arXiv subjects

Ranjeev Misra

Publications and source records attributed to Ranjeev Misra.

At least 73 records · Page 4Linked to original sources

arXiv2023

Weak secondary cyclotron line in eclipsing High Mass X-ray Binary Cen X-3

We report the time resolved spectroscopy result from two observations of Cen X-3, over one binary orbit with ASTROSAT and two binary orbit with NuSTAR. NuSTAR covered two intensity states where the light curve showed transition in count rate from first to second binary orbit by a factor of $\sim$ 3. A phenomenological model comprising of partially absorbed powerlaw with smoothed high energy cutoff, cyclotron absorption $\sim$ 24 keV and 6.4 keV iron emission gave good fit for ASTROSAT observation. NuSTAR spectra required two additional emission components, a broad one $\sim$ 5.7 keV and a narrow one $\sim$ 6.9 keV. A weak secondary absorption feature at $\sim$ 11.6 keV and $\sim$ 14.5 keV was seen in the residuals of the spectral fit for ASTROSAT and NuSTAR data respectively. The secondary absorption energy showed no correlation with the cutoff energy. Its strength varied within 0.1 to 0.6 keV with its width $\sim$ 1.6 keV. Its energy and optical depth showed linear positive correlation with the fundamental cyclotron line energy and depth respectively. The cyclotron line energy showed anti-correlation to flux described by a powerlaw with negative index and the secondary absorption also showed similar trend to flux. Depth of secondary absorption was $\sim$ 45 \% and centroid energy was $\sim$ 54 \% of fundamental. Depth and energy ratio of secondary to fundamental lied within 2$σ$ deviation from 0.5. We suggest this secondary absorption to be a redshifted dipolar cyclotron resonance feature exhibiting sub-harmonic behaviour.

astro-ph.HE↗

arXiv2023

A Search for X-ray/UV Correlation in the Reflection-Dominated Seyfert 1 Galaxy Mrk 1044

Correlated variability between coronal X-rays and disc optical/UV photons provides a very useful diagnostic of the interplay between the different regions around an active galactic nucleus (AGN) and how they interact. AGN that reveal strong X-ray reflection in their spectra should normally exhibit optical/UV to X-ray correlation consistent with reprocessing -- where the optical/UV emission lag behind the X-rays. While such correlated delay has been seen in some sources, it has been absent in others. \rm{Mrk~1044} is one such source that has been known to reveal strong X-ray reflection in its spectra. In our analysis of three long \textit{XMM-Newton} and several \textit{Swift} observations of the source, we found no strong evidence for correlation between its UV and X-ray lightcurves both on short and long time scales. Among other plausible causes for the non-detection, we posit that higher X-ray variability than UV and strong general relativistic effects close to the black hole may also be responsible. We also present results from the spectral analysis based on \textit{XMM-Newton} and \textit{NuSTAR} observations, which show the strong soft X-ray excess and iron K$α$ line in the 0.3--50 keV spectrum that can be described by relativistic reflection.

astro-ph.HE↗

arXiv2023

Investigating the Energy-Dependent Temporal Nature of Black Hole Binary System H 1743-322

Black hole X-ray binaries routinely exhibit Quasi Periodic Oscillations (QPOs) in their Power density spectrum. Studies of QPOs have demonstrated immense ability to understand these dynamical systems although their unambiguous origin still remains a challenge. We investigate the energy-dependent properties of the Type-C QPOs detected for H 1743-322 as observed with AstroSat in its two X-ray outbursts of 2016 and 2017. The combined broadband LAXPC and SXT spectrum is well modelled with a soft thermal and a hard Comptonization component. The QPO exhibits soft/negative lags i.e. variation in soft band lags the variation in hard band, although the upper harmonic shows opposite behaviour i.e. hard/positive lags. Here, we model energy-dependent properties (fractional root mean square and time-lag variation with energy) of the QPO and its upper harmonic individually with a general scheme that fits these properties by utilizing the spectral information and consequently allows to identify the radiative component responsible for producing the variability. Considering the truncated disk picture of accretion flow, a simple model with variation in inner disk temperature, heating rate and fractional scattering with time delays is able to describe the fractional RMS and time-lag spectra. In this work, we show that this technique can successfully describe the energy-dependent features and identify the spectral parameters generating the variability.

