arXiv subjects

Divya Rawat

Publications and source records attributed to Divya Rawat.

24 records · Page 2Linked to original sources

arXiv2021

Electron-Phonon Coupling and Quantum Correction to Topological Magnetoconductivity in Bi2GeTe4

We report on structure, vibrational properties and weak-antilocalization-(WAL-) induced quantum correction to magnetoconductivity in single crystal Bi2GeTe4. Surface band structure calculations show a single Dirac cone corresponding to topological surface states in Bi2GeTe4. An estimated phase coherence length, l_ϕ ~ 143 nm and prefactor α ~ - 1.54 from Hikami-Larkin-Nagaoka fitting of magnetoconductivity describe the quantum correction to conductivity. An anomalous temperature dependence of A1g Raman modes confirms enhanced electron-phonon interactions. Our results establish the involvement of vibrations of Bi-Te with existence of topological surface states and WAL in Bi2GeTe4.

cond-mat.mtrl-sci↗

arXiv2021

Testing evolution of LFQPOs with mass accretion rate in GRS 1915+105 with Insight-HXMT

Using the Insight-HXMT observations of GRS 1915+105 when it exhibits low frequency quasi-periodic oscillations (QPOs), we measure the evolution of the QPO frequency along with disk inner radius and mass accretion rate. We find a tight positive correlation between the QPO frequency and mass accretion rate. Our results extend the finding of previous work with AstroSat to a larger range of accretion rate with independent instruments and observations. Treating the QPO frequency of GRS 1915+105 as the relativistic dynamic frequency of a truncated disk, we are able to confirm the high spin nature of the black hole in GRS 1915+105. We also address the potential of our finding to test general relativity in the future.

astro-ph.HE↗

arXiv2020

Identification of QPO frequency of GRS 1915+105 as the relativistic dynamic frequency of a truncated accretion disk

We have analyzed AstroSat observations of the galactic micro-quasar system GRS 1915+105, when the system exhibited C-type Quasi-periodic Oscillations (QPOs) in the frequency range of 3.4-5.4 Hz. The broad band spectra (1-50 keV) obtained from simultaneous LAXPC and SXT can be well described by a dominant relativistic truncated accretion disk along with thermal Comptonization and reflection. We find that while the QPO frequency depends on the inner radii with a large scatter, a much tighter correlation is obtained when both the inner radii and accretion rate of the disk are taken into account. In fact, the frequency varies just as the dynamic frequency (i.e. the inverse of the sound crossing time) as predicted decades ago by the relativistic standard accretion disk theory for a black hole with spin parameter of ~0.9. We show that this identification has been possible due to the simultaneous broad band spectral coverage with temporal information as obtained from AstroSat.

astro-ph.HE↗

arXiv2018

Study of timing evolution from non-variable to structured large-amplitude variability transition in GRS 1915+105 using AstroSat

In this work, we present a $\sim$90 ks continuous monitoring of the Galactic micro-quasar GRS 1915+105 with AstroSat when the source undergoes a major transition from a non-variable, $χ$ class (similar to radio-quiet $χ$ class) to a structured, large amplitude, periodic heartbeat state (similar to $ρ$ class). We show that such transition takes place via an intermediate state when the large-amplitude, irregular variability of the order of hundreds of seconds in the soft X-ray band turned into a 100-150 sec regular, structured, nearly periodic flares. The properties of a strong low-frequency quasi-periodic oscillations (LF QPO) in the frequency range 3-5 Hz also evolve marginally during these variability transitions. We also study time-lag and rms spectra at the QPO and harmonic component and the dynamic power spectra. We note few important differences between the heartbeat state and the $ρ$ class. Interestingly, the time-averaged LF QPO properties in the hard X-ray band is relatively stable in three states when compared to the significant evolution observed in the slow variability properties at mHz frequencies. Such relative stability of LF QPOs implies the inner disk-corona coupled accretion flow which determines the LF QPO properties, may be uninterrupted by the launch of long, large-amplitude flares.

astro-ph.HE↗

arXiv2018

Data-Driven Application Maintenance: Views from the Trenches

In this paper we present our experience during design, development, and pilot deployments of a data-driven machine learning based application maintenance solution. We implemented a proof of concept to address a spectrum of interrelated problems encountered in application maintenance projects including duplicate incident ticket identification, assignee recommendation, theme mining, and mapping of incidents to business processes. In the context of IT services, these problems are frequently encountered, yet there is a gap in bringing automation and optimization. Despite long-standing research around mining and analysis of software repositories, such research outputs are not adopted well in practice due to the constraints these solutions impose on the users. We discuss need for designing pragmatic solutions with low barriers to adoption and addressing right level of complexity of problems with respect to underlying business constraints and nature of data.

cs.SE↗

arXiv2018

Extensive broadband X-ray monitoring during the formation of a giant radio jet base in Cyg X-3 with AstroSat

We present X-ray spectral and timing behavior of Cyg X-3 as observed by AstroSat during the onset of a giant radio flare on 01-02 April 2017. Within a time-scale of few hours, the source shows a transition from the hypersoft state (HPS) to a more luminous state (we termed as the very high state) which coincides with the time of the steep rise in radio flux density by an order of magnitude. Modeling the SXT and LAXPC spectra jointly in 0.5-70.0 keV, we found that the first few hours of the observation is dominated by the HPS with no significant counts above 17 keV. Later, an additional flat powerlaw component suddenly appeared in the spectra which extends to very high energies with the powerlaw photon index of 1.49 +/- 0.04. Such a flat powerlaw component has never been reported from Cyg X-3. Interestingly the fitted powerlaw model in 25-70 keV, when extrapolated to the radio frequency, predicts the radio flux density consistent with the trend measured from RATAN-600 telescope at 11.2 GHz. This provides a direct evidence of the synchrotron origin of flat X-ray powerlaw component and the most extensive monitoring of the broadband X-ray behavior at the moment of decoupling the giant radio jet base from the compact object in Cyg X-3. Using SXT and LAXPC observations, we determine the giant flare ejection time as MJD 57845.34 +/- 0.08 when 11.2 GHz radio flux density increases from ~100 to ~478 mJy.

astro-ph.HE↗