. Our spectral fits indicate that the disk evolves with the source luminosity, where the inner disk radius decreases with increasing luminosity. However, the disk remains substantially truncated throughout all the observations at the source luminosity of $\sim2-8\%\times$ of the Eddington luminosity. We note that our assumption of the soft X-ray excess emission as disk blackbody may not be realistic, and this kind of soft excess may arise due the non-homogeneity in the disk/corona geometry. Our temporal analysis deriving the power density spectra suggests that the break frequency increases with the source luminosity. Furthermore, our analysis demonstrates a consistency between the inner disk radii estimated from break frequency of the power density spectra and those obtained from the reflection modelling, supporting the truncated disk geometry in the hard state.

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arXiv2024

A multi-wavelength study of the hard and soft states of MAXI J1820+070 during its 2018 outburst

We present a comprehensive multi-wavelength spectral analysis of the black hole X-ray binary MAXI J1820+070 during its 2018 outburst, utilizing AstroSat far UV, soft and hard X-ray data, along with (quasi-)simultaneous optical and X-ray data from Las Cumbres Observatory and NICER, respectively. In the soft state, we detect soft X-ray and UV/optical excess components over and above the intrinsic accretion disk emission ($kT_{\rm in}\sim 0.58$ keV) and a steep X-ray power-law component. The soft X-ray excess is consistent with a high-temperature blackbody ($kT\sim 0.79$ keV), while the UV/optical excess is described by UV emission lines and two low-temperature blackbody components ($kT\sim 3.87$ eV and $\sim 0.75$ eV). Employing continuum spectral fitting, we determine the black hole spin parameter ($a=0.77\pm0.21$), using the jet inclination angle of $64^{\circ}\pm5^{\circ}$ and a mass spanning $5-10M_{\odot}$. In the hard state, we observe a significantly enhanced optical/UV excess component, indicating a stronger reprocessed emission in the outer disk. Broad-band X-ray spectroscopy in the hard state reveals a two-component corona, each associated with its reflection component, in addition to the disk emission ($kT_{\rm in}\sim 0.19$ keV). The softer coronal component dominates the bolometric X-ray luminosity and produces broader relativistic reflection features, while the harder component gets reflected far from the inner disk, yielding narrow reflection features. Furthermore, our analysis in the hard state suggests a substantial truncation of the inner disk ($\gtrsim 51$ gravitational radii) and a high disk density ($\sim 10^{20}\ \rm cm^{-3}$).

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arXiv2022

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

We present a 140ks observation of NGC 4593 with XMM-Newton providing simultaneous and continuous PN X-ray and OM UV (UVW1 2910Å) lightcurves which sample short-timescale variations better than previous observations. These observations were simultaneous with 22d of Swift X-ray and UV/optical monitoring, reported previously, and 4d of AstroSat X-ray (SXT), far (FUV 1541Å), and near (NUV 2632Å) 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$\pm$1.3ks, $\sim$half the lag previously determined from the Swift monitoring. Re-examination of the \textit{Swift} data reveals a bimodal lag distribution, with evidence for both the long and short lags. However if we detrend the Swift lightcurves by LOWESS filtering with a 5d width, only the shorter lag (23.8$\pm$21.2ks) remains. The NUV observations, compared to PN and SXT, confirm the $\sim$30ks lag found by XMM and, after 4d filtering is applied to remove the long-timescale component, the FUV shows a lag of $\sim$23ks. 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 $7.63\times10^{6}M_{\odot}$ and a spin approaching maximum. Previously noted lag contribution from the BLR in the Balmer and Paschen continua are still prominent.

astro-ph.HE↗

arXiv2022

The origin of the vanishing soft X-ray excess in the changing-look Active Galactic Nucleus Mrk 590

We have studied the nature and origin of the soft X-ray excess detected in the interesting changing-look AGN (CLAGN) Mrk~590 using two decades of multi-wavelength observations from \xmm{}, \suzaku{}, \swift{} and \nustar{}. In the light of vanishing soft excess in this CLAGN, we test two models, "the warm Comptonization" and "the ionized disk reflection" using extensive UV/X-ray spectral analysis. Our main findings are: (1) the soft X-ray excess emission, last observed in 2004, vanished in 2011, and never reappeared in any of the later observations, (2) we detected a significant variability ($\sim300\%$) in the observed optical-UV and power-law flux between observations with the lowest state ($L_{\rm bol} = 4.4\times 10^{43}\, erg\, s^{-1}$, in 2016) and the highest state ($L_{\rm bol} = 1.2\times 10^{44}\, erg\, s^{-1}$, in 2018), (3) the UV and power-law fluxes follow same temporal pattern, (4) the photon index showed a significant variation ($Γ=1.88^{+0.02}_{-0.08}$ and $Γ=1.58^{+0.02}_{-0.03}$ in 2002 and 2021 respectively) between observations, (5) no Compton hump was detected in the source spectra but a narrow Fe$K_α$ line is present in all observations, (6) we detected a high-energy cut-off in power-law continuum ($92^{+55}_{-25} \rm keV$ and $60^{+10}_{-08} \rm keV$) with the latest \nustar{} observations, (7) the warm Comptonization model needs an additional diskbb component to describe the source UV bump, (8) there is no correlation between the Eddington rate and the soft excess as found in other changing-look AGNs. We conclude that given the spectral variability in UV/X-rays, the ionized disk reflection or the warm Comptonization models may not be adequate to describe the vanishing soft excess feature observed in Mrk~590.

astro-ph.HE↗