The X-ray Globular Cluster Population in NGC 1399

We report on the {\it Chandra} observations of the elliptical galaxy NGC 1399, concentrating on the X-ray sources identified with globular clusters (GCs). A large fraction of the 2-10 keV X-ray emission in the $8' \times 8'$ {\it Chandra} image is resolved into point sources with luminosities $\ge 5 \times 10^{37}$ \ergsec. These sources are most likely Low Mass X-ray Binaries (LMXBs). In a region imaged by {\it HST} about 70% of the X-ray sources are located within GCs. This association suggests that in giant elliptical galaxies luminous X-ray binaries preferentially form in GCs. Many of the GC sources have super-Eddington luminosities (for an accreting neutron star) and their average luminosity is higher than the non-GC sources. The X-ray spectral properties of both GC and non-GC sources are similar to those of LMXBs in our Galaxy. Two of the brightest sources, one of which is in a GC, have an ultra-soft spectrum, similar to that seen in the high state of black hole candidates. The ``apparent'' super-Eddington luminosity in many cases may be due to multiple LMXB systems within individual GCs, but with some of the most extremely luminous systems containing massive black holes.Comment: accepted in ApJ letter. 10 pages 5 figure

Millenium Year X-ray Transients in Andromeda Galaxy

We study three transient X-ray sources, that were bright in the central region of M31 galaxy in the year 2000. Observations with Chandra and XMM-Newton allowed us for the first time in the history of X-ray astronomy, to build light curves of transient sources in M31 suitable for studying their variability on a time scale of months and, in some periods, weeks. The three sources demonstrate distinctly different types of X-ray variability and spectral evolution. XMMU J004234.1+411808 is most likely a black hole candidate based on the similarity of its X-ray light curve and spectra to typical transient low-mass X-ray binaries observed in our Galaxy. The outburst of CXO J004242.0+411608 lasted longer than a year, which makes the source an unusual X-ray transient. The supersoft transient XMMU J004319.4+ 411759 is probably a classical nova-like system containing a magnetized, rapidly-spinning white dwarf. We estimate a total rate of X-ray transient outbursts in the central bulge of M31 to be of the order ~10 per year. The rate of the hard X-ray transients (~5 1/year) in the central part of the Andromeda Galaxy appears to be comparable to that of the central part of our own Galaxy.Comment: ApJ Letters, submitte

The Dependence of the Soft X-ray Properties of LMXBs on the Metallicity of Their Environment

We determine the X-ray spectral properties of a sample of low-mass X-ray binaries (LMXBs) which reside in globular clusters of M31, as well as five LMXBs in Galactic globular clusters and in the Large Magellanic Cloud using the ROSAT PSPC. We find a trend in the X-ray properties of the LMXBs as a function of globular cluster metallicity. The spectra of LMXBs become progressively softer as the metallicity of its environment increases. The one M31 globular cluster LMXB in our sample which has a metallicity greater than solar has spectral properties similar to those of LMXBs in the bulge of M31, but markedly different from those which reside in low metallicity globular clusters, both in M31 and the Galaxy. The spectral properties of this high metallicity LMXB is also similar to those of X-ray faint early-type galaxies. This lends support to the claim that a majority of the X-ray emission from these X-ray faint early-type galaxies results from LMXBs and not hot gas, as is the case in their X-ray bright counterparts.Comment: 5 pages, 2 embedded Postscript figures, uses emulateapj.sty, Astrophysical Journal Letters, in pres

XMM-Newton discovery of 217 s pulsations in the brightest persistent supersoft X-ray source in M31

