THE 0.3–30 keV SPECTRA OF POWERFUL STARBURST GALAXIES:NuSTARANDCHANDRAOBSERVATIONS OF NGC 3256 AND NGC 3310

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 significantly 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) provide majority contributions to the emission at E > 1-3 keV. The NuSTAR galaxy-wide spectra of both galaxies follow steep power-law distributions with Γ ≈ 2.6 at E > 5-7 keV. Using new and archival Chandra data, we search for signatures of heavily obscured or low luminosity AGN. 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 (L 2−10 keV /L Edd < ∼ 10 −5 ) or non-AGN in nature (e.g., ULXs or crowded X-ray sources that reach L 2−10 keV ∼ 10 40 erg s −1 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 star-formation rate (SFR) normalized spectra of these starburst galaxies. The spectra of all four galaxies show sharply declining power-law slopes at energies above 3-6 keV primarily due to ULX populations. Our observations therefore constrain the average spectral shape of galaxy-wide populations of luminous accreting binaries (i.e., ULXs). Interestingly, despite a completely different galaxy sample selection, emphasizing here a range of SFRs and stellar masses, these properties are similar to those of 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 due to a corresponding overabundance of ULXs. We argue that the excess of ULXs in NGC 3310 is most likely explained by the relatively low metallicity of the young stellar population in this galaxy, a property that is expected to produce an excess of luminous X-ray binaries for a given SFR.

Diffuse X-Ray-emitting Gas in Major Mergers

“…Near/mid-infrared images reveal two nuclei, with the southern being much more obscured (Zenner & Lenzen 1993;Kotilainen et al 1996). The southern nucleus may be an AGN (Kotilainen et al 1996;Ohyama, Terashima, & Sakamoto 2015) although this is uncertain (Lira et al 2002;Lehmer et al 2015).…”

“…Our best-fit absorbing column for the MEKAL component is about 1/3 their value for their second thermal component, while our total absorption-corrected luminosity is about half theirs. Lehmer et al (2015) also studied the diffuse X-ray light from NGC 3256 as seen by Chandra. They fit its spectrum to two thermal components plus a power law.…”

A HARD X-RAY STUDY OF THE NORMAL STAR-FORMING GALAXY M83 WITH NuSTAR

“…Hard X-ray emission in star-forming galaxies lacking an active galactic nucleus (AGN) originates primarily from X-ray binaries: mainly high-mass X-ray binaries (HMXB) and ultraluminous X-ray sources (ULXs). There are other contributions, chiefly from low-mass X-ray binaries (LMXBs), but also possibly from accretion onto a supermassive black hole (SMBH) and perhaps diffuse non-thermal inverse Compton (IC) emission (Lehmer et al 2013(Lehmer et al , 2015Wik et al 2014b;Ptak et al 2015). However, the IC component results from the interaction of cosmic rays with IR and microwave background photons, and is likely to be negligible at moderately hard X-ray energies (e.g., see Wik et al 2014b for the case of NGC 253).…”

“…We are characterizing the 0.5-30 keV spectral energy distribution (SED) of these galaxies (Lehmer et al 2015;Ptak et al 2015), which is of great cosmological importance. For example, this SED is used to perform important k-corrections for high-z galaxies observed in the deepest Chandra X-ray surveys (Lehmer et al 2016, submitted to the ApJ).…”

“…It also appears that star-forming galaxies are dominated by ULXs and have overall spectra with steep turnovers at E 10 keV > (e.g., NGC 253; Wik et al 2014b; NGC 3310 and NGC 3256; Lehmer et al 2015). This hard-energy turn over around 6-10 keV is also seen for bright individual ULXs studied by XMM-Newton and NuSTAR (Stobbart et al 2006;Gladstone et al 2009;Walton et al 2013Walton et al , 2014Bachetti et al 2013;Rana et al 2015) and is interpreted as super-Eddington accretion onto a stellar-mass black hole instead of an intermediate-mass black hole in a low/hard accretion state (e.g., Shakura & Sunyaev 1973;King et al 2001;Poutanen et al 2007;Gladstone et al 2009;Sutton et al 2013).…”