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NuSTAR and Chandra Insight into the Nature of the 3-40 keV Nuclear Emission in NGC 253

Lehmer, B. D. and Wik, D. R. and Hornschemeier, A. E. and Ptak, A. and Antoniou, V. and Argo, M. K. and Bechtol, K. and Boggs, S. and Christensen, F. E. and Craig, W. W. and Hailey, C. J. and Harrison, F. A. and Krivonos, R. and Leyder, J.-C. and Maccarone, T. J. and Stern, D. and Venters, T. and Zezas, A. and Zhang, W. W. (2013) NuSTAR and Chandra Insight into the Nature of the 3-40 keV Nuclear Emission in NGC 253. Astrophysical Journal, 771 (2). Art. No. 134. ISSN 0004-637X. https://resolver.caltech.edu/CaltechAUTHORS:20130813-114356742

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Abstract

We present results from three nearly simultaneous Nuclear Spectroscopic Telescope Array (NuSTAR) and Chandra monitoring observations between 2012 September 2 and 2012 November 16 of the local star-forming galaxy NGC 253. The 3-40 keV intensity of the inner ~20 arcsec (~400 pc) nuclear region, as measured by NuSTAR, varied by a factor of ~2 across the three monitoring observations. The Chandra data reveal that the nuclear region contains three bright X-ray sources, including a luminous (L_(2-10) keV ~ few × 10^39 erg s^–1) point source located ~1 arcsec from the dynamical center of the galaxy (within the 3σ positional uncertainty of the dynamical center); this source drives the overall variability of the nuclear region at energies ≳3 keV. We make use of the variability to measure the spectra of this single hard X-ray source when it was in bright states. The spectra are well described by an absorbed (N_H ≈ 1.6 × 10^23 cm^–2) broken power-law model with spectral slopes and break energies that are typical of ultraluminous X-ray sources (ULXs), but not active galactic nuclei (AGNs). A previous Chandra observation in 2003 showed a hard X-ray point source of similar luminosity to the 2012 source that was also near the dynamical center (θ ≈ 0.4 arcsec); however, this source was offset from the 2012 source position by ≈1 arcsec. We show that the probability of the 2003 and 2012 hard X-ray sources being unrelated is ≫99.99% based on the Chandra spatial localizations. Interestingly, the Chandra spectrum of the 2003 source (3-8 keV) is shallower in slope than that of the 2012 hard X-ray source. Its proximity to the dynamical center and harder Chandra spectrum indicate that the 2003 source is a better AGN candidate than any of the sources detected in our 2012 campaign; however, we were unable to rule out a ULX nature for this source. Future NuSTAR and Chandra monitoring would be well equipped to break the degeneracy between the AGN and ULX nature of the 2003 source, if again caught in a high state.


Item Type:Article
Related URLs:
URLURL TypeDescription
http://dx.doi.org/10.1088/0004-637X/771/2/134DOIArticle
http://iopscience.iop.org/0004-637X/771/2/134/PublisherArticle
http://arxiv.org/abs/1306.2639arXivDiscussion Paper
ORCID:
AuthorORCID
Lehmer, B. D.0000-0003-2192-3296
Wik, D. R.0000-0001-8952-676X
Hornschemeier, A. E.0000-0001-8667-2681
Ptak, A.0000-0001-5655-1440
Antoniou, V.0000-0001-7539-1593
Boggs, S.0000-0001-9567-4224
Christensen, F. E.0000-0001-5679-1946
Harrison, F. A.0000-0003-2992-8024
Krivonos, R.0000-0003-2737-5673
Stern, D.0000-0003-2686-9241
Zezas, A.0000-0001-8952-676X
Zhang, W. W.0000-0002-1426-9698
Additional Information:© 2013 American Astronomical Society. Received 2013 March 14; accepted 2013 May 17; published 2013 June 25. We thank the anonymous referee for helpful comments, which have improved the quality of this paper. This work was supported under NASA Contract No. NNG08FD60C, and 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 theASI Science Data Center (ASDC, Italy) and the California Institute of Technology (USA). We thank the Chandra X-ray Center staff for providing faster than usual processing of the Chandra data.
Group:NuSTAR, Space Radiation Laboratory
Funders:
Funding AgencyGrant Number
NASANNG08FD60C
Subject Keywords:galaxies: active; galaxies: individual (NGC 253); galaxies: star formation; galaxies: starburst; X-rays: galaxies
Issue or Number:2
Record Number:CaltechAUTHORS:20130813-114356742
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:20130813-114356742
Official Citation:NuSTAR and Chandra Insight into the Nature of the 3-40 keV Nuclear Emission in NGC 253 B. D. Lehmer et al. 2013 ApJ 771 134
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:39895
Collection:CaltechAUTHORS
Deposited By: Jason Perez
Deposited On:14 Aug 2013 16:54
Last Modified:14 Nov 2019 23:32

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