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Highly penetrating cosmic ray muons constantly shower the earth at a rate of about 1 muon per cm2 per minute. We have developed a technique which exploits the multiple Coulomb scattering of these particles to perform nondestructive inspection without the use of artificial radiation. In prior work [1]-[3], we have described heuristic methods for processing muon data to create reconstructed images. In this paper, we present a maximum likelihood/expectation maximization tomographic reconstruction algorithm designed for the technique. This algorithm borrows much from techniques used in medical imaging, particularly emission tomography, but the statistics of muon scattering dictates differences. We describe the statistical model for multiple scattering, derive the reconstruction algorithm, and present simulated examples. We also propose methods to improve the robustness of the algorithm to experimental errors and events departing from the statistical model.

Abstract. We present the results of a study based on an XMM-Newton Performance Verification observation of the central 30 of the nearby spiral galaxy M 31. In the 34-ks European Photon Imaging Camera (EPIC) exposure, we detect 116 sources down to a limiting luminosity of 6 10 35 erg s −1 (0.3-12 keV, d = 760 kpc). The luminosity distribution of the sources detected with XMM-Newton flattens at luminosities below ∼2.5 10 37 erg s −1 . We make use of hardness ratios for the detected sources in order to distinguish between classes of objects such as super-soft sources and intrinsically hard or highly absorbed sources. We demonstrate that the spectrum of the unresolved emission in the bulge of M 31 contains a soft excess which can be fitted with a ∼0.35-keV optically-thin thermal-plasma component clearly distinct from the composite point-source spectrum. We suggest that this may represent diffuse gas in the centre of M 31, and we illustrate its extent in a wavelet-deconvolved image.

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Abstract. We describe the first XMM-Newton observations of the starburst galaxy NGC 253. As known from previous X-ray observations, NGC 253 shows a mixture of extended (disk and halo) and point-source emission. The high XMM-Newton throughput allows a detailed investigation of the spatial, spectral and variability properties of these components simultaneously. We characterize the brightest sources by their hardness ratios, detect a bright X-ray transient ∼70 SSW of the nucleus, and show the spectrum and light curve of the brightest point source (∼30 S of the nucleus, most likely a black-hole X-ray binary, BHXRB). The unresolved emission of two disk regions can be modeled by two thin thermal plasma components (temperatures of ∼0.13 and 0.4 keV) plus residual harder emission, with the lower temperature component originating from above the disk. The nuclear spectrum can be modeled by a three temperature plasma (∼0.6, 0.9, and 6 keV) with the higher temperatures increasingly absorbed. The high temperature component most likely originates from the starburst nucleus, as no non-thermal component, that would point at a significant contribution from an active nucleus (AGN), is needed. Assuming that type IIa supernova remnants (SNRs) are mostly responsible for the E > 4 keV emission, the detection with EPIC of the 6.7 keV line allows us to estimate a supernova rate within the nuclear starburst of 0.2 yr −1 . The unprecedented combination of RGS and EPIC also sheds new light on the emission of the complex nuclear region, the X-ray plume and the disk diffuse emission. In particular, EPIC images reveal that the limbbrightening of the plume is mostly seen in higher ionization emission lines, while in the lower ionization lines, and below 0.5 keV, the plume is more homogeneously structured. The plume spectrum can again be modeled by a three temperature thermal plasma containing the two low temperature nuclear components (though less absorbed) plus an unabsorbed 0.15 keV component similar to the disk spectra. This points to new interpretations as to the make up of the starburst-driven outflow.

We report the discovery of seven new fossil systems in the 400d cluster survey. Our search targets nearby, z ≤ 0.2, and X-ray bright, L X ≥ 10 43 erg sec −1 , clusters of galaxies. Where available, we measure the optical luminosities from Sloan Digital Sky Survey images, thereby obtaining uniform sets of both X-ray and optical data. Our selection criteria identify 12 fossil systems, out of which five are known from previous studies. While in general agreement with earlier results, our larger sample size allows us to put tighter constraints on the number density of fossil clusters. It has been previously reported that fossil groups are more X-ray bright than other X-ray groups of galaxies for the same optical luminosity. We find, however, that the X-ray brightness of massive fossil systems is consistent with that of the general population of galaxy clusters and follows the same L X − L opt scaling relation.

Abstract. We present the results of a study of the variability of X-ray sources in the central 30 of the nearby Andromeda Galaxy (M 31) based on XMM-Newton Performance Verification observations. Two observations of this field, with a total exposure time of about 50 ks, were performed in June and December of 2000. We found 116 sources brighter than a limiting luminosity of 6×10 35 erg s −1 (0.3-12 keV, d = 760 kpc). For the ∼60 brightest sources, we searched for periodic and non-periodic variability; at least 15% of these sources appear to be variable on a time scale of several months. We discovered a new bright transient source ∼2.9 from the nucleus in the June observation; this source faded significantly and was no longer detected in December. The behaviour of the object is similar to a handful of Galactic LMXB transients, most of which are supposed to harbor black holes. We detected pulsations with a period of ∼865 s from a source with a supersoft spectrum. The flux of this source decreased significantly between the two XMM observations. The detected period is unusually short and points to a rapidly spinning magnetized white dwarf. The high luminosity and transient nature of the source suggest its possible identification with classical or symbiotic nova, some of which were observed earlier as supersoft sources.

We present our analysis of the high-energy radiation from black hole (BH) transients, using archival data obtained primarily with the Rossi X-ray Timing Explorer (RXTE), and a comprehensive test of the bulk motion Comptonization (BMC) model for the high-soft state continuum. The emergent spectra of over 30 separate measurements of GRO J1655-40, GRS 1915+105, GRS 1739 1630-47 XTE J1755-32, and EXO 1846-031 X-ray sources are successfully fitted by the BMC model, which has been derived from basic physical principles in previous work. This in turn provides direct physical insight into the innermost observable regions where matter impinging upon the event horizon can effectively be directly viewed. The BMC model is characterized by three parameters: the disk color temperature, a geometric factor related to the illumination of the black hole (BH) site by the disk and a spectral index related to the efficiency of the bulk motion upscattering. For the case of GRO J1655-40, where there are distance and mass determinations, a self consistency check of the BMC model has been made, in particular, the assumption regarding the dominance of gravitational forces over the pressure forces within the inner few Schwarzschild radii. We have also examined the time behavior of these parameters which can provide information on the source structure. Using our inferred model parameters: color temperature, spectral index and an absolute normalization we present new, independently derived, constraints on the black hole mass, mass accretion rate and the distance for the aforementioned sources. Also notable is the relationship between the color temperature and flux, which for GRO J1655-40 is entirely distinct from a simple T 4 dependence, and strikingly consistent with the disk model we have invoked -standard Shakura-Sunyaev's disk with the modification to the electron scattering. This -3provides insight into the origin of the seed soft photons, and allows us to impose an important estimation of the hardness parameter, T h , which is the ratio of the color temperature to the effective temperature -we find T h ≃ 2.6, higher than previous estimates used in the literature. Subject headings: accretion -black hole physics -binaries: close -radiation mechanisms: nonthermal -Compton and inverse Compton-relativitystars: individual (GRO J1655-40, GRS 1915+105, GRS 1739-278, 4U 1630-47 XTE J1755-32, and EXO 1846-031)