Quasisoft X-Ray Sources: Unusual States of Stellar-Mass Objects, or Intermediate Mass Black Holes?

Abstract

Chandra observations of nearby galaxies have revealed a number of X-ray sources characterized by high luminosities ($L_X > \sim10^{36}$ erg s$^{-1}$) and $k T$ in the range $100 - 350$ eV. These ``quasisoft X-ray sources'' (QSSs) are harder than luminous supersoft X-ray sources (SSSs), whose characteristic temperatures are tens of eV, but, with little or no emission above 2 keV, they are significantly softer than most canonical X-ray sources. They are likely to include a range of physical systems; some may be common systems in unusual states: neutron stars or stellar-mass black holes (BHs), or SNRs. Others may be accreting BHs of intermediate mass. We have analyzed {\it Chandra} data from $19 $ near-by galaxies to identify QSSs with sufficiently high count rates to allow spectral fits. Six of these galaxies have been studied in great detail; in these we find 89 SSSs and 122 QSSs. In this paper we present spectra for QSSs those with more than 50 counts. We also use data from six of these galaxies to study the broadband spectral distributions of QSSs and to compare them with those of other X-ray sources. Since QSSs in the Milky Way are likely to be missed by our selection procedures, we discuss other signatures by which they can be found. If some of the QSSs for which we have spectra are accreting BHs, the lower bounds for the BH mass range from roughly $30 M_\odot$ to $1000 M_\odot$.