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Discovery of X-ray pulsations from the Be/X-ray binary XMMU J010429.4-723136 in the SMC with SRG/eROSITA

ATel #13312; F. Haberl (MPE), S. Carpano (MPE), C. Maitra (MPE), M. Freyberg (MPE), K. Dennerl (MPE), A. Schwope (AIP), A. Merloni (MPE), P. Predehl (MPE), H. Brunner (MPE), D. A.H. Buckley (SAAO), I. M. Monageng (SAAO/UCT)
on 25 Nov 2019; 20:29 UT
Credential Certification: Frank Haberl (fwh@mpe.mpg.de)

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

During the calibration and verification phase of eROSITA (Merloni et al. 2012, arXiv:1209.3114) on board the Russian/German SRG spacecraft, XMMU J010429.4-723136 was in the field of view during two observations of 1E0102.2-7219. This supernova remnant in the Small Magellanic Cloud (SMC) is used as calibration standard by many X-ray missions (Plucinsky et al. 2017, A&A 597, A35). Here, we report the discovery of pulsations with a period of ~164 s in the X-ray flux of XMMU J010429.4-723136, which we therefore also designate SXP 164, following standard conventions for SMC X-ray pulsars.

XMMU J010429.4-723136 is a known high-mass X-ray binary (HMXB) which shows high long-term flux variations by more than a factor of 400 (Maggi et al. 2013, ATel#5674). McBride et al. 2017 (MNRAS 467, 1526) classified the optical counterpart as B1V, suggesting a Be/X-ray binary sub type of the source. This is supported by their temporal analysis of the OGLE data which suggests an orbital period of ~30 days (see also Rajoelimanana et al. 2011, ATel#3154). The discovery of pulsations with eROSITA provides the final confirmation for an HMXB pulsar (number 66 in the SMC, Haberl & Sturm 2016, A&A 586, A81).

The eROSITA observations were performed from UTC 2019-11-07 17:13 to 2019-11-08 09:53 and 2019-11-09 02:41 to 2019-11-09 19:21 (~60 ks each). During the first observation, the source was far off-axis and covered by only four cameras. During the second observation the source was located in the field of view of all seven cameras. From preliminary analyses based on ground calibration data and without barycentric correction of the photon arrival times, we derived the following parameters from our timing and spectral analysis (using a simple absorbed power-law model). Because systematic uncertainties dominate we do not provide errors.

Obs. Period   Column density  Photon     Flux           Lx   
#      (s)       (H cm-2)     Index   (erg cm-2 s-1)  (erg s-1) 
1     163.9       3x1021       0.7      1.2x10-12     5.3x1035 
2     163.8       4x1021       0.7      1.1x10-12     4.9x1035 
The (observed) flux and the luminosity (absorption corrected, assuming 60 kpc distance) are provided for the 0.2-10 keV band and are at a similar level as observed during the Swift observations in Dec. 2013 (ATel#5674).

Optical spectroscopy of XMMU J010429.4-723136 was undertaken on 2019-11-24 using the Robert Stobie Spectrograph on the Southern African Large Telescope (SALT) under the SALT transient followup program. The PG2300 VPH grating was used, which covered the spectral region 6100-6900 at a resolution of 1.2. A single 1200 s exposure was obtained, starting at 19:40:08 UTC. The spectrum was dominated by the single-peaked, slightly asymmetric H-alpha emission line, with a steeper gradient on the red-ward side. The measured line properties are: E.W. = -5.12 ± 0.14 and FWHM = 7.224 ± 0.023 .

Other known Be/X-ray pulsars in the eROSITA field are detected and pulse periods were determined:
SXP 1323: 1005 s; SXP 726: 798 s; SXP 522: 517 s; SXP 445: 448 s; SXP 304: 302.5 s;