arXiv subjects

Mayukh Pahari

Publications and source records attributed to Mayukh Pahari.

At least 19 recordsLinked to original sources

arXiv2026

Investigating the disappearance of a type-C QPO in black hole X-ray binary Swift J151857-572147 with AstroSat

We present a timing and broadband spectral study of the AstroSat observation of the black hole low mass X-ray binary Swift J151857.0--572147, focusing on its evolution across two temporal segments, S1 and S2. During the first segment S1 lasting $\sim$ 50.6 ks, the source shows a clear type-C QPO at $\sim8.02$ Hz and a low-frequency bump (LFB) in the power density spectrum. By the later segment S2, the strength of the QPO feature decreases significantly and is no longer consistent with a coherent QPO, along with the simultaneous disappearance of the LFB. An energy-dependent analysis reveals that both the RMS amplitude and the time lag of the QPO in S1 increase monotonically with energy, suggesting a low inclination nature of the source. The QPO and LFB centroid frequencies show a strong positive correlation across orbital segments, with a Pearson coefficient of 0.98. Our variability analysis and hardness-intensity diagram reveal a spectral state transition of the source coincident with the turn-off of the QPO and the LFB, a conclusion supported by the evolution of broadband spectral parameters. Notably, the orbit-resolved spectral analysis reveals that the non-thermal parameters, photon index, and electron temperature of the corona exhibit significant deviation as the source traverses from S1 to S2, whereas the thermal parameters scatter without any systematic variation between the two segments. The simultaneous turn-off of the type-C QPO and the low-frequency broadband noise component, both originating from the hot inner flow, indicates a rapid reorganization of the inner accretion geometry. Our results strongly suggest a coronal origin for the QPO and demonstrate that the disappearance of both features reflects a substantial transformation of the Comptonizing region across the spectral transition.

astro-ph.HE↗

arXiv2026

X-ray and Radio campaign of the Z-source GX 340+0: discovery of X-ray polarization and its implications

We present the discovery of X-ray polarization from the neutron star low-mass X-ray binary and Z-source, GX~340$+$0, using an Imaging X-ray Polarimetry Explorer (IXPE) observation in March 2024. Along with the IXPE observation, we conducted an extensive X-ray and radio monitoring campaign to ascertain the source properties during and around the IXPE observation. The source was within the horizontal branch throughout the multiwavelength campaign. We measured a significant X-ray polarization in 2--8 keV with polarization degree (PD) = $4.02 \pm 0.35$% and polarization angle (PA) = $37.6 \pm 2.5^\circ$. The energy-dependent polarization indicates that in the 2-2.5 keV energy range, the PA is much lower, $\sim9\pm8^\circ$, while other energy bands are consistent with the PA found over 2.5--8 keV. The simultaneous AstroSat-IXPE spectro-polarimetric observations provide some evidence for independent polarization from various spectral components, hinting at a disparity in the PA from the accretion disk and the Comptonized emission, while suggesting an unpolarized emission from the blackbody component. Radio observations in the 0.7--9 GHz frequency range reveal a non-detection of radio emission in 0.7-1.5 GHz and a significant detection in 5.5--9 GHz, suggesting the presence of a spectral break in 1.5-5.5 GHz. Using ATCA observation we place upper limits on the radio polarization at $<$6% on the linear polarization and $<$4% on the circular polarization at 3$σ$ level. We discuss the origin of the X-ray polarization and its implications on the geometry of the spectral components.

astro-ph.HE↗

arXiv2026

A long-term spectro-temporal study of Jovian X-ray and Ultraviolet response to solar activity

We present results from a multi-decade investigation of solar activity-driven variability in Jupiter's emissions, using solar X-ray flux and sunspot numbers as activity indicators and ultraviolet (UV) and X-ray observations from the International Ultraviolet Explorer (IUE; 1978-1996) and the Chandra X-ray Observatory (2011-2021). Analysis of 51 high-SNR UV spectra spanning two solar cycles shows that Jupiter's Ly$α$ emission contains narrow and broad components, likely associated with the disk and auroral regions, respectively. The Ly$α$ line and the 1330-1400 Angstrom continuum flux closely follow variations in solar X-ray flux and sunspot numbers throughout all phases of two consecutive solar cycles, indicating a direct connection with solar irradiation processes, including resonant scattering of solar Ly$α$ photons and photoelectron-driven atmospheric excitation. In contrast, ionised UV lines such as Fe II (1608 Angstrom and 1575 Angstrom) show no correlation with solar activity over a solar cycle, suggesting an internal or magnetospheric origin, potentially linked to Io-derived charged particles or UV/X-ray radiation. To determine whether Jupiter's X-ray response resembles its UV response to solar activity, we analysed 29 Chandra/HRC observations obtained during 2014-2021 and two Chandra/ACIS observations from 2011. Significant X-ray flares are detected in both ACIS and HRC lightcurves 7-15 days after major reported coronal mass ejections (CMEs). Post-CME ACIS auroral spectra reveal a significant ($\geq 3σ$) Ne$^{8+}$ emission feature ($\sim$0.94-0.98 keV) near 70-80$^\circ$ latitude at Jupiter's north pole. Owing to the high ionisation energy required ($\sim$1.19 keV for the Ne VIII to Ne IX bound-bound transition), this feature is unlikely to arise from local interactions, supporting CME-driven auroral excitation on Jupiter.

