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Published April 21, 2017 | Submitted
Journal Article Open

X-ray flux variability of active galactic nuclei observed using NuSTAR


We present results of a systematic study of flux variability on hourly time-scales in a large sample of active galactic nuclei (AGN) in the 3−79 keV band using data from Nuclear Spectroscopic Telescope Array. Our sample consists of four BL Lac objects (BL Lacs), three flat spectrum radio quasars (FSRQs) 24 Seyfert 1, 42 Seyfert 2 and eight narrow line Seyfert 1 (NLSy1) galaxies. We find that in the 3−79 keV band, about 65 per cent of the sources in our sample show significant variations on hourly time-scales. Using the Mann–Whitney U-test and the Kolmogorov–Smirnov test, we find no difference in the variability behaviour between Seyfert 1 and 2 galaxies. The blazar sources (FSRQs and BL Lacs) in our sample are more variable than Seyfert galaxies that include Seyfert 1 and Seyfert 2 in the soft (3−10 keV), hard (10−79 keV) and total (3−79 keV) bands. NLSy1 galaxies show the highest duty cycle of variability (87 per cent), followed by BL Lacs (82 per cent), Seyfert galaxies (56 per cent) and FSRQs (23 per cent). We obtained flux doubling/halving time in the hard X-ray band less than 10 min in 11 sources. The flux variations between the hard and soft bands in all the sources in our sample are consistent with zero lag.

Additional Information

© 2016 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. Accepted 2016 December 8. Received 2016 December 8; in original form 2016 September 26. Published: 12 December 2016. We thank the anonymous referee for his/her detailed comments that helped us to improve the manuscript. The comments by Dr. Markus Bottcher on an initial version of the manuscript is thankfully acknowledged. This research has made use of data from the NuSTAR mission, a project led by the California Institute of Technology, managed by the Jet Propulsion Laboratory and funded by the National Aeronautics and Space Administration. We thank the NuSTAR Operations, Software and Calibration teams for support with the execution and analysis of these observations. This research has made use of the NuSTAR Data Analysis Software (NUSTARDAS) jointly developed by the ASI Science Data Center (ASDC, Italy) and the California Institute of Technology(USA). Note added in proof: We thank Dr. Madsen of the NuSTAR operations team for bringing to our attention improved background filtering for South Atlantic Anomaly (SAA) passage made available since the work in this paper as well as the ways to identify flares due to SAA from inspection of the background light curves. All data have now been checked for the existence of such events in our analysis and the updated results are given in this version of the manuscript.

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