Abstract
We present a detailed timing and spectral study of an extremely variable narrow-line Seyfert 1 galaxy NGC 4748 using observations in the year 2017 and 2014 performed with AstroSat and XMM-Newton, respectively. Both observations show extremely variable soft and hard X-ray emission that are correlated with each other. In the 2014 data set, the source retains its general behaviour of “softer when brighter” while the 2017 observation exhibits a “harder when brighter” nature. Such changing behaviour is rare in AGNs and is usually observed in the black hole binary systems. The “harder when brighter” is confirmed with the anti-correlation between the photon index and the 0.3–10 keV power-law flux. This suggests a possible change in the accretion mode from standard to the advection-dominated flow. Additionally, both the observations show soft X-ray excess below 2 keV over the power-law continuum. This excess was fitted with a single or multiple blackbody component(s). The origin of soft excess during the 2017 observation is likely due to the cool Comptonization as the photon index changes with time. On the other hand, the broad iron line and delayed UV emission during the 2014 observation strongly suggest that X-ray illumination onto the accretion disk and reflection and reprocessing play a significant role in this AGN.
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Acknowledgements
We are thankful to the anonymous referee for his/her useful comments which have improved the manuscript significantly. MP is thankful for financial support from the UGC, India through the Dr. D. S. Kothari Post-Doctoral Fellowship (DSKPDF) program (Grant No. BSR/2017-2018/PH/0111). PK acknowledges support from the Aryabhatta Post-Doctoral Fellowship (A-PDF) grant (AO/A-PDF/770). This publication uses the data from AstroSat mission of the ISRO, archived at the Indian Space Science Data Centre (ISSDC). We thank members of the SXT team for their contribution to the development of the instrument and analysis software. We also acknowledge the contributions of the AstroSat project team at ISAC and IUCAA. This work has been performed utilising the calibration data-bases and auxiliary analysis tools developed, maintained and distributed by AstroSat/SXT team with members from various institutions in India and abroad. This research has also used data from the XMM-Newton, operated by the European Space Agency.
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This article is part of the Special Issue on "AstroSat: Five Years in Orbit".
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Pal, M., Kumari, N., Kushwaha, P. et al. Spectro-timing analysis of a highly variable narrow-line Seyfert 1 galaxy NGC 4748 with AstroSat and XMM-Newton. J Astrophys Astron 42, 81 (2021). https://doi.org/10.1007/s12036-021-09719-1
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DOI: https://doi.org/10.1007/s12036-021-09719-1