A Chandra view of the normal S0 galaxy NGC 1332: I. An unbroken, steep power law luminosity function for the LMXB population

Abstract

Chandra ACIS-S3 observations of the S0 galaxy NGC 1332 resolve much of the emission into 73 point-sources, of which 37 lie within the D_25 isophote. The unresolved emission is discussed in two companion papers. The source luminosity function (XLF) shows the break seen in other early-type galaxies at \~2x10^{38}ergs/s. After correcting for detection incompleteness due to source confusion and contamination from diffuse emission, the break vanishes and the data are well-described as a single power law. This casts further doubt on there being a ``universal'' XLF break in such galaxies dividing neutron-star and black-hole systems. The slope of the differential XLF (dN/dL), beta=2.7+/-0.5, is marginally (~2.5 sigma) steeper than has been found for analogous fits of other early-type galaxies but resembles what is generally seen at high luminosities. Two of the sources within D_25 are ULX, although neither have L_X>2x 10^{39} ergs/s. The absence of very luminous ULX in early-type galaxies suggests a break in the XLF slope at ~1-2x 10^{39} ergs/s, but we cannot constrain it in NGC 1332. The sources have a spatial distribution consistent with the optical light and display a range of properties expected for a LMXB population. The spectra of the individual sources, as well as the composite source spectra, agree 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 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) point-source coincident with the centre of the companion dwarf galaxy NGC 1331. (Abridged)