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arXiv2023

Probing the soft state evolution of 4U 1543-47 during its 2021 outburst using AstroSat

4U 1543-47 underwent its brightest outburst in 2021 after two decades of inactivity. During its decay phase, AstroSat conducted nine observations of the source spanning from July $1^{st}$ to September $26^{th}$, 2021. The first three observations were performed with an offset of 40 arcmin with AstroSat/LAXPC, while the remaining six were on-axis observations. In this report, we present a comprehensive spectral analysis of the source as it was in the High/Soft state during the entire observation period. The source exhibited a disk-dominated spectra with a weak high-energy tail (power-law index $\geq2.5$) and a high inner disk temperature ($\sim$0.84 keV). Modelling the disk continuum with non-relativistic and relativistic models, we find inner radius to be significantly truncated at $>$$10~R_g$. Alternatively, to model the spectral evolution with the assumption that the inner disk is at the ISCO, it is necessary to introduce variation in the spectral hardening in the range $\sim$1.5-1.9.

astro-ph.HE↗

arXiv2023

Testing the dynamic origin of Quasi-periodic Oscillations in MAXI J1535-571 and H 1743-322

We investigate spectro-temporal properties for two black hole X-ray binary sources, MAXI J1535$-$571 and H 1743$-$322, during their hard and hard-intermediate states. For MAXI J1535$-$571, we analyze Swift/XRT, NuSTAR and NICER observations, specifically focusing on the occurrence of type-C Quasi-periodic Oscillations (QPOs). Regarding H 1743$-$322, we analyze multi-epoch observations of NICER and AstroSat, identifying a type-C QPO with centroid frequency ranging from 0.1--0.6 Hz. In both sources, we fit the spectra with a relativistic truncated disk and a power law component. In MAXI J1535$-$571, we also observe an additional relativistically smeared iron line. Through temporal and spectral analysis, we estimate the QPO centroid frequency and spectral parameters, such as the accretion rate and inner disc radii. We test the origin of type-C QPOs as relativistic precession frequency and dynamic frequency (i.e. the inverse of the sound crossing time $\frac{r}{c_s(r)}$). The dependence of QPO frequency on both the accretion rate and inner disc radii favours the QPO origin as dynamic frequency. We discuss the implications of these results.

astro-ph.HE↗

arXiv2023

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

In this work, we study the correlation between Quasi-periodic Oscillation (QPO) frequency and the spectral parameters during various X-ray states in the black hole binary GRS 1915+105 which matches well with the predicted relativistic dynamic frequency (i.e. the inverse of the sound crossing time) at the truncated radii. We have used broadband data of LAXPC and SXT instruments onboard AstroSat. Spectral fitting shows that the accretion rate varies from $\sim 0.1$ to $\sim 5.0 \times 10^{18}$ gm/s and the truncated radius changing from the last stable orbit of an almost maximally spinning black hole, $\sim$ 1.2 to $\sim$ 19 Gravitational radii. For this wide range, the frequencies of the C-type QPO (2 - 6 Hz) follow the trend predicted by the relativistic dynamical frequency model and interestingly, the high-frequency QPO at $\sim$ 70 Hz also follows the same trend, suggesting they originate from the innermost stable circular orbit with the same mechanism as the more commonly observed C-type QPO. While the qualitative trend is as predicted, there are quantitative deviations between the data and the theory, and the possible reasons for these deviations are discussed.

