An Extremely Luminous and Variable Ultraluminous X-Ray Source in the Outskirts of Circinus Observed with NuSTAR

摘要: Following a serendipitous detection with the Nuclear Spectroscopic Telescope Array (NuSTAR), we present multi-epoch spectral and temporal analysis of an extreme ultraluminous X-ray source (ULX) located in outskirts Circinus galaxy, hereafter ULX5, including coordinated XMM-Newton+NuSTAR follow-up observations. The NuSTAR data presented here represent one first instances ULX reliably detected at hard (E > 10 keV) X-rays. ULX5 is variable on long time scales by least factor ~5 flux, was caught historically bright state during our 2013 observations (0.3-30.0 keV luminosity 1.6 × 10^(40) erg s^(–1)). During this epoch, displayed curved 3-10 spectrum, broadly similar to other ULXs. Although pure thermal models result high energy excess data, too weak be modeled disk reflection interpretation previously proposed explain curvature In addition flux variability, clear variability also observed. While many cases components ULXs uncertain, properties all quality sets currently available strongly support simple disk-corona model reminiscent that invoked for Galactic binaries, accretion becoming more prominent as increases. However, although temperature are well correlated across probed, observed follows L ∝ T^(1.70±0.17), flatter than expected blackbody radiation. highly from known black hole binaries (BHBs) luminosities. This comparison implies mass ~90 M_⊙ ULX5. given diverse behavior BHB disks, estimate still uncertain. Finally, limits placed any undetected iron absorption features set imply not viewing central regions through super-Eddington outflow.