XMM-Newton observations of the Small Magellanic Cloud: X-ray outburst of the 6.85 s pulsar XTE J0103-728

A bright X-ray transient was seen during an XMM-Newton observation in the direction of the Small Magellanic Cloud (SMC) in October 2006. The EPIC data allow us to accurately locate the source and to investigate its temporal and spectral behaviour. X-ray spectra covering 0.2-10 keV and pulse profiles in different energy bands were extracted from the EPIC data. The detection of 6.85 s pulsations in the EPIC-PN data unambiguously identifies the transient with XTE J0103-728, discovered as 6.85 s pulsar by RXTE. The X-ray light curve during the XMM-Newton observation shows flaring activity of the source with intensity changes by a factor of two within 10 minutes. Modelling of pulse-phase averaged spectra with a simple absorbed power-law indicates systematic residuals which can be accounted for by a second emission component. For models implying blackbody emission, thermal plasma emission or emission from the accretion disk (disk-blackbody), the latter yields physically sensible parameters. The photon index of the power-law of ~0.4 indicates a relatively hard spectrum. The 0.2-10 keV luminosity was 2x10^{37} with a contribution of ~3% from the disk-blackbody component. A likely origin for the excess emission is reprocessing of hard X-rays from the neutron star by optically thick material near the inner edge of an accretion disk. From a timing analysis we determine the pulse period to 6.85401(1) s indicating an average spin-down of ~0.0017 s per year since the discovery of XTE J0103-728 in May 2003. The X-ray properties and the identification with a Be star confirm XTE J0103-728 as Be/X-ray binary transient in the SMC.Comment: 5 pages, 4 figures, submitted to A&A on 21 Dec. 200

A Coherent Timing Solution for the Nearby Isolated Neutron Star RX J0720.4-3125

We present the results of a dedicated effort to measure the spin-down rate of the nearby isolated neutron star RX J0720.4-3125. Comparing arrival times of the 8.39-sec pulsations for data from Chandra we derive an unambiguous timing solution for RX J0720.4-3125 that is accurate to 5 years. Adding data from XMM and ROSAT, the final solution yields Pdot=(6.98+/-0.02)x10^(-14) s/s; for dipole spin-down, this implies a characteristic age of 2 Myr and a magnetic field strength of 2.4e13 G. The phase residuals are somewhat larger than those for purely regular spin-down, but do not show conclusive evidence for higher-order terms or a glitch. From our timing solution as well as recent X-ray spectroscopy, we concur with recent suggestions that RX J0720.4-3125 is most likely an off-beam radio pulsar with a moderately high magnetic field.Comment: 5 pages, 1 figure. Accepted for publication in ApJ

RXJ0123.4-7321, a Be/X-ray binary in the wing of the SMC

To confirm faint Be/X-ray binary candidates from the XMM-Newton survey of the Small Magellanic Cloud, we searched for X-ray outbursts in archival ROSAT observations. We found that RXJ0123.4-7321 was much brighter when detected with ROSAT than seen 16 years later by XMM-Newton. We analysed the ROSAT observations and the OGLE I-band light curve of the optical counterpart to investigate the nature of the system. High long-term variability in the X-ray flux of a factor of ~150 was found between the ROSAT and XMM-Newton detections, indicating strong outburst activity during the ROSAT observations. The I-band light curve reveals long-term variability and regular outbursts with a period of (119.9+-2.5) days indicating the orbital period of the binary system. The large X-ray flux variations and the properties of the optical counterpart confirm RXJ0123.4-7321 as a new Be/X-ray binary in the wing of the Small Magellanic Cloud.Comment: 5 pages, 8 figures, accepted for publication in A&

A ROSAT PSPC X-Ray Survey of the Small Magellanic Cloud

We present the results of a systematic search for point-like and moderately extended soft (0.1-2.4 keV) X-ray sources in a raster of nine pointings covering a field of 8.95 deg^2 and performed with the ROSAT PSPC between October 1991 and October 1993 in the direction of the Small Magellanic Cloud. We detect 248 objects which we include in the first version of our SMC catalogue of soft X-ray sources. We set up seven source classes defined by selections in the count rate, hardness ratio and source extent. We find five high luminosity super-soft sources (1E 0035.4-7230, 1E 0056.8-7146, RX J0048.4-7332, RX J0058.6-7146 and RX J0103-7254), one low-luminosity super-soft source RX J0059.6-7138 correlating with the planetary nebula L357, 51 candidate hard X-ray binaries including eight bright hard X-ray binary candidates, 19 supernova remnants, 19 candidate foreground stars and 53 candidate background active galactic nuclei (and quasars). We give a likely classification for ~60% of the catalogued sources. The total count rate of the detected point-like and moderately extended sources in our catalogue is 6.9+/-0.3 s^-1, comparable to the background subtracted total rate from the integrated field of ~6.1+/-0.1 s^-1.Comment: Accepted by A&AS, 13 pages, 2 Postscript figure