astro-ph.EP↗

arXiv2026

Study of UV line and continuum variabilities in the Broadline Seyfert 1 Galaxy ESO 141-G55

We present the results from a 3-year-long Ultraviolet monitoring campaign of the broad line Seyfert 1 galaxy ESO 141-G55 using International Ultraviolet Explorer (IUE). By modelling all individual, extinction-corrected UV spectra in 1150-1978 A and 1850-3348 A wavelength range, we have observed a significant variability in both UV continuum and line fluxes. Variabilities due to ionised UV lines like SiIV, CIV and HeII are delayed with respect to the UV continuum by 2.92$^{+0.54}_{-0.61}$, 4.41$^{+0.44}_{-0.54}$, 4.11$^{+0.35}_{-0.81}$ days, respectively. At a distance of $\sim$0.004c, an outer accretion disc can be a possible site for the origin of UV lines.

astro-ph.HE↗

arXiv2026

Black hole-Neutron star distinction based on long-term MAXI and Swift study of 42 low mass X-ray binaries

In this study, we analysed about $\sim$13 years of publicly available data from MAXI and Swift/BAT to examine the long-term source evolution of 42 transient low-mass X-ray binaries. The sample consists of 11 confirmed black hole X-ray binaries (BHXBs), 10 black hole candidates (BHC), and 21 neutron star X-ray binaries (NSXBs). Outbursts and flaring activities studied over 13 years show that 19/21 NSXBs spend significantly longer time in the hard state (observations for which hardness ratio is $\geq$ 0.2) while 15/21 BHXB+XRC spend substantially longer time in the soft state (observations for which hardness ratio is $<$ 0.2). The frequency distribution of the hardness ratio clearly shows two distinct distributions for BHXBs and NSXBs, with their peaks separated: NSXBs prefer harder values, while BHXBs prefer softer values of hardness. Our model-independent analysis for 42 transient sources shows that statistically NSXBs do not prefer to show a canonical high soft state as observed in BHXBs. Additionally, the probability distribution of the duration of the 2-20 keV X-ray outburst is observed to peak at a significantly longer duration ($>$100 days) for BHXBs than for NSXBs (15-60 days). Our analysis shows that among candidate sources, Swift J1728.9-3613, MAXI J1535-571, MAXI J1659-152, EXO 1846-031 show a `q' diagram in the HID and prefer to align with the HID frequency distribution of BHXBs that show `q' diagram, MAXI J1305-704 and MAXI J1836-194 align with frequency distribution of black hole sources without `q' diagram while MAXI J1848-015 shows the HID distribution similar to NSXBs, indicating a neutron star accretor. Therefore, a long-term statistical study of MAXI and Swift/BAT X-ray outbursts from a large sample of transient sources may be used to distinguish BHXB from NSXB.

astro-ph.HE↗

arXiv2026

Understanding coronal geometry in NGC 4593 using Fourier frequency-resolved covariance and time-lag spectral analysis

Understanding disc-corona geometry through X-ray reverberation variability studies in Seyfert galaxies is crucial, yet our knowledge mostly relies on flux-averaged mean spectral analysis. In this study, we investigate the origin of the large X-ray variability of the Seyfert 1 galaxy NGC 4593 using two \xmm{} observations, which are at least 65 ksec long and have a 0.3-10 keV X-ray flux difference by a factor of $\sim$2.5. We extracted mean spectra, Fourier-frequency resolved covariance, and time-lag spectra and performed modelling of all spectra in a self-consistent manner. From the best-fit covariance spectra, we have shown that energy-dependent covariance during low flux shows dominances of direct powerlaw continuum over reflection continuum at all Fourier frequencies (2.1 $-$ 390 $\times$ 10$^{-5}$ Hz). However, during high flux, the variabilities are dominated by the reflection components most of the time. Our results are further supported by the Fourier frequency-dependent time-lag (between soft: 0.3-1 keV and hard: 1-5 keV bands) spectral modeling during high and low fluxes. A significant change is observed in the X-ray reverberation delay timescale from 483 $\pm$ 135 sec (during high flux) to $<$96 sec (during low flux), indicating a change in coronal size at least by a factor of $\sim$2 (from $<$3.3 R$_g$ to $>$7.2 R$_g$) during low to high flux transitions.

