A CHANDRA VIEW OF THE NORMAL SO GALAXY NGC 1332. I. AN UNBROKEN, STEEP POWER-LAW LUMINOSITY FUNCTION FOR THE LOW-MASS X-RAY BINARY POPULATION

摘要

Chandra ACIS -S3 observations of the nearby SO galaxy NGC 1332 resolve much of the X-ray emission inlo 73 point sources, of which 37 lie within the D_(25) isophote. The remaining galaxy emission comprises hot, diffuse gas and unresolved sources and is discussed in two companion papers. The point-source X-ray luminosity function (XLF) shows the characteristic break seen in other early-type galaxies at ～2 x 10~(38) ergs s~(-1). After applying corrections for detection incompleteness at low luminosities due to source confusion and contamination from diffuse galactic emission, the break vanishes and the data are well described as a single power law. This result casts further doubt on there being a "universal" XLF break in early-type galaxies, marking the division between neutron star and black hole systems. The logarithmic slope of the differential XLF (dN/dL), β = 2.7 +- 0.5, is marginally (～2.5 σ) steeper than has been found for analogous completeness-corrected fits of other early-type galaxies but closely matches the behavior seen at high luminosities in these systems. Two of the sources within D_(25) are ultraluminous X-ray sources (ULXs), although neither have L_X > 2 x 10~(39) ergs s~(-1). The absence of very luminous ULXs in early-type galaxies suggests a break in the XLF slope at ～(1 -2) x 10~(39) ergs s~(-1), although the data were not of sufficient quality to constrain such a feature in NGC 1332. The sources have a spatial distribution consistent with the optical light and display a range of characteristics that are consistent with an LMXB population. The general spectral characteristics of the individual sources, as well as the composite source spectra, are in good agreement with observations of other early-type galaxies, although a small number of highly absorbed sources are seen. Two sources have very soft spectra, two show strong variability, indicating compact binary nature, and one source shows evidence of an extended radial profile. We do not detect a central source in NGC 1332, but we find a faint [L_X = (2 +- 1) x 10~(38) ergs s~(-1)] point source coincident with the center of the companion dwarf galaxy NGC 1331.