Astrophys. J., 777, 7 (2013/November-1)
Discovery of a large population of ultraluminous X-ray sources in the bulgeless galaxies NGC 337 and ESO 501-23.
SOMERS G., MATHUR S., MARTINI P., WATSON L., GRIER C.J. and FERRARESE L.
Abstract (from CDS):
We have used Chandra observations of eight bulgeless disk galaxies to identify new ultraluminous X-ray source (ULX) candidates, study their high-mass X-ray binary (HMXB) population, and search for low-luminosity active galactic nuclei (AGNs). We report the discovery of 16 new ULX candidates in our sample of galaxies. Eight of these are found in the star forming galaxy NGC 337, none of which are expected to be background contaminants. The HMXB luminosity function of NGC 337 implies a star formation rate (SFR) of 6.8–3.5+4.4 M_☉/yr, consistent at 1.5σ with a recent state of the art SFR determination. We also report the discovery of a bright ULX candidate (X-1) in ESO 501-23. X-1's spectrum is well fit by an absorbed power law with Γ = 1.18–0.11+0.19 and NH= 1.13–1.13+7.07/cm2, implying a 0.3-8 keV flux of 1.08–0.07+0.05 x 10–12 erg/s/cm2. Its X-ray luminosity (LX ) is poorly constrained due to uncertainties in the host galaxy's distance, but we argue that its spectrum implies LX> 1040 erg/s. An optical counterpart to this object may be present in an Hubble Space Telescope image. We also identify ULX candidates in IC 1291, PGC 3853, NGC 5964, and NGC 2805. We find no evidence of nuclear activity in the galaxies in our sample, placing a flux upper limit of 4x10–15 erg/s/cm2 on putative AGN. Additionally, the Type II-P supernova SN 2011DQ in NGC 337, which exploded two months before our X-ray observation, is undetected.
galaxies: nuclei - X-rays: binaries - X-rays: galaxies
VizieR on-line data:
<Available at CDS (J/ApJ/777/7): table1.dat table3.dat table4.dat table5.dat table6.dat table7.dat>
Table 7: [SMM2013] IC 1291 X-N, [SMM2013] ESO 501-23 X-NN, [SMM2013] UGC 6446 X-1, [SMM2013] NGC NNNN X-NN, [SMM2013] PGC 3853 X-N N=40.
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