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Spectral and Temporal Properties of the Ultraluminous X-Ray Pulsar in M82 from 15 years of Chandra Observations and Analysis of the Pulsed Emission Using NuSTAR

Brightman, Murray and Harrison, Fiona A. and Walton, Dominic J. and Fuerst, Felix and Hornschemeier, Ann E. and Zezas, Andreas and Bachetti, Matteo and Grefenstette, Brian W. and Ptak, Andrew F. and Tendulkar, Shriharsh P. and Yukita, Mihoko (2016) Spectral and Temporal Properties of the Ultraluminous X-Ray Pulsar in M82 from 15 years of Chandra Observations and Analysis of the Pulsed Emission Using NuSTAR. Astrophysical Journal, 816 (2). Art. No. 60. ISSN 0004-637X. http://resolver.caltech.edu/CaltechAUTHORS:20150801-082902636

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Abstract

The recent discovery by Bachetti et al. of a pulsar in M82 that can reach luminosities of up to 10^(40) erg s^(−1), a factor of ~100 times the Eddington luminosity for a 1.4 M⊙ compact object, poses a challenge for accretion physics. In order to better understand the nature of this source and its duty cycle, and in light of several physical models that have been subsequently published, we conduct a spectral and temporal analysis of the 0.5–8 keV X-ray emission from this source from 15 years of Chandra observations. We analyze 19 ACIS observations where the point-spread function (PSF) of the pulsar is not contaminated by nearby sources. We fit the Chandra spectra of the pulsar with a power-law model and a disk blackbody model, subjected to interstellar absorption in M82. We carefully assess for the effect of pile-up in our observations, where four observations have a pile-up fraction of >10%, which we account for during spectral modeling with a convolution model. When fitted with a power-law model, the average photon index when the source is at high luminosity (L_X > 10^(39) erg s^(−1)) is Γ = 1.33 ± 0.15. For the disk blackbody model, the average temperature is T_(in) = 3.24 ± 0.65 keV, the spectral shape being consistent with other luminous X-ray pulsars. We also investigated the inclusion of a soft excess component and spectral break, finding that the spectra are also consistent with these features common to luminous X-ray pulsars. In addition, we present spectral analysis from NuSTAR over the 3–50 keV range where we have isolated the pulsed component. We find that the pulsed emission in this band is best fit by a power-law with a high-energy cutoff, where Γ = 0.6 ± 0.3 and E_C = 14^(+5)_(-3) keV. While the pulsar has previously been identified as a transient, we find from our longer-baseline study that it has been remarkably active over the 15-year period, where for 9/19 (47%) observations that we analyzed, the pulsar appears to be emitting at a luminosity in excess of 10^(39) erg s^(−1), greater than 10 times its Eddington limit.


Item Type:Article
Related URLs:
URLURL TypeDescription
http://arxiv.org/abs/1507.06014arXivDiscussion Paper
http://dx.doi.org/10.3847/0004-637X/816/2/60DOIArticle
http://iopscience.iop.org/article/10.3847/0004-637X/816/2/60/metaPublisherArticle
ORCID:
AuthorORCID
Harrison, Fiona A.0000-0003-2992-8024
Walton, Dominic J.0000-0001-5819-3552
Bachetti, Matteo0000-0002-4576-9337
Ptak, Andrew F.0000-0001-5655-1440
Additional Information:© 2016 The American Astronomical Society. Received 2015 July 21; accepted 2015 November 18; published 2016 January 5. This work made significant use of archival observations made by the Chandra X-ray observatory, for which we thank the builders and operators, as well as the software package ciao. The data were obtained from the High Energy Astrophysics Science Archive Research Center (HEASARC), which is a service of the Astrophysics Science Division at NASA/GSFC and the High Energy Astrophysics Division of the Smithsonian Astrophysical Observatory. This work also 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 NASA. 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). A.Z. acknowledges funding from the European Research Council under the European Union's Seventh Framework Programme (FP/2007–2013)/ERC grant agreement No. 617001. Facilities: CXO (ACIS), NuSTAR - Chandra X-ray Observatory satellite
Group:NuSTAR, Space Radiation Laboratory
Funders:
Funding AgencyGrant Number
NASA/JPL/CaltechUNSPECIFIED
European Research Council (ERC)617001
Subject Keywords:galaxies: individual (M82) – stars: neutron – X-rays: binaries
Other Numbering System:
Other Numbering System NameOther Numbering System ID
Space Radiation Laboratory2016-12
Record Number:CaltechAUTHORS:20150801-082902636
Persistent URL:http://resolver.caltech.edu/CaltechAUTHORS:20150801-082902636
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:59132
Collection:CaltechAUTHORS
Deposited By: Joy Painter
Deposited On:06 Aug 2015 18:30
Last Modified:30 Nov 2017 17:46

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