arXiv subjects

K. Sriram

Publications and source records attributed to K. Sriram.

At least 19 recordsLinked to original sources

arXiv2026

X-ray Timing and Spectral studies of the bare AGN Mrk 110

The origin of the soft X-ray excess below 2 keV in active galactic nuclei (AGNs) remains debated, with relativistic reflection from the inner accretion disk and warm Comptonization in an optically thick corona being the leading explanations. We investigate the timing and spectral properties of the Seyfert galaxy Mrk 110 using six XMM-Newton observations. A frequency-dependent lag analysis in the 7-9 $\times 10^{-5}$ Hz range reveals a soft X-ray lag of 889-3000s in the combined 2019 data, detected with a significance of 80%. The cross-correlation function analysis, supported by simulations, also detects lags of similar nature. Spectral modeling performed by adopting both proposed black hole masses in the literature for Mrk 110 confirms the presence of a warm corona in all observations, along with a weak relativistic reflection component and the reflection fraction remains low (Rf < 1). Interpreting the measured soft lag in terms of light travel time implies an emission radius 4.5 Rg for a supermassive black hole mass of $M = 1.4 \times 10^8$ solar mass , favoring a reflection scenario. However, if a lower mass of $M = 2 \times 10^7$ solar mass is adopted, the inferred radius increases, and both relativistic reflection and warm Comptonization can plausibly contribute to the observed soft lag. The warm corona radius appears larger in the high accretion state and smaller in a lower accretion state, although no trend can be established. The persistently low reflection fraction suggests an outflowing inner corona in Mrk 110, consistent with the recent detection of jet activity in this source.

astro-ph.HE↗

arXiv2025

Asymmetric Cross-Correlation functions with delays in Sco X-1: Evidence of possible Jet triggering

The formation and origin of Jets from Z sources are not well understood, although an X-ray-radio correlation has been observed. We analyzed a few observations of Sco X-1 using the Rossi X-ray Timing Experiment Satellite. Out of the 17 observations, 5 showed lags of a few 10s of seconds with an asymmetry in the CCF between the soft and hard bands in their cross-correlation function (CCF) analysis. Interestingly, during these observations, a ballistic-type radio jet of Ultra-relativistic(UR) nature was reported. The observed lags and associated cross-correlation coefficients were validated using simulations. The rest of the 12 observations' CCFs were symmetric, and their associated Power Density Spectrum (PDS) displayed Normal Branch(NBO)/Normal + Horizontal Branch Oscillations(NBO+HBO). The X-ray spectral study of 2 obs. where radio core emission was seen with abrupt variation in both PDS and CCF showed a black-body flux variation of 10-20%, but no spectral parameter varied. We suggest that the ballistic jet caused a disturbance in the inner accretion region, viz., the Boundary Layer plausibly along with the Corona, that caused the lags observed in the CCFs along with the absence of any oscillatory features in the PDS tracing only a flat-topped noise. Whereas the regions with no lags showed a persistent NBO/NBO+HBO feature, suggesting a steady accretion flow. Although the UR jet can't be related to NBO or HBO, we suggest it could be related to the phenomena that cause NBO since the majority of PDSs displayed NBO. We also constrain the inner accretion region size to 20-30 km, which is responsible for the accretion ejecta in Sco X-1.

astro-ph.HE↗

arXiv2022

Dependency of Optical/UV parameters on X-ray spectral index in AGNs

The study of Active Galactic Nuclei (AGNs) is vital in order to understand their respective nuclei bounded activity primarily triggered by the accretion disk and the associated corona. The hard X-ray emission characterised by the spectral index $Γ$ emitted by these sources in 2-10 keV band ionizes the nearby physical features like disk, BLR, NLR and provides a radiative configuration. However, based on previous studies, there is degeneracy in the evolution of $Γ$ across the redshift, wherein few studies display a systematic trend in the evolution and others rule out the systematic variation. In the present work, we study the evolution of $Γ$ across the redshift for a very large sample and perform further studies on the variation of $Γ$ with optical parameters using SDSS data viz. H$_β$ \& Mg II and their luminosities. Based on analysis we find that there is no change in $Γ$ across the redshift and it does not show any correlation with the optical parameters. This strongly suggests that the $Γ$ is influenced by the soft excess and/or reflection component and is affecting the disk and BLR region at the same strength across the redshift.

