ChandraDiscovery of Luminous Supersoft X‐Ray Sources in M81

Abstract

A Chandra ACIS-S imaging observation of the nearby galaxy M81 (NGC 3031) reveals nine luminous soft X-ray sources. The local environments, X-ray spectral properties, and X-ray light curves of the sources are presented and discussed in the context of prevailing physical models for supersoft sources. It is shown that the sample falls within expectations based on population synthesis models taken from the literature, although the high observed luminosities (L obs $ 2 Â 10 36 -3 Â 10 38 ergs s À1 in the 0.2-2.0 keV band) and equivalent blackbody temperatures (T eff $ 40 80 eV) place the brightest detected M81 objects at the high-luminosity end of the class of supersoft sources defined by previous ROSAT and Einstein studies of nearby galaxies. This is interpreted as a natural consequence of the higher sensitivity of Chandra to hotter and more luminous systems. Most of the sources can be explained as canonical supersoft sources: accreting white dwarfs powered by steady surface nuclear burning with X-ray spectra well fitted by hot white dwarf local thermodynamic equilibrium atmosphere models. An exceptionally bright source is scrutinized in greater detail since its estimated bolometric luminosity, L bol $ 1:5 Â 10 39 ergs s À1 , greatly exceeds theoretical estimates for supersoft sources. This source may be beyond the stability limit and undergoing a phase of mass outflow under extreme conditions. Alternatively, a model in which the observed X-ray spectrum arises from an accretion disk around a black hole of mass $1200=ðcos iÞ 1=2 M (viewed at an inclination angle i) cannot be excluded.

Models of magnetized neutron star atmospheres: thin atmospheres and partially ionized hydrogen atmospheres with vacuum polarization

“…(Kurucz 1970(Kurucz , 1993, modified to deal with strong magnetic fields. A nonmagnetic version of this modified code was previously used to model atmospheres of super-soft X-ray sources (Swartz et al 2002;Ibragimov et al 2003), atmospheres of non-magnetized NSs (Suleimanov & Werner 2007;Rauch et al 2008), and atmospheres of spreading layers on the surface of accreting NSs (Suleimanov & Poutanen 2006). The scheme of calculations is as follows.…”

The Luminous X‐Ray Source Population in M51 Observed withChandra

“…Notes.-In Col. (1), (1) and (2) Table 2 (NGC 5194) and Table 3 The luminosities, however, are much higher than those of the typical SSS (10 36 -10 38 ergs s À1 ). Such a soft source with a large luminosity ($10 39 ergs s À1 or higher) is known in M81 (Source N1, Swartz et al 2002), the Antennae (CXO Ant J120151.6À185231.9; Fabbiano et al 2003a), and M101 (P098; Mukai et al 2003).…”

Nature of the Soft Ulx in NGC 247: Super-Eddington Outflow and Transition Between the Supersoft and Soft Ultraluminous Regimes

“…Well studied examples include those in M51 (Di Terashima & Wilson 2004;Dewangan et al 2005;Terashima et al 2006), M81 (Swartz et al 2002(Swartz et al , 2003Liu 2008;Liu & Di Stefano 2008;Liu et al 2015), M101 (Pence et al 2001;Mukai et al 2003;Jenkins et al 2004;Kong et al 2004;Kong & Di Stefano 2005;Mukai et al 2005;Liu 2009;Liu et al 2013;Shen et al 2015;Soria & Kong 2016), NGC 247 (Winter et al 2006;Jin et al 2011;Tao et al 2012a), NGC 300 (Read & Pietsch 2001;, the Antennae (NGC 4038/4039; Fabbiano et al 2003), and NGC 4631 (Carpano et al 2007;Soria & Ghosh 2009). Multi-epoch observations of the canonical soft ULX in M101 revealed dramatic variability in the observed X-ray flux by two to threeorders of magnitude (Soria & Kong 2016).…”