astro-ph.HE↗

arXiv2022

Broadband Spectral properties of MAXI J1348--630 using AstroSat Observations

We present broadband X-ray spectral analysis of the black hole X-ray binary MAXI J1348--630, performed using five AstroSat observations. The source was in the soft spectral state for the first three and in the hard state for the last two observations. The three soft state spectra were modelled using a relativistic thin accretion disc with reflection features and thermal Comptonization. Joint fitting of the soft state spectra constrained the spin parameter of the black hole $a_*$ $>$ 0.97 and the disc inclination angle $i$ = 32.9$\substack{+4.1 \\ -0.6}$ degrees. The bright and faint hard states had bolometric flux a factor of $\sim 6$ and $\sim 10$ less than that of the soft state. Their spectra were fitted using the same model except that the inner disc radius was not assumed to be at the last stable orbit. However, the estimated values do not indicate large truncation radii and the inferred accretion rate in the disc was an order of magnitude lower than that of the soft state. Along with earlier reported temporal analysis, AstroSat data provides a comprehensive picture of the evolution of the source.

astro-ph.HE↗

arXiv2022

Correlated variability of the reflection fraction with the X-ray flux and spectral index for Mkn 478

The X-ray spectrum of Mkn 478 is known to be dominated by a strong soft excess which can be described using relativistic blurred reflection. Using observations from {\it XMM-Newton}, {\it AstroSat} and {\it Swift}, we show that for the long-term ($\sim$ years) and intermediate-term (days to months) variability, the reflection fraction is anti-correlated with the flux and spectral index, which implies that the variability is due to the hard X-ray producing corona moving closer to and further from the black hole. Using flux-resolved spectroscopy of the {\it XMM-Newton} data, we show that the reflection fraction has the same behaviour with flux and index on short time-scales of hours. The results indicate that both the long-term and short-term variability of the source is determined by the same physical mechanism of strong gravitational light bending causing enhanced reflection and low flux as the corona moves closer to the black hole.

astro-ph.HE↗

arXiv2022

Correlations between X-ray spectral parameters of Mkn 421 using long-term Swift-XRT data

We have performed a detailed analysis of the X-ray spectra of the blazar Mkn 421 using Swift-XRT observations taken between 2005 and 2020, to quantify the correlations between spectral parameters for different models. In an earlier work, it has been shown that such spectral parameter correlations obtained from a single short flare of duration $\sim$ 5-days of Mkn 421, can be used to distinguish spectrally degenerate models and provide estimates of physical quantities. In this work, we show that the results from the long-term spectral parameter correlations are consistent with those obtained from the single flare. In particular, that the observed spectral curvature is due to maximum cutoff energy in the particle distribution is ruled out. Instead, models where the curvature is due to the energy dependence of escape or acceleration time-scale of the particles are favored. The estimated values of the physical parameters for these models are similar to the ones obtained from the single flare analysis and are somewhat incompatible with the physical assumption of the models, suggesting that more complex physical models are required. The consistency of the results obtained from the long and short-term evolution of the source, underlines the reliability of the technique to use spectral parameter correlations to distinguish physical models.

astro-ph.HE↗

arXiv2022

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

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

astro-ph.HE↗

arXiv2022

On the energy dependence of the QPO phenomenon in the black hole system MAXI J1535-571

Previous analysis of AstroSat observations of the black hole system MAXI J1535-571, have revealed the presence of a strong Quasi-Periodic Oscillation (QPO) whose frequency is correlated with the high energy spectral index. Here, we fit the spectra as emitted from a truncated disc with an inner hot corona, study the QPO frequency dependence on other spectral parameters and model the energy dependent r.m.s and time-lag of the QPO to identify the physical spectral parameters whose variation are responsible for the QPO. The QPO frequency is found to also correlate with the scattering fraction (i.e. the fraction of the soft photons Comptonized) and its dependence on the accretion rate and inner disc radii is consistent with it being the dynamical frequency. The time-lag between the hard and soft photons is negative for QPO frequency > 2.2 Hz and is positive for lesser values, making this the second black hole system to show this behaviour after GRS 1915+105. Modelling the energy dependent time-lag and r.m.s requires correlated variation of the accretion rate, inner disc radii and the coronal heating rate, with the latter having a time-lag compared to the other two for QPO frequencies less than < 2.2 Hz and which changes sign (i.e. the coronal heating variation precedes the accretion rate one) for higher values. The implications of the results are discussed.