We report on the discovery of a periodic modulation in the bright supersoft X-ray source XMMU J004252.5+411540 detected in the 2000-2004 XMM-Newton observations of M31. The source exhibits X-ray pulsations with a period P~217.7 s and a quasi-sinusoidal pulse shape and pulsed fraction ~7-11%. We did not detect statistically significant changes in the pulsation period on the time scale of 4 years. The X-ray spectra of XMMU J004252.5+411540 are extremely soft and can be approximated with an absorbed blackbody of temperature 62-77 eV and a weak power law tail of photon index ~1.7-3.1 in the 0.2-3.0 keV energy band. The X-ray properties of the source and the absence of an optical/UV counterpart brighter than 19 mag suggest that it belongs to M31. The estimated bolometric luminosity of the source varies between ~2e38 and ~8e38 ergs/s at 760 kpc, depending on the choice of spectral model. The X-ray pulsations and supersoft spectrum of XMMU J004252.5+411540 imply that it is almost certainly an accreting white dwarf, steadily burning hydrogen-rich material on its surface. We interpret X-ray pulsations as a signature of the strong magnetic field of the rotating white dwarf. Assuming that the X-ray source is powered by disk accretion, we estimate its surface field strength to be in the range 4e5 G <B_{0}<8e6 G. XMMU J004252.5+411540 is the second supersoft X-ray source in M31 showing coherent pulsations, after the transient supersoft source XMMU J004319.4+411758 with 865.5 s pulsation period.Comment: 11 pages, 4 figures, uses emulateapj style. Submitted to Ap

Optical Constraints on an X-ray Transient Source in M31

We have detected a transient X-ray source in the M31 bulge through a continuing monitoring campaign with the Chandra ACIS-I camera. The source was detected at R.A.=00:42:33.428 +/- 0.11'', Dec.=+41:17:03.37 +/- 0.11'' in only a single observation taken 2004 May 23. Fortuitous optical HST/ACS imaging of the transient location prior to the X-ray outburst, along with follow-up HST/ACS imaging during and after the outburst, reveals no transient optical source brighter than B (equivalent) = 25.5. The location of the source and its X-ray properties suggest it is a low mass X-ray binary (LMXB). Assuming the transient is similar to many Galactic X-ray novae, the X-ray luminosity of (3.9 +/- 0.5) X 10$^{37}$ erg s$^{-1}$ and the upper-limit on the optical luminosity provide a prediction of <1.6 days for the orbital period of the binary system.Comment: 15 pages, 3 figures, accepted for publication in Ap

Supersoft X-ray sources in M31: II. ROSAT-detected supersoft sources in the ROSAT, Chandra and XMM eras

We have performed Chandra observations during the past 3 years of 5 of the M31 supersoft X-ray sources (SSS) discovered with ROSAT. Surprisingly, only one of these sources has been detected, despite a predicted detection of about 20-80 counts. This has motivated a thorough check of the ROSAT M31 survey I data, including a relaxation of the hardness ratio requirement used to select SSS. This increases the number of SSS identified in survey I by 7. We then carried out a comparison with the ROSAT M31 survey II dataset which had hitherto not been explicitly investigated for SSS. We find that most of the ROSAT survey I sources are not detected, and only two new SSS are identified. The low detection rate in the ROSAT survey II and our Chandra observations implies that the variability time scale of SSS is a few months. If the majority of these sources are close-binary SSS with shell hydrogen burning, it further implies that half of these sources predominantly experience large mass transfer rates.Comment: accepted for publ. in ApJ; 2 ps-figures; high-quality figures available at http://www.mpe.mpg.de/~jcg/publis.htm

Discovery of Bright Variable X-ray Sources in NGC 1569 with Chandra

From the analysis of a ~100 ks Chandra observation of the dwarf starburst galaxy NGC 1569, we have found that the X-ray point sources, CXOU 043048.1+645050 and CXOU 043048.6+645058, showed significant time variability. During this observation, the X-ray flux of CXOU 043048.1+645050 increased by 10 times in only 2 x 10^4 s. Since the spectrum in its bright phase was fitted with a disk blackbody model with kT_in ~0.43 keV and the bolometric luminosity is L_bol ~10^38 ergs s^-1, this source is an X-ray binary with a stellar mass black-hole. Since the spectrum in its faint phase was also fitted with a disk blackbody model, the time variability can be explained by a change of the accretion rate onto the black hole. The other variable source, CXOU 043048.6+645058, had a flat spectrum with a photon index of ~1.6. This source may be an X-ray binary with an X-ray luminosity of several x 10^37 ergs s^-1. In addition, three other weak sources showed possible time variability. Taking all of the variability into account may suggest an abundant population of compact X-ray sources in NGC 1569.Comment: 15 pages including 4 Postscript figures; accepted for publication in ApJ

