Soft X-ray emission from the inner disc of M33

We present a study, based on archival XMM–Newton observations, of the extended X-ray emission associated with the inner disc of M33. After the exclusion of point sources with LX > 2 × 1035 erg s−1 (0.3–6 keV), we investigate both the morphology and spectrum of the residual X-ray emission, comprising the integrated signal from unresolved discrete sources and diffuse components. This residual emission has a soft X-ray spectrum which can be fitted with a two-temperature thermal model, with kT≈ 0.2 keV and ≈0.6 keV, the cooler component accounting for the bulk of the luminosity. There is evidence that the X-ray emitting plasma has a subsolar metal abundance. The soft X-ray surface brightness distribution shows a strong correlation with far-ultraviolet (FUV) emission and since the latter serves as a tracer of the inner spiral arms of M33 this is indicative of a close connection between recent star formation activity and the production of soft X-rays. Within 3.5 kpc of the nucleus of M33, the soft X-ray and FUV surface brightness distributions exhibit similar radial profiles. The implication is that the ratio of the soft X-ray luminosity (0.3–2.0 keV) per unit disc area to the star formation rate per unit disc area remains fairly constant within this inner disc region. We derive a value for this ratio of 1–1.5 × 1039 erg s−1 (M⊙ yr−1)−1, which is towards the top of the range of similar estimates for several other nearby face-on spiral galaxies (e.g. M51, M83). In the same region, the ratio of soft X-ray luminosity to stellar mass (the latter derived from K-band photometry) is 4 × 1028 erg s−1 M−1⊙, a factor of 5–10 higher than is typical of dwarf elliptical galaxies (e.g. M32, NGC 3379), suggesting that 10–20 per cent of the unresolved emission seen in M33 may originate in its old stellar population. The remainder of the soft X-ray emission is found to be equally split between two spatial components, one which closely traces the spiral arms of the galaxy and the other more smoothly distributed across the inner disc of M33. The former must represent a highly clumped low-filling-factor component linked to sites of recent or ongoing star formation, encompassing H ii regions, X-ray source complexes and X-ray superbubbles, whereas the distribution of latter gives few clues as to its exact origin

X-ray emission from the extended discs of spiral galaxies

We present a study of the X-ray properties of a sample of six nearby late-type spiral galaxies based on XMM–Newton observations. Since our primary focus is on the linkage between X-ray emission and star formation in extended, extranuclear Galactic discs, we have selected galaxies with near face-on aspect and sufficient angular extent so as to be readily amenable to investigation with the moderate spatial resolution afforded by XMM–Newton. After excluding regions in each galaxy dominated by bright point sources, we study both the morphology and spectral properties of the residual X-ray emission, comprising both diffuse emission and the integrated signal of the fainter discrete source populations. The soft X-ray morphology generally traces the inner spiral arms and shows a strong correlation with the distribution of UV light, indicative of a close connection between the X-ray emission and recent star formation. The soft (0.3–2 keV) X-ray luminosity to star formation rate (SFR) ratio varies from 1–5 × 1039 erg s−1 (M⊙ yr−1)−1, with an indication that the lower range of this ratio relates to regions of lower SFR density. The X-ray spectra are well matched by a two-temperature thermal model with derived temperatures of typically ∼0.2 and ∼0.65 keV, in line with published results for other normal and star-forming galaxies. The hot component contributes a higher fraction of the soft luminosity in the galaxies with highest X-ray/SFR ratio, suggesting a link between plasma temperature and X-ray production efficiency. The physical properties of the gas present in the galactic discs are consistent with a clumpy thin-disc distribution, presumably composed of diffuse structures such as superbubbles together with the integrated emission of unresolved discrete sources including young supernova remnants

A Chandra observation of the interacting pair of galaxies NGC 4485/4490

We report the results of a 20-ks Chandra ACIS-S observation of the galaxy pair NGC 4485/4490. This is an interacting system containing a late-type spiral with an enhanced star formation rate (NGC 4490), and an irregular companion that possesses a disturbed morphology. A total of 29 discrete X-ray sources are found coincident with NGC 4490, but only one is found within NGC 4485. The sources range in observed X-ray luminosity from ∼2 × 1037 to 4 × 1039 erg s−1. The more luminous sources appear, on average, to be spectrally harder than the fainter sources, an effect that is attributable to increased absorption in their spectra. Extensive diffuse X-ray emission is detected coincident with the disc of NGC 4490, and in the tidal tail of NGC 4485, which appears to be thermal in nature and hence the signature of a hot interstellar medium in both galaxies. However, the diffuse component accounts for only ∼10 per cent of the total X-ray luminosity of the system (2 × 1040 erg s−1, 0.5–8 keV), which arises predominantly in a handful of the brightest discrete sources. This diffuse emission fraction is unusually low for a galaxy pair which has many characteristics that would lead it to be classified as a starburst system, possibly as a consequence of the small gravitational potential well of the system. The discrete source population, on the other hand, is similar to that observed in other starburst systems, possessing a flat luminosity function slope of ∼−0.6 and a total of six ultraluminous X-ray sources (ULX). Five of the ULX are identified as probable black hole X-ray binary systems, and the sixth (which is coincident with a radio continuum source) is identified as an X-ray luminous supernova remnant. The ULX all lie in star formation regions, providing further evidence of the link between the ULX phenomenon and active star formation. Importantly, this shows that even in star-forming regions, the ULX population is dominated by accreting systems. We discuss the implications of this work for physical models of the nature of ULX, and in particular how it argues against the intermediate-mass black hole hypothesis