Two long-period X-ray pulsars detected in the SMC field around XTE J0055-727

An XMM-Newton target of opportunity observation of the field around the transient 18.37 s pulsar XTE J0055-727 in the Small Magellanic Cloud (SMC) revealed two bright, long-period X-ray pulsars in the EPIC data. A new pulsar, XMMU J005517.9-723853, with a pulse period of 701.7 +/- 0.8 s was discovered and 500.0 +/- 0.2 s pulsations were detected from XMMU J005455.4-724512 (= CXOU J005455.6-724510), confirming the period found in Chandra data. We derive X-ray positions of RA = 00h5455.88s, Dec = -72d45m10.5s and RA = 00h55m18.44s, Dec = -72d38m51.8s (J2000.0) with an uncertainty of 0.2'' utilizing optical identification with OGLE stars. For both objects, the optical brightness and colours and the X-ray spectra are consistent with Be/X-ray binary systems in the SMC.Comment: A&A Letters, in pres

Spectroscopy of the brightest optical counterparts of X-ray sources in the direction of M~31 and M~33

Recent surveys of the Local Group spiral Galaxies M31 and M33 with XMM-Newton yielded a large number of X-ray sources. As part of the effort to identify and classify the objects responsible for this X-ray emission, we have obtained optical spectra of the brightest optical counterparts of the identified X-ray sources, using the 1.3m Skinakas Telescope. Most of these objects are foreground star candidates. The purpose of the present study is to confirm this identification and to explore the compatibility between the optical spectral classification and the observed X-ray properties of the sources. We have obtained optical spectra for the 14 brightest optical counterparts of X-ray sources identified by XMM-Newton in the direction of M31 and for 21 optical counterparts in the direction of M33, using the 1.3m Skinakas telescope in Crete, Greece. All of the M31 sources and all but one of the M33 sources were confirmed to be foreground stars, of spectral types between A and M. One of the stars is a late M dwarf with H-alpha emission, a flare star, also displaying strong X-ray variability. One of the M~33 sources (lying within the D25 ellipse) corresponds to a previously known background galaxy, LEDA 5899.Comment: 9 pages, 12 figures, accepted in A&

X-ray source populations in the Magellanic Clouds

Early X-ray surveys of the Magellanic Clouds (MCs) were performed with the imaging instruments of the Einstein, ASCA and ROSAT satellites revealing discrete X-ray sources and large-scale diffuse emission. Large samples of supernova remnants, high and low mass X-ray binaries and super-soft X-ray sources could be studied in detail. Today, the major X-ray observatories XMM-Newton and Chandra with their advanced angular and spectral resolution and extended energy coverage are ideally suited for detailed population studies of the X-ray sources in these galaxies and to draw conclusions on our own Galaxy. We summarize our knowledge about the X-ray source populations in the MCs from past missions and present first results from systematic studies of the Small Magellanic Cloud (SMC) using the growing number of archival XMM-Newton observations.Comment: 6 pages, 3 figures, Fig. 2 in low resolution, to be published in the proceedings of the ESAC workshop "X-rays from Nearby Galaxies

On the nature of the flux variability during an expansion stage of a type I X-ray burst: Constraints on Neutron Star Parameters for 4U 1820-30

Powerful Type I X-ray burst with strong radial expansion was observed from the low mass X-ray binary 4U 1820-30 with Rossi X-ray Timing Explorer on May 2, 1997. We investigate closely the flux profile during the burst expansion stage. Applying a semi-analytical model we are able to uncover the behavior of a photospheric radius and to simulate the evolution of neutron star (NS)-accretion disk system. The bottom flux L_{bot} is a few times the Eddington limit L_{Edd} for outer layers, because the electron cross-section is a few times less than the Thomson cross-section at such a high temperatures. The surplus of energy flux with respect to the Eddington, $L_{bot}-L_{Edd}$, goes into the potential energy of the expanded envelope. As cooling of the burning zone starts the surplus decreases and thus the envelope shrinks while the emergent photon flux stays the same $L=L_{Edd}$. At a certain moment the NS low-hemisphere, previously screened by the disk, becomes visible to the observer. Consequently, the flux detected by the observer increases. Indeed, we observe to the paradoxical situation when the burning zone cools, but the apparent flux increases because of the NS-accretion disk geometry. We demonstrate a strong observational evidence of NS-accretion disk occultation in the behavior of the observed bolometric flux. We estimate the anisotropy due to geometry and find that the system should have a high inclination angle. Finally, we apply an analytical model of X-ray spectral formation in the neutron star atmosphere during burst decay stage to infer the neutron star (NS) mass-radius relation.Comment: 15 pages, 3 figures, accepted to ApJ

AGN in the XMM-Newton first-light image as probes for the interstellar medium in the LMC

The XMM-Newton first-light image revealed X-ray point sources which show heavily absorbed power-law spectra. The spectral indices and the probable identification of a radio counterpart for the brightest source suggest AGN shining through the interstellar gas of the Large Magellanic Cloud (LMC). The column densities derived from the X-ray spectra in combination with HI measurements will allow to draw conclusions on HI to H_2 ratios in the LMC and compare these with values found for the galactic plane.Comment: 4 pages, LaTex, 4 figures, Accepted for publication in A&A Letter