An X-ray spectral survey of the disc of M31 with XMM-Newton

We present the results of a complete spectral survey of the X-ray point sources detected in five XMM-Newton observations along the major axis of M31 but avoiding the central bulge, aimed at establishing the population characteristics of X-ray sources in this galaxy. We obtained background subtracted spectra and lightcurves for each of the 335 X-ray point sources detected across the five observations from 2002. We also correlate our source list with those of earlier X-ray surveys and radio, optical and infra-red catalogues. Sources with more than 50 source counts are individually spectrally fit in order to create the most accurate luminosity functions of M31 to date. Based on the spectral fitting of these sources with a power law model, we observe a broad range of best fit photon index. From this distribution of best fit index, we identify 16 strong high mass X-ray binary system candidates in M31. We show the first cumulative luminosity functions created using the best fit spectral model to each source with more than 50 source counts in the disc of M31. The cumulative luminosity functions show a prominent, statistically significant flattening in the X-ray luminosity LX interval 37.0 \lesssim log LX erg s-1 \lesssim 37.5. Such a feature may also be present in the X-ray populations of several other galaxies, but at a much lower statistical significance. We investigate the number of AGN present in our source list and find that above LX ~1.4x1036 erg s-1 the observed population is statistically dominated by the point source population of M31.Comment: accepted by A&A. 24 pages, 6 figures, 7 table

A method for the stochastic modeling of karstic systems accounting for geophysical data: an example of application in the region of Tulum, Yucatan Peninsula (Mexico)

The eastern coast of the Yucatan Peninsula, Mexico, contains one of the most developed karst systems in the world. This natural wonder is undergoing increasing pollution threat due to rapid economic development in the region of Tulum, together with a lack of wastewater treatment facilities. A preliminary numerical model has been developed to assess the vulnerability of the resource. Maps of explored caves have been completed using data from two airborne geophysical campaigns. These electromagnetic measurements allow for the mapping of unexplored karstic conduits. The completion of the network map is achieved through a stochastic pseudo-genetic karst simulator, previously developed but adapted as part of this study to account for the geophysical data. Together with the cave mapping by speleologists, the simulated networks are integrated into the finite-element flow-model mesh as pipe networks where turbulent flow is modeled. The calibration of the karstic network parameters (density, radius of the conduits) is conducted through a comparison with measured piezometric levels. Although the proposed model shows great uncertainty, it reproduces realistically the heterogeneous flow of the aquifer. Simulated velocities in conduits are greater than 1cm s−1, suggesting that the reinjection of Tulum wastewater constitutes a pollution risk for the nearby ecosystem

ROSAT X-ray Colors and Emission Mechanisms in Early-Type Galaxies

The X-ray colors and X-ray-to-optical luminosity ratios (L_X/L_B) of 61 early- type galaxies observed with the ROSAT PSPC are determined. The colors indicate that the X-ray spectral properties of galaxies vary as a function of L_X/L_B. The brightest X-ray galaxies have colors consistent with thermal emission from hot gas with roughly the same metallicity of 50% solar. The spatial variation of the colors indicates that the gas temperature in these galaxies increases radially. Galaxies with medium L_X/L_B also have spectral properties consistent with emission from hot gas. If a simple one-component thermal model is assumed to describe the 0.1-2.0 keV X-ray emission in these galaxies, then one possible explanation for the progressive decrease in L_X/L_B among galaxies of this class could be the progressive decrease in metal abundance of the X-ray emitting contained by the galaxies. Galaxies with the lowest L_X/L_B values appear to be lacking a hot interstellar component. Their X-ray colors are consistent with those derived from the bulges of the spiral galaxies M31 and NGC1291. In M31 the X-ray emission is resolved into discrete sources, and is apparently due primarily to low mass X-ray binaries (LMXBs). We therefore suggest that the bulk of the X-ray emission in the faintest ellipticals is also due to LMXBs. Previously, the X-ray spectra of X-ray faint galaxies had been found to be described by a hard component which was attributed to LMXB emission, and a very soft component of unknown origin. We show that the very soft component also likely results from LMXBs, as a very soft component is seen in the X-ray spectra of the nearby LMXB Her X-1 and LMXBs in the bulge of M31. (Abridged)Comment: 49 pages, 17 embedded Postscript figures, uses aaspp4.sty, Astrophysical Journal, volume 499, in pres