astro-ph.HE↗

arXiv2022

Association of Optical, Ultraviolet and Soft X-ray excess Emissions in AGNs

Soft X-ray emission (0.5--2.0 keV) plays a pivotal role in regulating the optical and UV emission in the AGNs. We collected a sample of 1811 AGNs from the SDSS database and obtained various parameters of Balmer lines, optical continuum, MgII line \& UV continuum and studied their dependencies on soft X-ray luminosity. Based on the linear regression analysis, we found that FWHM$_{\text{MgII}}$ $\propto$ FWHM$_{\text{H}β}^{0.554}$ suggesting that UV emission is arising from a region relatively outside the broad line region (BLR) associated to the H$β$ emission and found a strong correlation between optical and UV luminosities (L$_{\text{MgII}}$ $\propto$ L$_{\text{H}β}^{0.822}$). It was noticed that the dependency of optical continuum luminosities on soft excess changes with the redshift (L$_{\text{X}}$ $\propto$ L$^{0.596}_{5100\textÅ}$ for z < 0.5 and L$_{\text{X}}$ $\propto$ L$^{0.429}_{5100\textÅ}$ for z > 0.5). The FWHM components of H$β$ and MgII core components were found to be virialized and is not affected by the soft excess emission whereas the wings of MgII display a dependency. We estimated a relation viz. L$_{\text{X}}$ $\propto$L$^{0.520}_{3000\textÅ}$ FWHM$^{0.525}_{\text{MgII}}$ and found to be well in agreement with a proposed physical scenario. All the derived relations were used to understand the inter-modulating association of the BLR and disc in the AGNs.

astro-ph.HE↗

arXiv2022

Constraining the Coronal Heights and Readjustment Velocities Based on the Detection of a Few Hundred Seconds Delays in the Z Source GX 17+2

Neutron star Z type sources provide a unique platform in order to understand the structure of accretion disk-corona geometry emitting close to the Eddington luminosity. Using RXTE and NuSTAR satellite data, we performed cross correlation function (CCF) studies in GX 17+2 in order to constrain the size of corona responsible for hard X-rays. From the RXTE data, we found that during horizontal and normal branches, the CCFs show anti-correlated hard (16 - 30 keV) and soft (2 - 5 keV) X-ray delays of the order of a few tens to hundred seconds with a mean correlation coefficient of 0.42$\pm$0.11. Few observations shows correlated lags and on one occasion coincident with radio emission. We also report an anti-correlated hard X-ray delay of 113$\pm$51 s using the NuSTAR data of GX 17+2. Based on RXTE data, it was found that soft and hard X-ray fluxes are varying indicating the changes in the disk-corona structure during delays. We bridle the size of corona using relativistic precession, transition layer and boundary layer models. Assuming the delays to be readjustment time scale of disk-corona structure, height of the corona was estimated to be $\sim$ 17--100 km. Assuming that inner region of the truncated disk is occupied by the corona, we constrain the coronal readjustment velocities (v$_{corona}$=$β$ v$_{disk}$, where v$_{disk}$ is the radial velocity component of the disk) of the order of $β$=0.06-0.12. This study indicates that the observed delays are primarily dependent on the varying coronal readjustment velocities.

astro-ph.HE↗

arXiv2022

Anti-Correlated lags in a Neutron star Z source GX 5-1: AstroSat's View

We report the cross-correlation function studies of a Neutron star low mass X-ray binary, a Z source GX 5-1 using SXT and 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 keV 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 which were exhibiting Horizontal Branch Oscillation variations. It was observed that the bbody component assumed to be originating from the boundary layer over the NS, 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.

astro-ph.HE↗

arXiv2021

Influence of Comptonization region over the ambiance of accretion disc in Active Galactic Nucleus

Understanding the radiative and physical structures of inner region of a few 100 pc of AGNs is important to constrain the causes of their activities. Although the X-ray emission from the Comptonization region/corona and the accretion disc regulates the broad line emission regions and torus structures, the exact mutual dependency is not understood well. We performed correlation studies for X-ray, mid-infrared, and different components of Balmer emission lines for the selected sample of AGNs. Almost 10 different parameters and their inter-dependencies were explored in order to understand the underlying astrophysics. We found that the X-ray luminosity has a linear dependency on the various components of broad Balmer emission lines (e.g. L$_{\text{2-10 keV}}$ $\propto$ L$^{0.78}_{\text{H}β^{\text{B}}}$) and found a strong dependency on the optical continuum luminosity (L$_{\text{2-10 keV}}$ $\propto$ L$^{0.86}_{5100\,\textÅ}$). For a selected sample, we also observed a linear dependency between X-ray and mid-infrared luminosity (L$_{\text{2-10 keV}}$ $\propto$ L$^{0.74}_{6\,\upmu \text{m}}$). A break point was observed in our correlation studies for X-ray power-law index, $Γ$, and mass of black hole at $\sim$ log (M/M$_{\odot}$) = 8.95. Similarly the relations between $Γ$ and FWHM of H$α$ and H$β$ broad components show breaks at FWHM$_{\text{H}α}$= 7642$\pm$657 km s$^{-1}$ and FWHM$_{\text{H}β}$ = 7336$\pm$650 km s$^{-1}$. However, more data are required to confine the breaks locations exactly. We noted that $Γ$ and Eddington ratios are negatively correlated to Balmer decrements in our selected sample. We analyzed and discussed about the implications of new findings in terms of interaction AGN structures.

