| Name: | Prajjwal Majumder |
| Affiliation: | Rishi Bankim Chandra College, Naihati, West Bengal |
| Conference ID : | ASI2024_278 |
| Title : | Energy dependent spectro-temporal study of HFQPOs of GRS 1915+105 during softer variability state: A multi-mission approach |
| Authors : | Prajjwal Majumder1, Broja G. Dutta1, Anuj Nandi2 |
| Authors Affiliation: | 1 Department of Physics, Rishi Bankim Chandra College, Naihati, West Bengal 743165, India.
2 Space Astronomy Group, ISITE Campus, U. R. Rao Satellite Centre, Outer Ring Road, Marathahalli, Bangalore, 560037, India. |
| Mode of Presentation: | Poster |
| Abstract Category : | High Energy Phenomena, Fundamental Physics and Astronomy |
| Abstract : | The persistently bright X-ray black hole binary GRS 1915+105 exhibits HFQPOs several times during its ‘softer’ variability class. For the first time, we detect soft-lag associated with the ~ 67 Hz HFQPO of this source using AstroSat observations compared to the previous findings of hard-lag using RXTE. The soft-lag of the photons in the 6-25 keV band w.r.t 3-6 keV band is found to be 0.40 - 1.68 ms. We find a coherent lag-energy correlation for all variability classes where the soft-lag increases with the energy up to 18 keV. A generic time lag-rms correlation implies that the soft-lag increases with the rms amplitude of the HFQPO. The spectral analysis of all observations exhibits a linear correlation between the optical depth of the Comptonizing medium and the time-lag of the HFQPO. We interpret this observed soft-lag could be due to the dominant reflection mechanism of the comptonized photons in the ‘cooler’ Keplerian disc. A long-term observation of a softer variability class ‘γ’, using the RXTE, AstroSat and NuSTAR instruments from 1996 to 2017, reveals that HFQPOs evolve from ~ 69 Hz to ~ 66 Hz and then increase to ~ 71 Hz. Time-lag studies of these γ class observations show an evolution from hard-lag to soft-lag. Broadband spectral analysis indicates that the bolometric flux is higher, and the electron temperature (kTe) of the corona is lower during the periods with no HFQPO observations. We explain these spectro-temporal features in the context of the present accretion model. |
