THE 0.3–30 keV SPECTRA OF POWERFUL STARBURST GALAXIES: NuSTAR AND CHANDRA OBSERVATIONS OF NGC 3256 AND NGC 3310
THE 0.3–30 keV SPECTRA OF POWERFUL STARBURST GALAXIES: NuSTAR AND CHANDRA OBSERVATIONS OF NGC 3256 AND NGC 3310
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DOI：
10.1088/0004-637x/806/1/126
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发表时间：
2015-05
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通讯作者：
B. Lehmer;J. Tyler;A. Hornschemeier;D. Wik;M. Yukita;V. Antoniou;S. Boggs;F. Christensen;W. Craig;C. Hailey;F. Harrison;T. Maccarone;A. Ptak;D. Stern;A. Zezas;W. Zhang
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作者：
B. Lehmer;J. Tyler;A. Hornschemeier;D. Wik;M. Yukita;V. Antoniou;S. Boggs;F. Christensen;W. Craig;C. Hailey;F. Harrison;T. Maccarone;A. Ptak;D. Stern;A. Zezas;W. Zhang
我们给出了在50Mpc内几乎同时对两个活跃的恒星形成星系进行Chandra和NuSTAR观测：NGC3256和NGC3310。钱德拉和NuSTAR都显著地探测到了这两个星系，这两个星系一起提供了这两个星系有史以来第一次跨越0.3-30keV的光谱。钱德拉对这两个星系的X射线辐射进行了空间分辨；我们发现热气体主导着E1-3keV。在E>；5-7keV处，两个星系的核子星全星系光谱服从陡峭的幂定律分布，Γ≈为2.6.使用新的和存档的钱德拉数据，我们寻找严重遮挡或低亮度活动星系核(AGN)的特征。我们发现NGC3256和NGC3310都有与核区重合的X射线探测源；然而，两个星系陡峭的NuSTAR谱限制了这些源要么是低光度活动星系核(L2−10keV/≲10−5)，要么是自然界中的非活动星系核(例如，ULX或达到L2−10keV∼1040erg S−1的拥挤X射线源)。结合我们对NGC3256和NGC3310的0.3-30keV能谱的约束，以及附近恒星形成星系M83和NGC253的等效测量，我们分析了这些恒星爆发星系的恒星形成率(SFR)归一化光谱。在能量高于3-6keV时，所有四个星系的光谱都显示出幂定律斜率急剧下降，这主要是由于ULX布居造成的。因此，我们的观测限制了全星系发光吸积双星(即ULX)群体的平均光谱形状。有趣的是，尽管选择了完全不同的星系样本，这里强调了一系列SFR和恒星质量，但这些性质类似于已在目标NuSTAR ULX计划中单独研究的超级爱丁顿吸积型ULX的那些性质。我们还发现，与其他恒星形成星系相比，NGC3310的X射线辐射增加了3-10个≈，这是由于相应的ULX丰度过高。我们认为，NGC3310中ULX的过剩很可能是因为该星系中年轻恒星群体的金属丰度相对较低，这一特性预计会在给定的SFR下产生过剩的发光X射线双星。
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. 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 active galactic nuclei (AGNs). 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 AGNs (L2−10 keV/LEdd ≲ 10−5) or non-AGNs in nature (e.g., ULXs or crowded X-ray sources that reach L2−10 keV ∼ 1040 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.