astro-ph.HE↗

arXiv2021

Investigating the coronal structure by studying time lags in the Atoll source 4U 1705-44 using AstroSat

We performed a detailed timing study of the Atoll source 4U 1705-44 in order to understand the accretion disk geometry. Cross correlation function (CCF) studies were performed between soft (3-5 keV) and hard energy (15-30 keV) bands using the AstroSat LAXPC data. We detected hard as well as soft lags of the order of few ten to hundred seconds. A dynamical CCF study was performed in the same energy bands for one of the light curves and we found smaller lags of few tens of seconds ($<$ 50 s) suggesting that the variation is probably originating from the corona. We found a broad noise component around $\sim$ 13 Hz in the 3-10 keV band which is absent in 10-20 keV band. We interpret the observed lags as the readjustment timescales of the corona or a boundary layer around the neutron star and constrain the height of this structure to few tens of km. We independently estimated the coronal height to be around 15 km assuming that the 13 Hz feature in the PDS is originating from the oscillation of the viscous shell around the neutron star.

astro-ph.HE↗

arXiv2021

Exploring the inner-disk region of the atoll source 4U 1705-44 using AstroSat's SXT and LAXPC observations

For the first time, simultaneous broadband spectral and timing study of the atoll source 4U 1705-44 was performed using AstroSat Soft X-ray Telescope (SXT) and Large Area X-ray Proportional Counter (LAXPC) data (0.8-70 keV). Based on the HID, the source was in the soft banana state during these observations. Spectral modeling was performed using the full reflection framework and an inner disk radii of 14 Rg was obtained. A hard powerlaw tail was noticed in the soft state and hot component fluxes and varying powerlaw indices point towards a varying corona/sub-keplerian flow. Based on the spectral fits the boundary layer radius and magnetospheric radius were constrained to be $\sim$ 14-18 km and $\sim$ 9-19 km respectively. Cross- Correlation Function studies were performed between the 0.8-3 keV soft SXT lightcurve and 10-20 keV hard LAXPC lightcurve and correlated and anticorrelated lags were found, which was used to constrain the coronal height to 0.6-20 km (\b{eta}=0.1). Since the inner disk radius is not varying during the observations, we conclude that the detected lags are possibly caused by a varying structure of corona/boundary layer in the inner region of the accretion disk. Based on the observations, a geometrical model is proposed for explaining the detected lags in the atoll source 4U 1705-44.

astro-ph.HE↗

arXiv2021

Understanding the Inner Structure of Accretion disk in GX 17+2: AstroSat's Outlook

We performed the timing and spectral studies of a Z source GX 17+2 observed from Astrosat LAXPC instrument. Cross-Correlation function (CCF) was performed using soft (3-5 keV) and hard (16-40 keV) X -ray bands across the hardness intensity diagram and found correlated/anti-correlated hard and soft lags which seems to be a common feature in these sources. We performed spectral analysis for few of these observations and found no consistent variation in the spectral parameters during the lags, however 10-40% change was noticed in diskbb and power-law components in few of observations. For the first time, we report the detection of HBOs around $\sim$25 Hz and $\sim$ 33 Hz along with their harmonics using AstroSat LAXPC data. On comparison with spectral results of HB and other branches, we found that inner disk front is close to the last stable orbit and as such no systematic variations are observed. We suggest that the detected lags are readjustment time scales of corona close to the NS and constrained its height to be around few tens to hundreds of km. The detected lags and no significant variation of inner disk front across the HID strongly indicate that structural variation in corona is the most possible cause of Z track in HID.

astro-ph.HE↗

arXiv2021

Spectro-temporal studies of rapid transition of the quasi periodic oscillations in the black hole source H1743-322

