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      https://www.ias.ac.in/article/fulltext/joaa/042/0081

    • Keywords

       

      Accretion; accretion discs; galaxies: active; galaxies: individual: NGC 4748; galaxies: nuclei; X-rays: galaxies.

    • 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 anticorrelation 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 anddelayed 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.

    • Author Affiliations

       

      MAIN PAL1 NEERAJ KUMARI2 3 P. KUSHWAHA4 K. P. SINGH5 ALOK C. GUPTA4 SACHINDRA NAIK2 G. C. DEWANGAN6 P. TRIPATHI6 RATHIN ADHIKARI1 O. ADEGOKE7 H. NANDAN8

      1. Centre for Theoretical Physics, Jamia Millia Islamia University, New Delhi 110 025, India.
      2. Astronomy and Astrophysics Division, Physical Research Laboratory, Ahmedabad 380 009, India.
      3. Indian Institute of Technology Gandhinagar, Gandhinagar 382 355, India.
      4. Aryabhatta Research Institute of Observational Sciences, Manora Peak, Nainital 263 002, India.
      5. Indian Institute of Science Education and Research, Mohali, Manauli P.O. 140 306, India.
      6. Inter University Centre for Astronomy and Astrophysics, Pune 411 007, India.
      7. Department of Astronomy, University of Geneva, Versoix 1290, Geneva, Switzerland.
      8. Department of Physics, Gurukul Kangri University, Haridwar 249 404, India.
    • Dates

       
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