astro-ph.HE↗

arXiv2026

Spectro-timing origin of large amplitude X-ray variability in GRS 1915+105 using AstroSat/LAXPC and SXT

The origin of the large-amplitude, quasi-periodic X-ray flux variations in several classes of the Galactic microquasar GRS~1915+105 remains unresolved. We address this issue through flux-resolved, broadband (0.8-20 keV) spectral modelling and simultaneous covariance spectral analysis during two $κ$ and two $ω$ class observations using \textit{AstroSat}/SXT and LAXPC. The lightcurves show strong, quasi-periodic oscillations involving rapid transitions between bright bursts and deep dips on timescales of a few tens of seconds. Flux-resolved spectroscopy indicates that high-flux intervals in both classes are dominated by a hot, optically thick accretion disc with steep Comptonized emission, whereas low-flux intervals correspond to a cooler or partially recessed disc and a harder coronal continuum. These transitions involve a systematic 1-2 keV drop in disc temperature and a pronounced hardening of the Comptonized component, with flux reductions of up to a factor of five. Using covariance spectra across 0.015-5 Hz, we show that the rapid coherent variability arises almost entirely from the disc, which exhibits strong energy-dependent variations, while the Comptonized component contributes minimally. The combined results suggest that radiation-pressure-driven structural changes in the disc, with a slower coronal response, produce the observed oscillations, consistent with cyclic disc evacuation and refilling in the $κ$ and $ω$ classes.

astro-ph.HE↗

arXiv2025

Measuring accretion disc properties in the transitional millisecond pulsar PSR J1023+0038 using XMM-Newton, NuSTAR, NICER and Chandra

Whether the accretion disc in the X-ray high-mode of transitional millisecond pulsars (tMSP) reaches near the neutron star surface by penetrating the magnetosphere is a crucial question with many implications, including for continuous gravitational wave emission from the pulsar. We attempt to answer this question for the tMSP PSR J1023+0038 by segregating high-mode data and performing detailed spectral analysis using the XMM-Newton EPIC-PN+MOS1+MOS2 joint observations, XMM-Newton+NuSTAR joint observations, NICER and Chandra individual observations during different epochs. With the sum of longest exposures ($\sim$202 ksec of high mode data from $\sim$364 ksec of total exposure), we performed a self-consistent spectral analysis and constrain the inner disc radius 16.8 $\pm$ 3.8 km with at least 3$σ$ significance. Such a measurement is found consistent with best-fit spectral values of inner disc radius from other observatory like NICER and a joint observations with XMM-Newton and NuSTAR within 3$σ$ limits. We also detect a Fe emission line at 6.45 keV, for the first time from a tMSP, in the Chandra spectrum with 99% significance with an upper limit of the inner disc radius of 21 R$_g$, supporting independently the fact that inner disc extends into neutron stars's magnetosphere during high mode. All results from our analysis imply that the accretion disc is significantly present and extended within the corotation radius of the neutron star in PSR J1023+0038 during the X-ray high-mode of the tMSP PSR J1023+0038. The measured range of inner disc radius is fully consistent with an independent analysis by Bhattacharyya (2020), which suggests continuous gravitational wave emission from this neutron star, and the standard model of X-ray pulsations in accreting MSPs.

astro-ph.HE↗

arXiv2025

Distinct Fe-K Line Complexes in MAXI J1744-294 Revealed by XRISM High-Resolution Spectroscopy

The newly discovered Galactic transient MAXI J1744-294 went into its first X-ray outburst in 2025. We study the spectral properties of this source in the 2-10 keV energy band during this outburst using X-ray data from the XRISM satellite for both of its Resolve and Xtend instruments, taken on March 03, 2025. High-resolution spectroscopy has revealed, for the first time, complex iron line features in this source, corresponding to distinct components of Fe XXV emission and Fe XXVI absorption lines. Such a detailed structure has not been reported in other low-mass X-ray binaries to date, prior to the XRISM era. Our analysis shows that the line complexes arise from two highly ionized plasmas with ionization rate ~ 1000 erg-cm/s with distinct turbulent velocities: one broad (~2513 km/s) from hot gas at the inner accretion disk and one narrow (~153 km/s) scattered by nearby photoionized gas. These results offer new insight into the reprocessing of continuum in stratified media, either in the accretion disk or winds, or both, for XRBs in the soft state. The data are well described by models with spin, mass of the black hole, and accretion disk inclination 0.63-0.70, 5.7-10.1 Solar masses, and 19-24 degrees. The fitted spectral model parameters suggest that the source is in the soft spectral state. The source is situated in a crowded field near the Galactic center, resulting in a large hydrogen column density.

