The Statistical Properties of Galaxies Containing ULXs

We present a statistical analysis of the properties of galaxies containing ultraluminous X-ray objects (ULXs). Our primary goal is to establish the fraction of galaxies containing a ULX as a function of ULX luminosity. Our sample is based on ROSAT HRI observations of galaxies. We find that ~ 12% of galaxies contain at least one ULX with L_X > 10^39 erg/s and ~ 1% of galaxies contain at least one ULX with L_X > 10^40 erg/s. These ULX frequencies are lower limits since ROSAT HRI observation would miss absorbed ULXs (i.e., with N_H >~ 10^21 cm^-2) and those within ~ 10" of the nucleus (due to the positional error circle of the ROSAT HRI). The Hubble type distribution of galaxies with a ULX differs significantly from the distribution of types for nearby RC3 galaxies, but does not differ significantly from the galaxy type distribution of galaxies observed by the HRI in general. We find no increase in the mean FIR luminosity or FIR / K band luminosity ratio for galaxies with a ULX relative to galaxies observed by the HRI in general, however this result is also most likely biased by the soft bandpass of the HRI and the relatively low number of high SFR galaxies observed by the HRI with enough sensitivity to detect a ULX.Comment: Accepted by Apj. 5 pages with 4 figures formatted using emulateapj. Version with just b/w figures available at http://www.pha.jhu.edu/~ptak/paper

SHEEP: The Search for the High Energy Extragalactic Population

We present the SHEEP survey for serendipitously-detected hard X-ray sources in ASCA GIS images. In a survey area of $\sim 40$ deg$^{2}$, 69 sources were detected in the 5-10 keV band to a limiting flux of $\sim 10^{-13}$ erg cm$^{-2}$ s$^{-1}$. The number counts agree with those obtained by the similar BeppoSAX HELLAS survey, and both are in close agreement with ASCA and BeppoSAX 2-10 keV surveys. Spectral analysis of the SHEEP sample reveals that the 2-10 and 5-10 keV surveys do not sample the same populations, however, as we find considerably harder spectra, with an average $\Gamma\sim1.0$ assuming no absorption. The implication is that the agreement in the number counts is coincidental, with the 5-10 keV surveys gaining approximately as many hard sources as they lose soft ones, when compared to the 2-10 keV surveys. This is hard to reconcile with standard AGN ``population synthesis'' models for the X-ray background, which posit the existence of a large population of absorbed sources. We find no evidence of the population hardening at faint fluxes, with the exception that the few very brightest objects are anomalously soft. 53 of the SHEEP sources have been covered by ROSAT in the pointed phase. Of these 32 were detected. An additional 3 were detected in the RASS. As expected the sources detected with ROSAT are systematically softer than those detected with ASCA alone, and of the sample as a whole (truncated).Comment: 36 pages, 7 figs, to appear in Ap

The X-ray variability and the near-IR to X-ray spectral energy distribution of four low luminosity Seyfert 1 galaxies

We present the results from a study of the X-ray variability and the near-IR to X-ray spectral energy distribution of four low-luminosity, Seyfert 1 galaxies. We compared their variability amplitude and broad band spectrum with those of more luminous AGN in order to investigate whether accretion in low-luminosity AGN operates as in their luminous counterparts. We used archival XMM-Newton and, in two cases, ASCA data to estimate their X-ray variability amplitude and determine their X-ray spectral shape and luminosity. We also used archival HST data to measure their optical nuclear luminosity, and near-IR measurements from the literature, in order to construct their near-IR to X-ray spectra. The X-ray variability amplitude of the four Seyferts is what one would expect, given their black hole masses. Their near-IR to X-ray spectrum has the same shape as the spectrum of quasars which are 10^2-10^5 times more luminous. The objects in our sample are optically classified as Seyfert 1-1.5. This implies that they host a relatively unobscured AGN-like nucleus. They are also of low luminosity and accrete at a low rate. They are therefore good candidates to detect radiation from an inefficient accretion process. However, our results suggest that they are similar to AGN which are 10^2-10^5 times more luminous. The combination of a "radiative efficient accretion disc plus an X-ray producing hot corona" may persist at low accretion rates as well.Comment: 11 pages, 8 figures, accepted for publication in A&

Evidence for Elevated X-ray Emission in Local Lyman Break Galaxy Analogs

In this paper, we study the relationship between the 2-10 keV X-ray luminosity (L_X), assumed to originate from X-ray binaries (XRBs), and star formation rate (SFR) in UV-selected z<0.1 Lyman break analogs (LBAs). We present Chandra observations for four new GALEX-selected LBAs. Including previously studied LBAs, Haro 11 and VV 114, we find that LBAs demonstrate L_X/SFR ratios that are elevated by ~1.5sigma compared to local galaxies, similar to the ratios found for stacked LBGs in the early Universe (z>2). We show that these LBAs are unlikely to harbor AGN, based on their optical and X-ray spectra and the spatial distribution of the X-rays in three spatially extended cases. We expect that high-mass X-ray binaries (HMXBs) dominate the X-ray emission in these galaxies, based on their high specific SFRs (sSFRs=SFR/M* > 10^{-9}/yr), which suggest the prevalence of young stellar populations. Since both LBAs and LBGs have lower dust attenuations and metallicities compared to similar samples of more typical local galaxies, we investigate the effects of dust extinction and metallicity on the L_X/SFR for the broader population of galaxies with high sSFRs (>10^{-10}/yr). The estimated dust extinctions (corresponding to column densities of N_H<10^{22}/cm^2) are expected to have insignificant effects on observed L_X/SFR ratio for the majority of galaxy samples. We find that the observed relationship between L_X/SFR and metallicity appears consistent with theoretical expectations from X-ray binary population synthesis models. Therefore, we conclude that lower metallicities, related to more luminous HMXBs such as ultraluminous X-ray sources (ULXs), drive the elevated L_X/SFR observed in our sample of z<0.1 LBAs. The relatively metal-poor, active mode of star formation in LBAs and distant z>2 LBGs may yield higher total HMXB luminosity than found in typical galaxies in the local Universe.Comment: 11 pages, 7 figures, Submitted to ApJ (references updated in v2