An appearance or disappearance of QPOs associated with the variation of X-ray flux can be used to decipher the accretion ejection mechanism of black hole X-ray sources. We searched and studied such rapid transitions in H1743-322 using RXTE archival data and found eight such events, where QPO vanishes suddenly along with the variation of X-ray flux. The appearance/disappearance of QPOs were associated to the four events exhibiting type-B QPOs at $\sim$ 4.5 Hz, one with type-A QPO at $ν$ $\sim$ 3.5 Hz, and the remaining three were connected to type-C QPOs at $\sim$ 9.5 Hz. Spectral studies of the data unveiled that an inner disk radius remained at the same location around 2-9 r$_g$ , depending on the used model but power-law indices were varying, indicating that either corona or jet is responsible for the events. The probable ejection radii of corona were estimated to be around 4.2-15.4 r$_g$ based on the plasma ejection model. Our X-ray and quasi-simultaneous radio correlation studies suggest that the type-B QPOs are probably related to the precession of a weak jet though a small and weak corona is present at its base and the type-C QPOs are associated to the base of a relatively strong jet which is acting like a corona.

astro-ph.HE↗

arXiv2020

Coronal Vertical Structure Variations in Normal Branch of GX 17+2 : AstroSaťs SXT and LAXPC perspective

We performed spectro-temporal analysis in the 0.8--50 keV energy band of the neutron star Z source GX 17+2 using AstroSat Soft X-ray Telescope (SXT) and Large Area X-ray Proportional Counter (LAXPC) data. The source was found to vary in the normal branch of the Hardness Intensity Diagram. Cross-correlation studies of LAXPC light curves in soft and hard X-ray band unveiled anticorrelated lags of the order of few hundred seconds. For the first time, Cross-correlation studies were performed using SXT soft and LAXPC hard lightcurves and they exhibited correlated and anti-correlated lags of the order of a hundred seconds. Power density spectrum displayed NB oscillations of 6.7--7.8 Hz (quality factor 1.5--4.0). Spectral modeling resulted in inner disk radius of $\sim$ 12--16 km with $Γ$ $\sim$ 2.31--2.44 indicating that disk is close to the ISCO and a similar value of disk radius was noticed based on the reflection model. Different methods were used to constrain the corona size in GX 17+2. Using the detected lags, corona size was found to be 27-46 km ($β$ = 0.1, $β$ = v$_{corona}$/v$_{disk}$) and 138--231 km ($β$ = 0.5). Assuming the X-ray emission to be arising from the Boundary Layer (BL), its size was determined to be 57--71 km. Assuming that BL is ionizing the disk's inner region, it's size was constrained to $\sim$ 19--86 km. Using NBO frequency, the transition shell radius was found to be around 32 km. Observed lags and no movement of the inner disk front strongly indicates that the varying corona structure is causing the X-ray variation in the NB of Z source GX 17+2.

astro-ph.HE↗

arXiv2014

Photometry and H$alpha$ studies of a Low Mass Ratio Overcontact binary ASAS J082243+1927.0

Both high precision CCD photometric and H$alpha$ line studies are presented for an overcontact binary ASAS J082243+1927.0. The light curve exhibits a total eclipse at secondary minima along with an O'Connell effect. The light curve was modeled using the Wilson Devinney code and the best solution provides the mass ratio q = 0.106 and fill-out factor f = 72%. These parameters indicate that the system is a low mass ratio overcontact binary with a high degree of geometrical contact. The H alpha line equivalent width varied at different phases and it is found that the line is possibly filled-in at secondary minima. From a small sample of overcontact binaries, we found a correlation between the orbital period and H alpha line equivalent width of the primary component. Based on a sample of high filling factor and low mass ratio contact binaries, a mass ratio cut-off is observed at q = 0.085 in mass ratio - period plane. It was observed that below q $\le$ 0.085, period decreases with an increase in q and above it, period increases as the mass ratio increases. Interestingly, the observed mass ratio cut-off value lies close to the critical mass ratio range as predicted in the literature. The observational evidence of the cut-off of the mass ratio and its variation with orbital period are discussed in terms of mass transfer and angular momentum loss. Based on the results, we suggest that, ASAS J082243+1927.0 is at the verge of merger, eventually forming a fast rotating star.

astro-ph.SR↗

arXiv2013

Photometric study of an Eclipsing Binary in Praesepe

We present CCD photometric observations of an eclipsing binary in the direction of the open cluster Praesepe using the 2 m telescope of IUCAA Girawali Observatory, India. Though the system was classified as an eclipsing binary by Pepper et al.(2008),detail investigations were lacking. The photometric solutions using the Wilson-Devinney code suggest that it is a W-type W UMa system and interestingly, the system parameters were similar to another contact binary system SW Lac.