New light on the X-ray spectrum of the Crab Nebula

XMM-Newton observations of the Crab provide new information on its integrated X-ray spectrum and the variation of the spectral form across the nebula. The Crab pulsar and its surrounding torus exhibit the hardest spectra with power-law indices of $\Gamma = 1.6$ and 1.8. The jet and outer reaches of the nebula are significantly softer with $\Gamma = 2.1$ and 2.3 respectively. For the whole nebula, the huge number of recorded counts allows a detailed examination of the soft X-ray absorption due to cool gas in the foreground of the Crab. Absorption edges due to oxygen and neon are clearly identified. Oxygen and iron in the interstellar medium are underabundant by a factor of $0.63 \pm 0.01$. The average $N_{\rm H}=3.45\pm0.02 10^{21}$ cm-2 and varies by less than $\pm11\%$ on a scale equal to or larger than 20 arcsec over the face of the nebula. These observations of the Crab provide an excellent demonstration of the power of the EPIC cameras on XMM-Newton for spatial, spectral and timing studies

X-ray hiccups from Sagittarius A* observed by XMM-Newton: The second brightest flare and three moderate flares caught in half a day

Context. Our Galaxy hosts at its dynamical center Sgr A*, the closest supermassive black hole. Surprisingly, its luminosity is several orders of magnitude lower than the Eddington luminosity. However, the recent observations of occasional rapid X-ray flares from Sgr A* provide constraints on the accretion and radiation mechanisms at work close to its event horizon. Aims. Our aim is to investigate the flaring activity of Sgr A* and to constrain the physical properties of the X-ray flares. Methods. In Spring 2007, we observed Sgr A* with XMM-Newton with a total exposure of ~230 ks. We have performed timing and spectral analysis of the new X-ray flares detected during this campaign. To study the range of flare spectral properties, in a consistent manner, we have also reprocessed, using the same analysis procedure and the latest calibration, archived XMM-Newton data of previously reported rapid flares. The dust scattering was taken into account during the spectral fitting. We also used Chandra archived observations of the quiescent state of Sgr A* for comparison. Results. On April 4, 2007, we observed for the first time within a time interval of roughly half a day, an enhanced incidence rate of X-ray flaring, with a bright flare followed by three flares of more moderate amplitude. The former event represents the second brightest X-ray flare from Sgr A* on record with a peak amplitude of about 100 above the quiescent luminosity. This new bright flare exhibits similar light-curve shape (nearly symmetrical), duration (~3 ks) and spectral characteristics to the very bright flare observed in October 3, 2002 by XMM-Newton. The measured spectral parameters of the new bright flare, assuming an absorbed power law model taken into account dust scattering effect, are $N_{\rm H}$ = $12.3^{+2.1}_{-1.8}\times 10^{22}$ cm-2 and $\Gamma$ = 2.3 $\pm$ 0.3 calculated at the 90% confidence level. The spectral parameter fits of the sum of the three following moderate flares, while lower ($N_{\rm H}$ = $8.8^{+4.4}_{-3.2} \times 10^{22}$ cm-2 and $\Gamma = 1.7^{+0.7}_{-0.6}$), are compatible within the error bars with those of the bright flares. The column density found, for a power-law model taking into account the dust scattering, during the flares is at least two times higher than the value expected from the (dust) visual extinction toward Sgr A* ($A_{\rm V}$ ~ 25 mag), i.e., 4.5 $\times$ 1022 cm-2. However, our fitting of the Sgr A* quiescent spectra obtained with Chandra, for a power-law model taking into account the dust scattering, shows that an excess of column density is already present during the non-flaring phase. Conclusions. The two brightest X-ray flares observed so far from Sgr A* exhibited similar soft spectra

Detection of the high z GRB 080913 and its implications on progenitors and energy extraction mechanisms

Aims. We present multiwavelength observations of one of the most distant gamma-ray bursts detected so far, GRB 080913. Based on these observations, we consider whether it could be classified as a short-duration GRB and discuss the implications for the progenitor nature and energy extraction mechanisms. Methods. Multiwavelength X-ray, near IR and millimetre observations were made between 20.7 h and ~16.8 days after the event. Results. Whereas a very faint afterglow was seen at the 3.5m CAHA telescope in the nIR, the X-ray afterglow was clearly detected in both Swift and XMM-Newton observations. An upper limit is reported in the mm range. We have modeled the data assuming a collimated ≳ 3° blast wave with an energy injection at days carrying erg or approximately 12 times the initial energy of the blast wave. We find that GRB 080913 shares many of the gamma-ray diagnostics with the more recent burst GRB 090423 for being classified as short had they ocurred at low redshift. If the progenitor were a compact binary merger, it is likely composed by a NS and BH. The Blandford-Znajek (BZ) mechanism is the preferred one to extract energy from the central, maximally-rotating BH. Both the magnetic field close to the event horizon (B) and the BH mass () are restricted within a relatively narrow range, such that . Similar constraints on the central BH hold for collapsar-like progenitor systems if the BZ-mechanism works for the system at hand