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An Extremely Luminous and Variable Ultraluminous X-Ray Source in the Outskirts of Circinus Observed with NuSTAR

Walton, D. J. and Fuerst, F. and Harrison, F. and Stern, D. and Bachetti, M. and Barret, D. and Bauer, F. and Boggs, S. E. and Christensen, F. E. and Craig, W. W. and Fabian, A. C. and Grefenstette, B. W. and Hailey, C. J. and Madsen, K. K. and Miller, J. M. and Ptak, A. and Rana, V. and Webb, N. A. and Zhang, W. W. (2013) An Extremely Luminous and Variable Ultraluminous X-Ray Source in the Outskirts of Circinus Observed with NuSTAR. Astrophysical Journal, 779 (2). Art. No. 148. ISSN 0004-637X.

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Following a serendipitous detection with the Nuclear Spectroscopic Telescope Array (NuSTAR), we present a multi-epoch spectral and temporal analysis of an extreme ultraluminous X-ray source (ULX) located in the outskirts of the Circinus galaxy, hereafter Circinus ULX5, including coordinated XMM-Newton+NuSTAR follow-up observations. The NuSTAR data presented here represent one of the first instances of a ULX reliably detected at hard (E > 10 keV) X-rays. Circinus ULX5 is variable on long time scales by at least a factor of ~5 in flux, and was caught in a historically bright state during our 2013 observations (0.3-30.0 keV luminosity of 1.6 × 10^(40) erg s^(–1)). During this epoch, the source displayed a curved 3-10 keV spectrum, broadly similar to other bright ULXs. Although pure thermal models result in a high energy excess in the NuSTAR data, this excess is too weak to be modeled with the disk reflection interpretation previously proposed to explain the 3-10 keV curvature in other ULXs. In addition to flux variability, clear spectral variability is also observed. While in many cases the interpretation of spectral components in ULXs is uncertain, the spectral and temporal properties of all the high quality data sets currently available strongly support a simple disk-corona model reminiscent of that invoked for Galactic binaries, with the accretion disk becoming more prominent as the luminosity increases. However, although the disk temperature and luminosity are well correlated across all time scales currently probed, the observed luminosity follows L ∝ T^(1.70±0.17), flatter than expected for simple blackbody radiation. The spectral variability displayed here is highly reminiscent of that observed from known Galactic black hole binaries (BHBs) at high luminosities. This comparison implies a black hole mass of ~90 M_⊙ for Circinus ULX5. However, given the diverse behavior observed from Galactic BHB accretion disks, this mass estimate is still uncertain. Finally, the limits placed on any undetected iron absorption features with the 2013 data set imply that we are not viewing the central regions of Circinus ULX5 through any extreme super-Eddington outflow.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Walton, D. J.0000-0001-5819-3552
Fuerst, F.0000-0003-0388-0560
Harrison, F.0000-0003-2992-8024
Stern, D.0000-0003-2686-9241
Bachetti, M.0000-0002-4576-9337
Barret, D.0000-0002-0393-9190
Bauer, F.0000-0002-8686-8737
Boggs, S. E.0000-0001-9567-4224
Christensen, F. E.0000-0001-5679-1946
Fabian, A. C.0000-0002-9378-4072
Grefenstette, B. W.0000-0002-1984-2932
Madsen, K. K.0000-0003-1252-4891
Ptak, A.0000-0001-5655-1440
Rana, V.0000-0003-1703-8796
Zhang, W. W.0000-0002-1426-9698
Additional Information:© 2013 American Astronomical Society. Received 2013 August 18; accepted 2013 October 8; published 2013 December 3. The authors thank Koji Mukai for useful discussion regarding Galactic CVs, and Rubens Reis for discussion regarding Galactic BHBs. This research has made use of data obtained with the NuSTAR mission, a project led by the California Institute of Technology (Caltech), managed by the Jet Propulsion Laboratory (JPL) and funded by NASA, XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA, and the Suzaku observatory, a collaborative mission between the space agencies of Japan (JAXA) and the USA (NASA). In addition, this research has also made use of data obtained from NASA’s Swift, Chandra, and Spitzer satellites. 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 Caltech (USA). We also made use of the NASA/IPAC Extragalactic Database (NED), which is operated by JPL, Caltech, under contract with NASA. Some of the figures included in this work have been produced with the Veusz plotting package:, written and maintained by Jeremy Sanders. F.E.B. acknowledges support from Basal-CATA (PFB-06/2007) and CONICYT-Chile (under grants FONDECYT 1101024 and Anillo ACT1101). M.B. wishes to acknowledge the support from the Centre National D´Etudes Spatiales (CNES).
Group:NuSTAR, Space Radiation Laboratory
Funding AgencyGrant Number
Fondo Nacional de Desarrollo Científico y Tecnológico (FONDECYT)1101024
Comisión Nacional de Investigación Científica y Tecnológica (CONICYT)Anillo ACT1101
Centre National d'Études Spatiales (CNES)UNSPECIFIED
Subject Keywords:black hole physics; X-rays: binaries; X-rays: individual (Circinus ULX5)
Issue or Number:2
Record Number:CaltechAUTHORS:20140121-112443927
Persistent URL:
Official Citation:An Extremely Luminous and Variable Ultraluminous X-Ray Source in the Outskirts of Circinus Observed with NuSTAR D. J. Walton et al. 2013 ApJ 779 148
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:43453
Deposited By: Ruth Sustaita
Deposited On:21 Jan 2014 20:48
Last Modified:09 Mar 2020 13:19

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