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NuSTAR and Swift observations of the ultraluminous X-ray source IC 342 X-1 in 2016: witnessing spectral evolution

Shidatsu, M. and Ueda, Y. and Fabrika, S. (2017) NuSTAR and Swift observations of the ultraluminous X-ray source IC 342 X-1 in 2016: witnessing spectral evolution. Astrophysical Journal, 839 (1). Art. No. 46. ISSN 1538-4357. doi:10.3847/1538-4357/aa67e7.

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We report on an X-ray observing campaign of the ultraluminous X-ray source IC 342 X-1 with NuSTAR and Swift in 2016 October, in which we captured the very moment when the source showed spectral variation. The Swift/XRT spectrum obtained in October 9–11 has a power-law shape and is consistent with those observed in the coordinated XMM-Newton and NuSTAR observations in 2012. In October 16–17, when the 3–10 keV flux became ≈4 times higher, we performed simultaneous NuSTAR and Swift observations. In this epoch, the source showed a more round-shaped spectrum like that seen with ASCA23 years ago. Thanks to the wide energy coverage and high sensitivity of NuSTAR, we obtained hard X-ray data covering up to ~30 keV for the first time during the high-luminosity state of IC 342 X-1. The observed spectrum has a broader profile than the multi-color disk blackbody model. The X-ray flux decreased again in the last several hours of the NuSTARobservation, when the spectral shape approached those seen in 2012 and 2016 October 9–11. The spectra obtained in our observations and in 2012 can be commonly described with disk emission and its Comptonization in cool (T_e ≈ 4 keV), optically thick (τ ≈ 5) plasma. The spectral turnover seen at around 5–10 keV shifts to higher energies as the X-ray luminosity decreases. This behavior is consistent with that predicted from recent numerical simulations of super-Eddington accretion flows with Compton-thick outflows. We suggest that the spectral evolution observed in IC 342 X-1 can be explained by a smooth change in mass-accretion rate.

Item Type:Article
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Ueda, Y.0000-0001-7821-6715
Additional Information:© 2017 The American Astronomical Society. Received 2017 January 25; revised 2017 March 3; accepted 2017 March 17; published 2017 April 12. We are grateful to the NuSTAR and Swift operation teams for carrying out the ToO observations. We thank Kirill Atapin for helpful comments and Ken Ohsuga and Tomohisa Kawashima for useful discussion of latest results from numerical simulations. M.S. acknowledges support by the Special Postdoctoral Researchers Program at RIKEN. This work is partly supported by a Grant-in-Aid for Young Scientists (B) 16K17672 (M.S.) and for Scientific Research 26400228 (Y.U.). S.F. acknowledges support by the the Russian RFBR grants 16-02-00567, 15-42-02573, and the Russian Science Foundation grant 14-50-00043. This research has made use of data supplied by the UK Swift Science Data Centre at the University of Leicester and data obtained from the High Energy Astrophysics Science Archive Research Center (HEASARC), provided by NASA's Goddard Space Flight Center.
Subject Keywords:accretion, accretion disks – black hole physics – X-rays: binaries – X-rays: individual (IC 342 X-1)
Issue or Number:1
Record Number:CaltechAUTHORS:20170331-105554581
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Official Citation:M. Shidatsu et al 2017 ApJ 839 46
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:75586
Deposited By: Joy Painter
Deposited On:31 Mar 2017 18:10
Last Modified:15 Nov 2021 16:34

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