{
  "id": "1505.00789",
  "version": "v1",
  "published": "2015-05-04T20:01:15.000Z",
  "updated": "2015-05-04T20:01:15.000Z",
  "title": "The 0.3-30 keV Spectra of Powerful Starburst Galaxies: NuSTAR and Chandra Observations of NGC 3256 and NGC 3310",
  "authors": [
    "B. D. Lehmer",
    "J. B. Tyler",
    "A. E. Hornschemeier",
    "D. R. Wik",
    "M. Yukita",
    "V. Antoniou",
    "S. Boggs",
    "F. E. Christensen",
    "W. W. Craig",
    "C. J. Hailey",
    "F. A. Harrison",
    "T. J. Maccarone",
    "A. Ptak",
    "D. Stern",
    "A. Zezas",
    "W. W. Zhang"
  ],
  "comment": "13 pages, 6 figures, accepted for publication in ApJ",
  "categories": [
    "astro-ph.GA",
    "astro-ph.CO",
    "astro-ph.HE"
  ],
  "abstract": "We present nearly simultaneous Chandra and NuSTAR observations of two actively star-forming galaxies within 50 Mpc: NGC 3256 and NGC 3310. Both galaxies are detected by both Chandra and NuSTAR, which together provide the first-ever spectra of these two galaxies spanning 0.3-30 keV. The X-ray emission from both galaxies is spatially resolved by Chandra; we find that hot gas dominates the E < 1-3 keV emission while ultraluminous X-ray sources (ULXs) dominate at E > 1-3 keV. The NuSTAR galaxy-wide spectra of both galaxies follow steep power-law distributions with Gamma ~ 2.6 at E > 5-7 keV, similar to the spectra of bright individual ULXs and other galaxies that have been studied by NuSTAR. We find that both NGC 3256 and NGC 3310 have X-ray detected sources coincident with nuclear regions; however, the steep NuSTAR spectra of both galaxies restricts these sources to be either low luminosity AGN or non-AGN in nature (e.g., ULXs or crowded X-ray sources that reach L2-10 keV ~ 10^40 erg/s cannot be ruled out). Combining our constraints on the 0.3-30 keV spectra of NGC 3256 and NGC 3310 with equivalent measurements for nearby star-forming galaxies M83 and NGC 253, we analyze the SFR-normalized spectra of these starburst galaxies. The spectra of all four galaxies show sharply declining power-law slopes above 3-6 keV due to ULX populations. Our observations therefore constrain the average spectra of luminous accreting binaries (i.e., ULXs). This result is similar to the super-Eddington accreting ULXs that have been studied individually in a targeted NuSTAR ULX program. We also find that NGC 3310 exhibits a factor of ~3-10 elevation of X-ray emission over the other star-forming galaxies. We argue that the excess is most likely explained by the relatively low metallicity of the young stellar population in NGC 3310.",
  "revisions": [
    {
      "version": "v1",
      "updated": "2015-05-04T20:01:15.000Z"
    }
  ],
  "analyses": {
    "keywords": [
      "powerful starburst galaxies",
      "kev spectra",
      "chandra observations",
      "star-forming galaxies",
      "x-ray emission"
    ],
    "publication": {
      "doi": "10.1088/0004-637X/806/1/126"
    },
    "note": {
      "typesetting": "TeX",
      "pages": 13,
      "language": "en",
      "license": "arXiv",
      "status": "editable",
      "inspire": 1366131
    }
  }
}