astro-ph.HE↗

arXiv2022

Probing the shot behaviour in Cygnus X-1 using simultaneous AstroSat-NICER observation

We analyse the aperiodic flaring features, also known as shots, observed in Cyg X-1 in the 0.1-80 keV energy band using a 6.39 ks simultaneous observation with AstroSat and NICER. We detect 49 simultaneous shots in the soft and hard X-ray bands with NICER and AstroSat-LAXPC, respectively. We observe the shot profile for the first time in soft X-rays (0.1-3 keV), which shows a spectral peak at $\sim$2 keV. Using time-averaged spectroscopy, we measured the truncation of the inner accretion disk at $6.7\pm0.2$ gravitational radii. The shot-phase resolved spectroscopy allowed us to identify the origin of some of the brightest aperiodic peaks in the soft X-rays. We find that the accretion rate is consistent with a constant during the shots while the inner edge of the accretion disk moves inwards/outwards as these shots rise/decay. We discuss the possible mechanisms causing the swing in the inner radius.

astro-ph.HE↗

arXiv2022

Spectral properties of soft X-ray transient MAXI J0637$-$430 using AstroSat

Soft X-ray transients are systems that are detected when they go into an outburst, wherein their X-ray luminosity increases several orders of magnitude. These outbursts are markers of the poorly understood change in the spectral state of these systems from low/hard state to high/soft state. We report the spectral properties of one such soft X-ray transient: MAXI J0637$-$430, with data from the \textit{SXT} and \textit{LAXPC} instruments on-board \textit{AstroSat} mission. The source was observed for a total of $\sim$ 60 ks over two observations on 8$^{th}$ and 21$^{st}$ November, 2019 soon after its discovery. Flux resolved spectral analysis of the source indicates the presence of a multi-colour blackbody component arising from the accretion disk and a thermal Comptonization component. The stable low temperature ($\sim$ 0.55 $keV$) of the blackbody component, points to a cool accretion disk with an inner disk radius of the order of a few hundred $km$. In addition, we report the presence of a relativistically broadened Gaussian line at 6.4 $keV$. The disk dominated flux and photon power law index of $\gtrapprox 2$ and a constant inner disk radius indicate the source to be in the soft state. From the study we conclude that MAXI J0637$-$430 is a strong black hole X-ray binary candidate.

astro-ph.HE↗

arXiv2022

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

We present the spectro-timing analysis of the black hole binary system GX 339-4 using AstroSat data extracted at the beginning of outbursts in 2017 and 2019. The joint spectral fitting of LAXPC and SXT data revealed that the source was in a faint low/hard state for both 2017 and 2019 observations with a nearly equal photon index of $\sim$1.57 and $\sim$1.58 and with Eddington ratio, $L/L_{Edd}$, of 0.0011 and 0.0046 respectively. The addition of a reflection component into the spectral modelling improved the fit ($Δχ^2$ $\approx~6$ for 2017 and $Δχ^2$ $\approx~7$ for 2019), pointing towards the presence of weak reflection features arising due to irradiation of the accretion disk. The power density spectrum (PDS) consisted of strong band-limited noise with a break at low frequencies, described with a combination of few zero-centered Lorentzian. The fitting revealed a low-frequency break at $\sim$6~mHz for 2017 and at $\sim$11~mHz for 2019 observation, whose detection is validated by results from independent detectors (LAXPCs and SXT). The break frequency is roughly consistent with results obtained from earlier observations that showed an evolution of the frequency with flux, which is in accordance with the truncated disk model. Associating the break frequency with viscous time scale of the accretion disk, we estimated a truncation radius of $\sim$93 and $\sim$61 gravitational radius for 2017 and 2019 observation, respectively.

