XMM-Newton observations of the Small Magellanic Cloud: Be/X-ray binary pulsars active between October 2006 and June 2007

Abstract

Aims. We analysed eight XMM-Newton observations toward the Small Magellanic Cloud (SMC), performed between October 2006 and June 2007, to investigate high mass X-ray binary systems. Methods. We produced images from the European Photon Imaging Cameras (EPIC) and extracted X-ray spectra and light curves in different energy bands from sources which yielded a sufficiently high number of counts for a detailed temporal and spectral analysis. To search for periodicity we applied Fourier transformations and folding techniques and determined pulse periods using a Bayesian approach. To identify optical counterparts we produced X-ray source lists for each observation using maximum likelihood source detection techniques and correlated them with optical catalogues. The correlations were also used for astrometric boresight corrections of the X-ray source positions. Results. We found new X-ray binary pulsars with periods of 202 s (XMMU J005929.0-723703), 342 s (XMMU J005403.8-722632), 645 s (XMMU J005535.2-722906) and 325 s (XMMU J005252.1-721715), in the latter case confirming the independent discovery in Chandra data. In addition we detected sixteen known Be/X-ray binary pulsars and six ROSAT-classified candidate high mass Xray binaries. From one of the candidates, RX J0058.2-7231, we discovered X-ray pulsations with a period of 291 s which makes it the likely counterpart of XTE J0051-727. From the known pulsars, we revise the pulse period of CXOU J010206.6-714115 to 967 s, and we detected the 18.37 s pulsar XTE J0055-727 (= XMM J004911.4-724939) in outburst, which allowed us to localise the source. The pulse profiles of the X-ray pulsars show a large variety of shapes from smooth to highly structured patterns and differing energy dependence. For all the candidate high mass X-ray binaries optical counterparts can be identified with magnitudes and colours consistent with Be stars. Twenty of the Be/X-ray binaries were detected with X-ray luminosities in the range 1.5×10 35 erg s −1 -5.5×10 36 erg s −1 . The majority of the spectra is well represented by an absorbed power-law with an average power-law index of 0.93. The absorption (in addition to the Galactic foreground value) varies over a wide range between a few 10 20 cm −2 and several 10 22 cm −2 . An overall correlation of the absorption with the total SMC H I column density suggests that the absorption seen in the X-ray spectra is often largely caused by interstellar gas.

RX J0123.4-7321, a Be/X-ray binary in the wing of the Small Magellanic Cloud

“…We used a Bayesian odds ratio (Gregory & Loredo 1996;Haberl et al 2008) and a Rayleigh Z 2 1 (Buccheri et al 1983;Haberl & Zavlin 2002) Since this period has a significance of 3.3σ and could not be confirmed in the other observations, probably owing to the lower number of detected counts, the spin period needs further confirmation.…”

Long-term evolution of the neutron-star spin period of SXP 1062

“…3. Using a Bayesian detection method as described by Gregory & Loredo (1996) and Haberl et al (2008), the most probable spin period during the recent XMM-Newton observation is derived to P s = (1071.01 ± 0.16) s.…”

XMM-Newtonobservation of the persistent Be/NS X-ray binary pulsar RX J0440.9+4431

“…This excess is well-described by a thermal emission model (either blackbody, bremsstrahlung, or mekal) with low temperature (kT SE < 0.5 keV) and large emission area (R SE ≥ a few hundred km). This feature has been detected not only in the high-luminosity sources (with L X ∼ 10 37−38 erg s −1 ) but also in several low-luminosity (L X ∼ 10 35−36 erg s −1 ) XBPs observed in the Small Magellanic Cloud (SMC), where its detection is made easier by the low interstellar absorption (Sasaki et al 2003;Ueno et al 2004;Majid et al 2004;Haberl & Pietsch 2005;Haberl et al 2008). Only in a few cases has this low-energy component showed coherent pulses and the debate about its origin remains open.…”