C. Motch

The XMM-Newton serendipitous survey: V - The Second XMM-Newton serendipitous source catalogue

Aims. Pointed observations with XMM-Newton provide the basis for creating catalogues of X-ray sources detected serendipitously in each field. This paper describes the creation and characteristics of the 2XMM catalogue. Methods. The 2XMM catalogue has been compiled from a new processing of the XMM-Newton EPIC camera data. The main features of the processing pipeline are described in detail. Results. The catalogue, the largest ever made at X-ray wavelengths, contains 246 897 detections drawn from 3491 public XMM-Newton observations over a 7-year interval, which relate to 191 870 unique sources. The catalogue fields cover a sky area of more than 500 deg(2). The non-overlapping sky area is similar to 360 deg(2) (similar to 1% of the sky) as many regions of the sky are observed more than once by XMM-Newton. The catalogue probes a large sky area at the flux limit where the bulk of the objects that contribute to the X-ray background lie and provides a major resource for generating large, well-defined X-ray selected source samples, studying the X-ray source population and identifying rare object types. The main characteristics of the catalogue are presented, including its photometric and astrometric properties

The proper motion and energy distribution of the isolated neutron star RX J0720.4-3125

ESO 4 m class telescope and VLT deep imaging of the isolated neutron star RX J0720.4-3125 reveals a proper motion of µ = 97 ± 12 mas/yr and a blue U − B color index. We show that a neutron star atmosphere model modified to account for a limited amount of hydrogen on the stars surface can well represent both the optical and X-ray data without invoking any additional thermal component. The large proper motion almost completely excludes the possibility that accretion from the interstellar medium is the powering mechanism of the X-ray emission. It also implies that the proposed spin down is entirely due to magnetic dipole losses. RX J0720.4-3125 is thus very likely a middle aged cooling neutron star. Its overall properties are quite similar to some of the long period radio pulsars recently discovered, giving further support to the idea that RX J0720.4-3125 may be a pulsar whose narrow radio beam does not cross the Earth.

A broad absorption feature in the x-ray spectrum of the isolated neutron star RBS1223 (1RXS J130848.6+212708)

X-ray spectra of the isolated neutron star RBS1223 obtained with the instruments on board XMM-Newton in December 2001 and January 2003 show deviations from a Planckian energy distribution at energies below 500 eV. The spectra are well fit when a broad, Gaussian-shaped absorption line with σ = 100 eV and centered at an energy of 300 eV is added to an absorbed blackbody model. The resulting equivalent width of the line is −150 eV. However, the spectral resolution at these low energies of the EPIC detectors and the lower statistical quality and restricted energy band of the RGS instruments are not sufficient to exclude even broader lines at energies down to 100 eV or several unresolved lines. The most likely interpretation of the absorption feature is a cyclotron absorption line produced by protons in the magnetic field of the neutron star. In this picture line energies of 100-300 eV yield a magnetic field strength of 2-6 × 10 13 G for a neutron star with canonical mass and radius. Folding light curves from different energy bands at a period of 10.31 s, which implies a double peaked pulse profile, shows different hardness ratios for the two peaks. This confirms that the true spin period of RBS1223 is twice as long as origi- nally thought and suggests variations in cyclotron absorption with pulse phase. We also propose that changes in photo-electric absorption seen in phase resolved spectra of RX J0720.4−3125 by Cropper et al. (2001), when formally fit with an absorbed blackbody model, are caused instead by cyclotron absorption varying with pulse phase.

IDENTIFICATION OF SELECTED SOURCES FROM THE ROSAT GALACTIC PLANE SURVEY. I.

We report on optical searches in the error circles of 93u2000ROSAT survey sources located at low galactic latitudes ( H . This relatively good segregation offers the possibility to build source samples with enhanced probability of identification with a given class. Complete optical identification of such subsamples could eventually be used to compute meaningful probabilities of identification for all sources using as basis a restricted set of multi-wavelength information.