astro-ph.SR↗

arXiv2012

Anti-correlated time lags in the Z source GX 5-1: Possible evidence for a truncated accretion disk

We investigate the nature of the inner accretion disk in the neutron star source GX 5-1 by making a detailed study of time lags between X-rays of different energies. Using the cross-correlation analysis, we found anti-correlated hard and soft time lags of the order of a few tens to a few hundred seconds and the corresponding intensity states were mostly the horizontal branch (HB) and upper normal branch (NB). The model independent and dependent spectral analysis showed that during these time lags the structure of accretion disk significantly varied. Both eastern and western approaches were used to unfold the X-ray continuum and systematic changes were observed in soft and hard spectral components. These changes along with a systematic shift in the frequency of quasi-periodic oscillations (QPOs) made it substantially evident that the geometry of the accretion disk is truncated. Simultaneous energy spectral and power density spectral study shows that the production of the horizontal branch oscillations (HBOs) are closely related to the Comptonizing region rather than the disk component in the accretion disk. We found that as the HBO frequency decreases from the hard apex to upper HB, the disk temperature increases along with an increase in the coronal temperature which is in sharp contrast with the changes found in black hole binaries where the decrease in QPO frequency is accompanied by a decrease in the disk temperature and a simultaneous increase in the coronal temperature. We discuss the results in the context of re-condensation of coronal material in the inner region of the disk.

astro-ph.HE↗

arXiv2012

Fast transition of type-B QPO in the black hole transient XTE J1817-330

The evolution of different types of quasi-periodic oscillations (QPOs) and the coupled radiative/physical changes in the accretion disk are still poorly understood. In a few black hole binaries it was found that fast evolution of QPOs is associated with spectral variations. Such studies in other black hole binaries are important to understand the QPO phenomenon. For the black hole transient XTE J1817-330, we study fast QPO transitions and accompanying spectral variations to investigate what causes the spectral variation during the QPO transition. Roy et al. (2011) found QPOs in ten RXTE observations of XTE J1817-330. We found that, among the ten observations, only one observation shows erratic dips in its X-ray light curve. The power density spectra and the corresponding energy spectra were extracted and analyzed for the dip and non-dip sections of the light curve. We found that type-B $\sim$6 Hz QPO changes into type-A QPO in a few tens of seconds along with a flux decrease. This transient evolution is accompanied with a significant spectral variation. We report a transient QPO feature and accompanying spectral variation in XTE J1817-330. Based on our findings, we discuss the origin of fast evolution of QPOs and spectral variations.

astro-ph.HE↗

arXiv2011

Coupled HBO and NBO variations in the Z source GX 5-1: inner accretion disk as the location of QPOs

The simultaneous and coupled evolution of horizontal branch oscillation (HBO) and normal branch oscillation (NBO) in Z-type sources suggests that the production of HBO is connected to NBO and is caused by changes in the physical/radiative properties of the inner accretion disk, although there is a lack of substantial spectral evidence to support this. In this {\it Letter}, we present the results of an analysis of a RXTE observation of a Z source GX~5-1, where the 6 Hz NBO is simultaneously detected along with a HBO at 51 Hz. The variations in the intensity and the associated power density spectrum indicate that the HBO and NBO are strongly coupled, originating from the same location in the inner accretion disk. The absence of HBO and NBO in the lower energy bands, an increase in the rms amplitude with energy and a smooth transition among them suggest that they are produced in the hot inner regions of the accretion disk. Based on a spectral analysis, we found a signature of changing or physically modified inner disk front during the coupled HBO and NBO evolution. We explore the various models to explain the observed phenomenon and propose that the NBO is affiliated to the oscillations in the thick/puffed-up inner region of the accretion disk.

astro-ph.HE↗

arXiv2011

XMM-Newton confirmation of a new intermediate polar: XMMU J185330.7-012815

We report the results from a detailed spectro-imaging and temporal analysis of an archival XMM-Newton observation of a new intermediate polar XMMU J185330.7-012815. Its X-ray spectrum can be well-described by a multi-temperature thermal plasma model with the K-lines of heavy elements clearly detected. Possible counterparts of XMMU J185330.7-012815 have been identified in optical and UV bands. The low value of the inferred X-ray-to-UV and X-ray-to-optical flux ratios help to safely rule out the possibility as an isolated neutron star. We confirm the X-ray periodicity of ~238 s, but different from the previous preliminary result, we do not find any convincing evidence of phase-shift in this observation. We further investigate its properties through an energy-resolved temporal analysis and find the pulsed fraction monotonically increases with energy.

astro-ph.HE↗