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Black Holes and Neutron Stars in Nearby Galaxies: Insights from Nustar

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Black Holes and Neutron Stars in Nearby Galaxies: Insights from Nustar Draft version August 20, 2018 Typeset using LATEX twocolumn style in AASTeX62 Black Holes and Neutron Stars in Nearby Galaxies: Insights from NuSTAR N. Vulic,1,2 A. E.Hornschemeier,1,3 D. R. Wik,4,1 M. Yukita,3,1 A. Zezas,5,6 A. F. Ptak,3,1 B. D. Lehmer,7 V. Antoniou,6 T. J. Maccarone,8 B. F. Williams,9 and F. M. Fornasini6 1Laboratory for X-ray Astrophysics, Code 662, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA 2Department of Astronomy and Center for Space Science and Technology (CRESST), University of Maryland, College Park, MD 20742-2421, USA 3Department of Physics & Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218, USA 4Department of Physics & Astronomy, University of Utah, Salt Lake City, UT 84112-0830, USA 5Physics Department & Institute of Theoretical & Computational Physics, University of Crete, 71003 Heraklion, Crete, Greece 6Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA 7Department of Physics, University of Arkansas, 825 West Dickson Street, Fayetteville, AR 72701, USA 8Department of Physics & Astronomy, Box 41051, Science Building, Texas Tech University, Lubbock, TX 79409-1051, USA 9Department of Astronomy, Box 351580, University of Washington, Seattle, WA 98195, USA (Received; Revised; Accepted) Submitted to ApJ ABSTRACT Nearby galaxy surveys have long classified X-ray binaries (XRBs) by the mass category of their donor stars (high-mass and low-mass). The NuSTAR observatory, which provides imaging data at E > 10 keV, has enabled the classification of extragalactic XRBs by their compact object type: neutron star (NS) or black hole (BH). We analyzed NuSTAR/Chandra/XMM-Newton observations from a NuSTAR-selected sample of 12 galaxies within 7 11 1 5 Mpc having stellar masses (M⋆) 10 − M and star formation rates (SFR) 0.01 15 M yr− . We detect ⊙ ≈ − ⊙ 128 NuSTAR sources to a sensitivity of 1038 erg s 1. Using NuSTAR color-intensity and color-color diagrams ≈ − we classify 43 of these sources as candidate NS and 47 as candidate BH. We further subdivide BH by accretion states (soft, intermediate, and hard) and NS by weak (Z/Atoll) and strong (accreting pulsar) magnetic field. Using 8 normal (Milky Way-type) galaxies in the sample, we confirm the relation between SFR and galaxy X-ray point source luminosity in the 4 25 and 12 25 keV energy bands. We also constrain galaxy X-ray point − − source luminosity using the relation LX = αM⋆ + βSFR, finding agreement with previous work. The XLF of all sources in the 4 25 and 12 25 keV energy bands matches with the α = 1.6 slope for high-mass XRBs. We − − find that NS XLFs suggest a decline beginning at the Eddington limit for a 1.4 M NS, whereas the BH fraction ⊙ shows an approximate monotonic increase in the 4 25 and 12 25 keV energy bands. We calculate the overall − − ratio of BH to NS to be 1 for4 25 keV and 2 for 12 25 keV. ≈ − ≈ − Keywords: pulsars: general — stars: black holes — stars: neutron — X-rays: binaries — X-rays: luminosity function — X-rays: galaxies arXiv:1808.05617v1 [astro-ph.GA] 16 Aug 2018 1. INTRODUCTION the total X-ray emission of a galaxy above 2 keV is dom- Until the launch of the first focusing telescope to oper- inated by X-ray binaries (XRBs), classified as low-mass ate at E > 10 keV, the Nuclear Spectroscopic Telescope (LMXB) or high-mass (HMXB) based on their donor star. Array (NuSTAR; Harrison et al. 2013), we knew very lit- Previous studies of nearby galaxies in the soft X-ray band (0.5 10 keV) by, e.g. Chandra and XMM-Newton (e.g. tle about the behaviour and nature of extragalactic black − hole (BH) and neutron star (NS) populations at harder en- Stiele et al. 2011; Mineo et al. 2012, 2014; Long et al. 2014; ergies. In the absence of an X-ray bright supermassive BH, Haberl & Sturm 2016; Peacock & Zepf 2016) have revealed important new information on compact object populations, such as strong correlations between properties of XRBs and Corresponding author: N. Vulic galaxy star formation rate (SFR), stellar mass, and metal- [email protected] licity (e.g. Basu-Zych et al. 2016). Extrapolation of these 2 N. Vulic et al. local-Universe measurements as well as supporting measure- tween galaxy properties such as the stellar mass and recent ments at high-redshift (Lehmer et al. 2016) have indicated a star formation rate/history and compact object type/accretion possible significant role of XRBs in heating the Intergalactic state as determined from hard X-ray diagnostics? To estimate Medium (IGM) of the early Universe (e.g. Fragos et al. 2013; the number of BH and NS that will be formed in a galaxy Mesinger et al. 2014; Pacucci et al. 2014; Madau & Fragos requires binary population synthesis, and a detailed under- 2017; Sazonov & Khabibullin 2017; Das et al. 2017). standing of concepts such as supernova explosions, which is However, there are questions about the extragalactic XRB not well understood (e.g. Pejcha & Thompson 2015). Alter- population that are difficult to answer at E < 10 keV, in- natively, we can use observational data and methods to deter- cluding whether compact objects are BH or NS. The rich mine the BH fraction and its dependenceon X-ray luminosity suite of thousands of Rossi X-ray Timing Explorer (RXTE) and specific star formation rate (sSFR). PCA spectra of BH/NS XRBs in the Milky Way galaxy pro- With NuSTAR we can measure local-galaxy SEDs over vide critical diagnostics in the 4 25 keV band of both 0.5 30 keV that are applicable to high-z galaxies detected − − compact object type (BH vs. NS) and accretion state (e.g. by Chandra. One of our goals is to determine what sources Maccarone 2003; McClintock & Remillard 2006; Done et al. are contributing to the 0.5 30 keV emission. Furthermore, − 2007). With NuSTAR, for the first time, we are able to lever- we would like to be able to predict, based on galaxy prop- age the knowledge gained from compact objects in our own erties such as star formation rate/history and stellar mass, galaxy by applying these harder X-ray diagnostics to extra- what the distribution of binaries and their emitting proper- galactic populations. ties are. Achieving this goal is rather complicated, as there The hard X-ray coverage with NuSTAR is crucial for dis- are parameters such as the duty cycle that result in a broad tinguishing different types of accreting binaries, such as range of population properties for different stellar ages, etc. BH/NS XRBs and accreting pulsars. Compact object di- One approach to this complicated problem is to make di- agnostics have already been successfully applied to charac- rect measurements over a variety of galaxy properties. Each terize XRBs in several nearby galaxies observed by NuSTAR. snapshot view of an individual galaxy measures the state of These studies include simultaneous NuSTAR/Chandra/XMM- the overall population, giving us a constraint on duty cy- Newton/Swift studies of the nearby star-forming galaxies cles (Binder et al. 2017). Hard X-ray diagnostics allow us NGC 253 (Lehmer et al. 2013; Wik et al. 2014a) and M83 to determine the distribution of BH spectral states, similar (Yukita et al. 2016), as well as Local Group galaxy M31 to Galactic BH studies (e.g. Tetarenko et al. 2016). Using (Maccarone et al. 2016; Yukita et al. 2017; Lazzarini et al. this approach, we can obtain baseline estimates of XRB for- 2018); for a description of the NuSTAR galaxy program mation rate, duty cycles, spectral states, and galaxy SEDs. please see Hornschemeier et al. (2016). Using 4 25 keV Understanding these properties at E > 10 keV is critical to − color-color and color-intensity diagnostics, these studies compare to the results of XRB evolution in the 0.5 10 keV − have shown that the starburst galaxies are dominated by lu- bandpass. NuSTAR is well-matched to the rest-frame ener- minous BH-XRB systems, mostly in intermediate accretion gies of high-z galaxies at z = 3 4 probed by Chandra and is − states. Specifically, ultraluminous X-ray sources (ULXs) thus a new window into XRB evolution. with 3 30 keV spectra indicative of super-Eddington accre- The X-ray luminosity function (XLF) represents the dis- − tion (e.g. Gladstone et al. 2009) appear to dominate the hard tribution of sources in a galaxy based on their luminos- X-ray emission of starburst galaxies (Walton et al. 2013; ity. Seminal studies of LMXBs in elliptical galaxies (e.g. Bachetti et al. 2014a; Rana et al. 2015; Lehmer et al. 2015). Gilfanov 2004a; Zhang et al. 2012) and HMXBs in spiral Meanwhile, M31 has a significant contribution from NS galaxies (e.g. Grimm et al. 2003; Mineo et al. 2012) found accretors (pulsars and low-magnetic field Z-type sources; that their XLFs were (approximately) universal when nor- Maccarone et al. 2016; Yukita et al. 2017). As expected, the malizing by the stellar mass and SFR of a galaxy, respec- pulsars trace the young stellar population in the spiral arms tively (see Gilfanov 2004b for a summary). Small variations and the Z-type sources are concentrated in globular clusters in the power law slope and cutoff are dependent on factors and the bulge/field of the galaxy. NuSTAR data were crucial such as metallicity (Basu-Zych et al. 2016) and star forma- to the reclassification of previously identified BH candidates tion history (Lehmer et al. 2017). We will investigate how in M31 globular clusters as NS, based on their hard X-ray scaling NuSTAR XLFs by SFR compares with results from spectra (Maccarone et al.
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