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Identification of IGR J01217-7257 with the transient pulsar XTE J0119-731 (SXP2.16) using XMM-Newton

ATel #8305; F. Haberl, G. Vasilopoulos, R. Sturm (MPE, Garching) and P. Maggi (CEA, Saclay)
on 19 Nov 2015; 16:47 UT
Credential Certification: Frank Haberl (fwh@mpe.mpg.de)

Subjects: X-ray, Binary, Neutron Star, Transient, Pulsar

Referred to by ATel #: 8312, 9299

We report first results from our XMM-Newton observation of IGR J01217-7257 performed on November 7, 2015. This transient in the Small Magellanic Cloud (Coe et al. 2014, ATel #5806) was found to be in a new outburst during Integral observations end of October 2015 (Coe et al. 2015, ATel #8246). The EPIC-pn data revealed X-ray pulsations with a period of 2.16501(1) s. The pulse period is almost identical to that of the transient XTE J0119-731 (2.1652(1) s), discovered by Corbet et al. (2003, IAU Circ. 8064) in RXTE data. This strongly suggests that IGR J01217-7257 and XTE J0119-731 are the same source, that has shown little net change of the spin period of the neutron star over 12.8 years. The preliminary position derived from the EPIC images of RA = 01:21:40.5, Dec = -72:57:32 (J2000) (uncertainty dominated by systematic errors) confirms the optical counterpart suggested for IGR J01217-7257, the emission-line star AzV 503 = [MA93] 1888 with spectral type B0-5IIe. OGLE-IV data of this star revealed a likely orbital period of 84 days (Coe et al. 2014, ATel #5806).

The pulse profile obtained from EPIC-pn data in the 0.2 - 10.0 keV energy band is complex with three major peaks. This leads to a power spectrum with the first (the strongest), second and third harmonics being stronger than the fundamental frequency at 0.46189 Hz.

The X-ray spectrum obtained by EPIC-pn (19.6 ks exposure) can be modelled by an absorbed powerlaw with an additional soft component. We found an acceptable fit (reduced chi-square of 1.13 for 1204 degrees of freedom) using a disk-blackbody model for the soft excess with the following parameters: photon index 0.42 ± 0.01; inner disk temperature (0.33 ± 0.01) keV; absorption consistent with a foreground column density of 4×1020 cm-2 (fixed in the fit) without additional SMC absorption. The normalisation of the disk-blackbody component indicates a lower limit for the inner-disk radius of 14.4 km (for an inclination of 0 degrees). The observed flux was 4.25×10-11 erg cm-2 s-1 and the source luminosity (assuming a distance of 60 kpc) 1.85×1037 erg s-1 (0.2 - 10.0 keV band).

Swift observations indicate a slow decay of the outburst. From the last observation on November 11, 3.2 days after the XMM-Newton observation, we derive a flux of 3.7×10-11 erg cm-2 s-1 and a source luminosity of 1.6×1037 erg s-1. This assumes the best-fit spectral model parameters from above, only fitting a normalisation factor.

We thank the XMM-Newton and Swift teams for scheduling the target of opportunity observations.