The isolated neutron star X-ray pulsars RX J0420.0-5022 and RX j0806.4-4123: New X-ray and optical observations

We report on the analysis of new X-ray data obtained with XMM-Newton and Chandra from two ROSAT-discovered X-ray dim isolated neutron stars (XDINs). RX J0806.4−4123 was observed with XMM-Newton in April 2003, 2.5 years after nthe first observation. The EPIC-pn data confirm that this object is an X-ray pulsar with 11.371 s neutron star spin period. The X-ray spectrum is consistent with absorbed black-body emission with a temperature kT = 96 eV and N H = 4 × 10 19 cm −2 without significant changes between the two observations. Four XMM-Newton observations of RX nJ0420.0−5022 between December 2002 and July 2003 did not confirm the 22.7 s pulsations originally indicated in ROSAT data, but clearly reveal a 3.453 s period. A fit to the X-ray spectrum using an absorbed black-body model yields kT = 45 eV, the lowest value found from the small group of XDINs and N H = 1.0 × 10 20 cm −2. Including a broad absorption line improves the quality of the spectral fits considerably for both objects and may indicate the presence of absorption features similar to those reported from RBS1223, RX J1605.3+3249 and RX J0720.4−3125. For both targets we derive accurate X-ray positions from the Chandra data and present an optical counterpart candidate for RX J0420.0−5022 with B = 26.6 ± 0.3 mag from VLT imaging.

Timing analysis of the isolated neutron star RX J0720.4−3125

We present a combined analysis of XMM-Newton, Chandra and ROSAT observations of the isolated neutron star RX J0720.4-3125, spanning a total period of similar to7 yr. We develop a maximum likelihood periodogram for our analysis based on the DeltaC statistic and the maximum likelihood method, which are appropriate for the treatment of sparse event lists. Our results have been checked a posteriori by folding a further BeppoSAX data set with the period predicted at the time of that observation: the phase is found to be consistent.The study of the spin history and the measure of the spin-down rate are of extreme importance in discriminating between the possible mechanisms suggested for the nature of the X-ray emission. The value of P., here measured for the first time, is approximate to10 (-14) s s(-1). This value cannot be explained in terms of torque from a fossil disc. When interpreted in terms of dipolar losses, it gives a magnetic field of B approximate to10(13) G, making it also implausible that the source is accreting from the underdense surroundings. On the other hand, we also find it unlikely that the field decayed from a much larger value (B approximate to10(15) G, as expected for a magnetar powered by dissipation of a superstrong field) since this scenario predicts a source age of approximate to10(4) yr, too young to match the observed X-ray luminosity. The observed properties are more compatible with a scenario in which the source is approximate to10(6) yr old, and its magnetic field has not changed substantially over the lifetime.

The pulsed X-ray light curves of the isolated neutron star RBS1223

We present a multi-epoch spectral and timing analysis of the isolated neutron star RBS1223. New XMM-Newton data obtained in January 2004 confirm the spin period to be twice as long as previously thought, P spin = 10.31 s. The combined ROSAT, Chandra, and XMM-Newton data (6 epochs) give, contrary to earlier findings, no clear indication of a spin evolution of the neutron star. The X-ray light curves are double-humped with pronounced hardness ratio variations suggesting an inhomogeneous surface temperature with two spots separated by about ∼160°. The sharpness of the two humps suggests a mildly relativistic star with a ratio between R ns , the neutron star radius at source, and rs, the Schwarzschild-radius, of R ns /r S > 3.5. Assuming Planckian energy distributions as local radiation sources, light curves were synthesized which were found to be in overall qualitative agreement with observed light curves in two different energy bands. The temperature distribution used was based on the crustal field models by Geppert et al. (2004) for a central temperature of T c = 10 8 K and an external dipolar field of B ∼ 10 13 G. This gives a mean atmospheric temperature of 55 eV. A much simpler model with two homogeneous spots with T∞ = 92 eV and 84 eV, and a cold rest star, T star, ∞ < 45 eV, invisible at X-ray wavelengths, was found to be similarly successful. The new temperature determination and the new P spin suggest that the star is older than previously thought, T 10 5, 6 yr. The model-dependent distance to RBS1223 is estimated between 76 pc and 380 pc (for R ns = 12km).

The low X-ray state of LS 5039/RX j1826.2-1450

Recent XMM-Newton and Chandra observations of the high mass X-ray binary LSu20095039u2009/u2009RXu2009J1826.2-1450 caught the source in a faint X-ray state. In contrast with previous RXTE observations, we fail to detect any evidence of iron line emission. We also fail to detect X-ray pulsations. The X-ray spectrum can be well fitted by a simple powerlaw, slightly harder than in previous observations, and does not require the presence of any additional disk or blackbody component. XMM-Newton data imply an X-ray photoelectric absorption (

XMM-Newton observations of two black hole X-ray transients in quiescence

N_{rm H} sim 7 times 10^{21}

X-ray flares from the ultra-luminous X-ray source in NGC 5408

u2009cm -2 ) consistent with optical reddening, indicating that no strong local absorption occurs at the time of these observations. We discuss possible source emission mechanisms and hypotheses on the nature of the compact object, giving particular emphasis to the young pulsar scenario.