astro-ph.HE↗

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

UV flux variation study in contact binary VW Cephei

Despite many attempts, the origin of UV emission line and continuum in contact binary stars remains unclear. We present a substantial UV spectroscopic analysis of VW Cephei, a late-type contact binary system, using 46 low-resolution spectra from the International Ultraviolet Explorer (IUE) in the wavelength range 1150-1978 Å. By modelling continuum and emissions lines in individual spectra, we report the significant detection of OIII] (1660 and 1666 Å) and SiIV (1393 and 1402 Å) line complexes. We observe that UV fluxes for both continuum and emission lines like CIV, OIII], CII and SiIV vary significantly (fractional rms variability up to 45%) from hours to years. In addition, line widths also change by hundreds of kilometres/sec. The UV flux variabilities observed in the continuum bands and line emissions are uncorrelated. However, most of the flux values follow the binary orbital period observed from optical data. Our analysis indicates that, while the variation in continuum flux may be attributed to a heated photosphere, the line width measurements indicate that the emission lines are likely formed in the dynamical clouds associated with Roche lobe overflow. We estimate the mass transfer rate of $ \dot{M} = (0.82 \pm 0.01) \times 10^{-7} \ M_{\odot} {yr^{-1}}$ from UV line fluxes, which is in good agreement with optical studies.

astro-ph.SR↗

arXiv2025

Astrophysics with Compact Objects: An Indian Perspective, Present Status and Future Vision

Astrophysical compact objects, viz., white dwarfs, neutron stars, and black holes, are the remnants of stellar deaths at the end of their life cycles. They are ideal testbeds for various fundamental physical processes under extreme conditions that are unique in nature. Observational radio astronomy with uGMRT and OORT facilities has led to several important breakthroughs in studies of different kinds of pulsars and their emission mechanisms. On the other hand, accretion processes around compact objects are at the core of Indian astronomy research. In this context, AstroSat mission revolutionized spectro-temporal observations and measurements of accretion phenomena, quasi-periodic oscillations, and jet behaviour in binary systems hosting compact objects. Moreover, recently launched XPoSat mission is set to provide an impetus to these high-energy phenomena around compact objects by enabling us to conduct polarization measurements in the X-ray band. Further, during the past decade, numerous gravitational wave signals have been observed from coalescing black holes and neutron stars in binary systems. Recent simultaneous observation of the GW170817 event in both gravitational waves and electromagnetic channels has ushered in the era of multi-messenger astronomy. In the future, synergistic efforts among several world-class observational facilities, e.g., LIGO-India, SKA, TMT, etc., within the Indian astrophysics community will provide a significant boost to achieve several key science goals that have been delineated here. In general, this article plans to highlight scientific projects being pursued across Indian institutions in this field, the scientific challenges that this community would be focusing on, and the opportunities in the coming decade. Finally, we have also mentioned the required resources, both in the form of infrastructural and human resources.

astro-ph.HE↗

arXiv2025

X-ray and Radio Campaign of the Z-source GX 340+0 II: the X-ray polarization in the normal branch

We present the first X-ray polarization measurement of the neutron star low-mass X-ray binary and Z-source, GX 340$+$0, in the normal branch (NB) using a 200 ks observation with the Imaging X-ray Polarimetric Explorer (IXPE). This observation was performed in 2024 August. Along with IXPE, we also conducted simultaneous observations with NICER, AstroSat, Insight-HXMT, ATCA, and GMRT to investigate the broadband spectral and timing properties in the X-ray and radio wavelengths. During the campaign, the source traced a complete Z-track during the IXPE observation but spent most of the time in the NB. We measure X-ray polarization degree (PD) of $1.22\pm0.25\%$ in the 2-8 keV energy band with a polarization angle (PA) of $38\pm6^\circ$. The PD in the NB is observed to be weaker than in the horizontal branch (HB) but aligned in the same direction. The PD of the source exhibits a marginal increase with energy while the PA shows no energy dependence. The joint spectro-polarimetric modeling is consistent with the observed X-ray polarization originating from a single spectral component from the blackbody or the Comptonized emission while the disk emission does not contribute towards the X-ray polarization. GMRT observations at 1.26 GHz during HB had a tentative detection at 4.5$\pm$0.7 mJy while ATCA observations a day later during the NB detected the source at 0.70$\pm$0.05 mJy and 0.59$\pm$0.05 mJy in the 5.5 & 9 GHz bands, respectively, suggesting an evolving jet structure depending on the Z-track position.

