XMMU J005527.6-721059 = CXOU J005527.9-721058: Be/X-ray binary pulsar in the SMC or background AGN?

ATel #1529; F. Haberl, P. Eger (Max-Planck-Institut fuer extraterrestrische Physik, MPE)
on 16 May 2008; 09:01 UT
Credential Certification: Frank Haberl (fwh@mpe.mpg.de)

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We report the detection of a point source designated XMMU J005527.6-721059 in an archival XMM-Newton observation in the direction of the SMC from March 2002. The position (see table below) is consistent with the Chandra source CXOU J005527.9-721058 that has been proposed as a Be/X-ray binary pulsar (Edge et al. 2004, ATEL #217; Edge et al. 2004, MNRAS 353,1286) with a pulse period of (34.08 +- 0.03) s (98.5% confidence). Although the source was more than a factor of two brighter during the two times longer XMM-Newton observation, we were not able to confirm this period. Moreover, the EPIC X-ray spectra are well represented by an absorbed powerlaw with a column density (1.6 +- 0.07)E21 cm-2, a photon index 1.65 +- 0.14 and an unabsorbed flux of (5.95 +- 0.43)E-13 erg cm**-2 s**-1. The powerlaw is steeper than observed with EPIC from any other Be/X-ray binary in the SMC (typically below 1.2), suggesting XMMU J005527.6-721059 more likely to be a background AGN.

	Position (J2000)	Error	distance to Be star
Chandra	00:55:27.9 -72:10:58	0.6"	3.1"
XMM	00:55:27.6 -72:10:59	1.0"	4.9"

The optical counterpart (from the catalog of Zaritsky et al. 2002, AJ 123, 855) proposed by Edge et al. 2004 lies ~5 sigma away from the Chandra source, which could be caused by the large off-axis position in the ACIS-I observation. However, also the XMM-Newton detection lies 4.9" away from the Be star, although the position of five other Be/X-ray binaries detected in the EPIC field are consistent with their optical counterparts (within 1.6", in the worst case at a large off-axis angle). The steep powerlaw, the non-detection of any periodicity in the EPIC data and the X-ray positions being inconsistent with the proposed optical counterpart suggest that the 2.5 sigma detection of pulsations might have been spurious and this object was misidentified with a Be star. We propose a faint object (Vmag = 20.3, Zaritsky et al. 2002), consistent with the X-ray positions as more likely optical counterpart and XMMU J005527.6-721059 = CXOU J005527.9-721058 as probable background AGN.