astro-ph.HE↗

arXiv2022

AstroSat Detection of a Quasi-periodic Oscillation at ~ 42 Hz in Cygnus X-2

We report the results of AstroSat observations of Cygnus X-2 during February 2016. The source's power density spectrum generated using LAXPC data revealed the presence of a prominent Quasi-periodic Oscillation (QPO) at $\sim42$ Hz with broadband continuum noise at lower frequencies at $\sim10$ Hz. The large effective area of LAXPC at $\gtrsim$30 keV allowed for an unprecedented study of the energy dependence of the QPO and the broad noise continuum. The fractional r.m.s increases with energy, and its shape is similar for both the QPO and the continuum noise, suggesting a common radiative origin. However, while the QPO exhibits hard time lags, with the high energy photons lagging the low ones by a few milliseconds, the continuum noise shows the opposite behavior. The photon spectrum from SXT and LAXPC in $0.7-30$ keV band comprises the soft component from a disc and a hard Comptonized component from a hot corona. While the energy dependence of the r.m.s shows that the QPO and the continuum noise variability are dominated by the Comptonized component, the change in sign of the time-lag suggests that the dynamic origin of the QPO may be in the disk while the noise continuum may originate from the corona.

astro-ph.HE↗

arXiv2022

Time-resolved spectroscopy on the heartbeat state of GRS 1915+105 using AstroSat

AstroSat spectra of the black hole system GRS 1915+105 during the heartbeat state (with a varying oscillation period from 150 to 100 secs) were analysed using a truncated relativistic disc model along with a Comptonization component. Spectra were fitted for segments of length ~ 24 secs. The oscillation can be described as coordinated variations of the accretion rate, Comptonised flux, and the inner disc radius, with the latter ranging from 1.235-5 gravitational radii. Comparison with results from the $χ$ and Intermediate states shows that while the accretion rate and the high energy photon index were similar, the inner disc radius and the fraction of Comptonised photons were larger for these states than for the heartbeat one. The coronal efficiency $η\equiv L_{ac}/\dot M c^2$, where $L_{ac}$ is the radiative luminosity generated in the corona is found to be approximately $\propto \dot M^{-2/3}$ for all the observations. The efficiency decreases with inner radii for the heartbeat state but has similar values for the $χ$ and Intermediate states where the inner radii is larger. The implications of these results are discussed.

astro-ph.HE↗

arXiv2021

Evidence for dynamical changes in Betelgeuse using multi-wavelength data

The reasons behind the Great Dimming and subsequent rising in the brightness of Betelgeuse between October 2019 and March 2020 still continue to baffle astronomers. It has been shown by George et. al. (2020) that critical slowing down preceded the dimming event. This suggested that the dimming was a result of the change in the nature of the nonlinear dynamics of the star. In this work we present additional evidence for dynamical changes in Betelgeuse prior to the Great Dimming event, using nonlinear time series analysis. We study the relations between the different bands in the photometry data collected from the Wing photometry (IR/near-IR) and Wasatonic observatory (V-band). We also analyse how the early warning signals studied previously changed during and after the Great Dimming.

astro-ph.SR↗

arXiv2021

Understanding the X-ray spectral curvature of Mkn 421 using broadband AstroSat observations

We present a time-resolved X-ray spectral study of the high energy peaked blazar Mkn 421 using simultaneous broadband observations from the LAXPC and SXT instruments on-board AstroSat. The ~ 400 ksec long observation taken during 3-8 January, 2017 was divided into segments of 10 ksecs. Each segment was fitted using synchrotron emission from particles whose energy distribution was represented by a log-parabola model. We also considered particle energy distribution models where (i) the radiative cooling leads to a maximum energy (ξ max model), (ii) the system has energy dependent diffusion (EDD) and (iii) has energy dependent acceleration (EDA). We found that all these models describe the spectra, although the EDD and EDA models were marginally better. Time resolved spectral analysis allowed for studying the correlation between the spectral parameters for different models. In the simplest and direct approach, the observed correlations are not compatible with the predictions of the ξ max model. While the EDD and EDA models do predict the correlations, the values of the inferred physical parameters are not compatible with the model assumptions. Thus, we show that spectrally degenerate models, can be distinguished based on spectral parameter correlations (especially those between the model normalization and spectral shape ones) making time-resolved spectroscopy a powerful tool to probe the nature of these systems.

astro-ph.GA↗