astro-ph.HE↗

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

Effect of Magnetic Field on the Formation of Radiatively Inefficient Accretion Flow around Black Holes

We study the effects of magnetic field in the formation of a radiatively inefficient accretion flow (RIAF) in the presence of Bremsstrahlung cooling, which facilitates the formation of a geometrically thin, optically thick accretion disk surrounded by a hot corona. We have performed axis-symmetric magnetohydrodynamic (MHD) simulations of an initial accretion torus with a $1/r$ dependant local poloidal field in the presence of a pseudo-Newtonian potential, taking into account optically thin cooling, resistivity and viscosity. We observe the formation of persistent jets and magnetised outflows from the corona surrounding a thin disk with an increase in the magnetic diffusivity parameter. We have defined an equivalent time scale ($τ_{eq}$) which takes into account the heating time scales due to viscosity, resistivity, magnetic reconnection and magneto-rotational instability turbulence such that the thin disk is formed if the cooling time scale ($τ_{cool}$) is lower than this equivalent time scale ($τ_{cool}/τ_{eq}<1$). Using this condition, for the first time, we found that the thin disk exists when the initial ratio of plasma pressure to magnetic pressure (plasma beta) exceeds a range of $600-800$ for the gas obeying a polytropic equation of state accreting at $10^{-5}\ M_{\odot}/year$

astro-ph.HE↗

arXiv2024

Probing the origin of the extended flaring branch of Z-type X-ray binaries GX 340+0 and GX 5-1 using AstroSat

`Z' type neutron star low-mass X-ray binaries typically show a `Z'-like three-branched track in their hardness intensity diagram. However, a few such `Z' sources show an additional branch known as the extended flaring branch (EFB). EFB has been poorly studied, and its origin is not known. It is thought to be an extension of the flaring branch (FB) or associated with Fe K$α$ complex or an additional continuum due to the radiative recombination continuum (RRC) process. Using AstroSat observations, we have detected the EFB from two `Z' sources, GX 340+0 and GX 5-1, and performed a broadband spectral analysis in the 0.5-22 keV energy range. During EFB, both sources show the presence of a significant RRC component with absorption edges at $7.91^{+0.16}_{-0.15}$ keV and $8.10^{+0.16}_{-0.17}$ keV, respectively along with blackbody radiation and thermal Comptonisation. No signature of RRC was detected during the FB, which is adjoint to the EFB. No Fe K$α$ complex is detected. Interestingly, inside EFB dips of GX 5-1, for the first time, we have detected flaring events of 30-60s, which can be modelled with a single blackbody radiation. During the FB to EFB transition, an increase in the blackbody radius by a factor of 1.5-2 is observed in both sources. Our analysis strongly suggests that EFB is not an extension of FB or caused by the Fe K$α$ complex. Rather, it is caused by a sudden expansion of the hot, thermalised boundary layer and subsequent rapid cooling.

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.

astro-ph.HE↗

arXiv2024

AstroSat and NICER timing view of the Z-type Neutron Star X-ray binary GX 340+0

The timing properties of the Z-type low-mass X-ray binaries provide insights into the emission components involved in producing the unique Z-shaped track in the hardness-intensity diagrams of these sources. In this work, we investigate the AstroSat and NICER observations of the GX 340+0 covering the complete 'Z'-track from the horizontal branch (HB) to the extended flaring branch (EFB). For the first time, we present the Z-track as seen in soft X-rays using the AstroSat/SXT and NICER (the soft colour is defined as a ratio of 3-6 keV to 0.5-3 keV). The shape of the track is distinctly different in soft X-rays, strongly suggesting the presence of additional components active in soft X-rays. The detailed timing analysis revealed significant quasi-periodic oscillation throughout the HB and the normal branch (NB) using LAXPC and the first NICER detection of 33.1 +/- 1.1 Hz horizontal branch oscillation (HBO) in 3-6 keV. The oscillations at the HB/NB vertex are observed to have higher frequencies (41-52 Hz) than the HB oscillations (16-31 Hz) and NB oscillations (6.2-8 Hz) but significantly lower rms (~1.6%). The HB oscillation is also limited to the energy range of 3-20 keV, indicating an association of HBO origin with the non-thermal component. It is also supported by earlier studies that found the strongest X-ray polarisation during HB.

astro-ph.HE↗