On the Integrated Spectrum of the X-ray Binaries and the Origin of Soft X-ray Emission from the Bulge of M31

Using ROSAT PSPC data, we have performed several tests aimed at understanding the origin of the soft X-ray spectral component detected from the bulge of M31. We find that a significant soft component in the spectrum of the bulge is spatially correlated with the unresolved X-ray emission near the core of M31, which is probably a hot interstellar medium or perhaps a population of multiple faint sources. For the first time, we extracted the spectrum of this unresolved emission, by removing point sources dominating the integral spectrum of the bulge, and found it to be responsible for the most of soft excess. A soft spectral component is not at all needed to fit the point source spectrum that remains after subtracting the unresolved emission. The integral spectra of bright point sources, both inside and outside of the M31 bulge, can be fitted with a single power-law in the ROSAT band. Our analysis rules out the previous suggestion that all bulge emission in M31 may be generated by low mass X-ray binaries (Irwin & Bregman, 1999).Comment: 11 pages incl. 2 figures, 2 tables, accepted to ApJ

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 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

Aeromagnetic anomaly images of Vulcano and Southern Lipari Islands (Aeolian Archipelago, Italy)

Newly acquired high-resolution, low-altitude aeromagnetic data over Vulcano Island and Southwestern Lipari in the Southern Tyrrhenian Sea resolve the major volcanic features in the area associated with the past and present activity. The magnetic character changes in amplitude and frequency from south-east to north-west. The Primordial Vulcano, the Lentia Complex, the Piano Caldera units, the Fossa Caldera deposits, and the currently active La Fossa cone and Vulcanello represent the main volcanic phases on Vulcano Island. They show a distinct magnetic anomaly pattern, whereas prior to this survey, no magnetic signatures of these features were found

Chandra Study of X-Ray Point Sources in the Early-Type Galaxy NGC 4552 (M89)

We present a Chandra ACIS study of the early-type galaxy NGC 4552. We detect 47 X-ray point sources, most of which are likely LMXBs, within 4 R_e. The brightest X-ray source coincides with the optical/UV/radio center of the galaxy, and shows variability on >1 hr timescales, indicating the possible existence of a LLAGN. The 46 off-center sources and the unresolved point sources contribute about 29% and 20% to the total luminosity of the galaxy, respectively. The corrected cumulative XLF of the off-center sources is best fit by a broken power-law model with a break at L_b=4.4+2.0-1.4 e38 ergs/s. We identified 210 GCs candidates in a HST WFPC2 optical image of the galaxy's central region. Of the 25 off-center LMXBs that fall within the WFPC2 FOV, 10 sources are coincident with a GC. Thus the fraction of the GCs hosting bright LMXBs and the fraction of the LMXBs associated with GCs are 4.8% and 40%, respectively. In the V and I bands, the GCs hosting bright LMXBs are typically 1-2 magnitudes brighter than the GCs with no detected LMXBs. There are about 1.9+-0.4 times as many LMXBs in the red, metal-rich GCs as there are in the blue, metal-poor ones. We find no obvious difference between the luminosity distributions of LMXBs in GCs and in field, but the cumulative spectrum of the LMXBs in GCs tends to be softer than that of the LMXBs in field. We detected 3 X-ray sources that have isotropic luminosities larger than 1e39 erg/s. The one located in the joint Chandra-HST field is found to be associated with a GC. By studying the ACIS spectra we infer that the this may be a candidate black hole system with a mass of 15-135 M_sun. One of the other sources with a luminosity brighter than 1e39 ergs/s reveals temporal variations in brightness on timescales greater than an hour.Comment: Accepted for publication in Ap

The Discovery of a Spatially-Resolved Supernova Remnant in M31 with Chandra

Chandra observations of M31 allow the first spatially resolved X-ray image of a supernova remnant (SNR) in an external spiral galaxy. CXOM31 J004327.7+411829 is a slightly elongated ring-shaped object with a diameter of ~11'' (42 pc). In addition, the X-ray image hints that the chemical composition of the SNR is spatial dependent. The X-ray spectrum of the SNR can be well fitted with a Raymond-Smith model or a non-equilibrium ionization model. Depending on the spectral model, the 0.3-7 keV luminosity is between 3.2x10^36 erg/s and 4.5x10^37 erg/s. The age of the SNR is estimated to be 3210-22300 years and the number density of ambient gas is ~0.003-0.3 cm^-3. This suggests that the local interstellar medium around the SNR is low.Comment: 5 pages, 3 figures; accepted for publication in ApJ

A complex geo-scientific strategy for landslide hazard mitigation ? from airborne mapping to ground monitoring

International audienceAfter a large landslide event in Sibratsgfäll/Austria several exploration methods were evaluated on their applicability to investigate and monitor landslide areas. The resulting optimised strategy consists of the combined application of airborne electromagnetics, ground geoelectrical measurements and geoelectrical monitoring combined with hydrological and geological mapping and geotechnical modelling. Interdisciplinary communication and discussion was the primary key to assess this complicated hazard situation

Application and reliability of techniques for landslide site investigation, monitoring and early warning – outcomes from a questionnaire study

The presented questionnaire study summarizes an evaluation of approaches, techniques and parameters of slope-instability investigation and monitoring of their occurrence, reliability and the applicability of the monitoring techniques for early warning. The study is based on information collected from 86 monitored landslides in 14 European and Asian countries. Based on the responses, lidar ALS (airborne laser scanners), geophysical logging, aerial photographs, resistivity surveying, GB InSAR (ground-based synthetic aperture radar interferometer) and the refraction seismic were considered the most reliable methods for investigation of structure and character of landslides. Especially lidar ALS and geophysical logging were ranked high despite their application at relatively few landslides. Precipitation amount, pore-water pressure and displacement monitored by wire extensometers, dGPS and total stations, followed by air temperature and EM-emissions monitoring and displacement monitored by the TM 71 crack gauge were considered the most promising parameters for early warning