Population synthesis and its connection to astronomical observables

by Sipior, Michael S.

In this thesis, I present a model used for binary population synthesis, and use it to simulate a starburst of 2 × 108 M? over a duration of 20 Myr. This population reaches a maximum 2–10 keV luminosity of ? 4 × 1040 erg s?1, attained at the end of the star formation episode, and sustained for a period of several hundreds of Myr by succeeding populations of XRBs with lighter companion stars. An important property of these results is the minimal dependence on poorly-constrained values of the initial mass function (IMF) and the average mass ratio between accreting and donating stars in XRBs. The peak X-ray luminosity is shown to be consistent with recent observationally-motivated correlations between the star formation rate and total hard (2–10 keV) X-ray luminosity. Recent calculations published by other groups fail to account for the aforementioned sustained high X-ray luminosity from different mass companions. Model cumulative luminosity functions show increasing steepness at the high end, as the most luminous systems die off. I also consider those XRBs with massive companions that survive the second supernova, and go on to become double compact object binaries. Depending upon the initial configuration at the time the second compact object is formed, the system may go on to experience a merger through iii the loss of orbital energy to gravitational radiation. We show that with a detection threshold of h ? 10?21 for gravitational radiation (comparable to the expected sensitivity of LIGO I), a total merger rate of 6×10?3–10?2 yr?1 can be expected. This means that detection of gravitational wave sources through this formation channel will have to wait for LIGO II, with an order of magnitude improvement in sensitivity, and a commensurate thousand-fold increase in search volume and event rates. In an attempt to compare model predictions with observations, I analyze a sample of 41 nearby mildly-active galaxies observed in a snapshot survey during Cycles 1 and 2 of the Chandra X-ray Observatory. Using the observed X-ray images, 33 nuclei are detected, and diffuse nuclear X-ray emission is found in 25% of the targets. Substantial XRB populations are detected in all but a few fields, many with luminosities in excess of 1039 erg s?1. Over four hundred sources were detected overall, with fourteen in the latter high luminosity category. All but one of these sources is found in a spiral host galaxy, implying that such sources are generally tied to higher star formation rates. iv