• V. K. AGRAWAL

      Articles written in Journal of Astrophysics and Astronomy

    • AstroSat observation of 2016 outburst of H 1743-322

      SWADESH CHAND V. K. AGRAWAL G. C. DEWANGAN PRAKASH TRIPATHI PARIJAT THAKUR

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      We present the detection of type C quasi-periodic oscillation (QPO) along with upper harmonic at respective frequencies of $\sim$0.6 Hz and $\sim$1.2 Hz in the single AstroSat observation taken during the 2016 outburst of the low-mass black hole X-ray binary H 1743-322. These frequencies are found to be shifted by $\sim$0.4 Hz for the QPO and $\sim$0.8 Hz for the upper harmonic with respect to that found in the simultaneous XMM-Newton and NuSTAR observation taken five days later than the AstroSat observation, indicating a certain geometrical change in the system. However, the centroid frequency of the QPO and the upper harmonic do not change with energy, indicating the energy-independent nature. The decreasing trend in thefractional rms of the QPO with energy is consistent with the previous results for this source in the low/hard state. The value of the photon index ($\Gamma \sim 1.67$) also indicates that the source was in the low/hard state duringthis particular observation. In addition, similar to the XMM-Newton observations during the same outburst, we find a hard lag of $\sim$21 ms in the frequency range of $\sim$1–5 Hz. The log-linear trend between the averaged time lag and energy indicates the propagation of fluctuations in the mass accretion rate from outer part of the accretion disk to the inner hot regions.

    • Understanding the inner structure of accretion disk in GX $17+2$: AstroSat’s outlook

      K. SRIRAM P. CHIRANJEEVI S. MALU V. K. AGRAWAL

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      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 stronglyindicate that structural variation in corona is the most possible cause of Z track in HID.

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    • Continuous Article Publication

      Posted on January 27, 2016

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      Posted on July 25, 2019

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