ATel #12224: XTE J0052-723 (SXP 4.78; Swift J005139.2-721704), a newly identified Be/X-ray binary pulsar

The ATel #12224 was released on 17 Nov 2018 (19:01 UT) regarding the identification of the proposed optical counterpart to the source XTE J0052-723 (SXP 4.78; Swift J005139.2-721704) as an Hα emitting OB star. The full text follows.

ATel #12224; G. Maravelias (NOA), V. Antoniou (TTU/SAO), A. Zezas (UoC/SAO), A. Strantzalis (UoA), D. Hatzidimitriou (UoA), F. Haberl (MPE)

on 17 Nov 2018; 19:01 UT /

ATel #12209 reported the detection of a new X-ray transient in the Small Magellanic Cloud (SMC), Swift J005139.2-721704, exhibiting outbursting activity. The system has been classified as a new SMC high-mass X-ray binary based on its identification with the B-type star [M2002]20671. NICER followed up this source immediately (ATel #12219), reporting an absorption-corrected luminosity of LX ~7×1037erg s-1 (0.5-8 keV; 60 kpc). Temporal analysis of NICER and Fermi/GBM observations (ATel #12222) identified Swift J005139.2-721704 with the known X-ray pulsar XTE J0052-723 (SXP 4.78). Evans et al. (2004, MNRAS, 353, 601) obtained a refined B0-3(III) spectral type for [M2002]20671 (= [2dFS]0811) based on optical spectroscopy, but they do not report Hα emission.

Here, we report the identification of the SMC star [M2002]20671, and thus XTE J0052-723, with an Hα emitting source, which confirms XTE J0052-723 as a Be/X-ray binary (Be-XRB) pulsar in the SMC. This source is identified in Hα and R-band observations covering the SMC almost completely (Maravelias et al. 2017, IAUS 329, 373) using the Wide Field Imager on the 2.2m MPG/ESO telescope at La Silla (16/17 November 2011) and the MOSAIC-II camera on the 4m Blanco telescope at Cerro Tololo (15/16 December 2011).

The Hα emitting source is found at RA=00:51:38.78 and DEC=-72:17:04.7 (better than 0.2″ relative astrometry with respect to the 2MASS catalog). It is located 2.2″ away from the Swift position (ATel #12209), with Hα and R-band magnitudes equal to 15.476±0.006 and 15.613±0.008, respectively. The resulting Hα-R color is -0.137±0.010 mag with an SNR~13 above the continuum and an Hα excess significance of ~6σ above the Hα-R baseline value. The only nearby source with significant Hα-R excess is much fainter (Hα and R ~22 mag), has a lower SNR (~4) and is located at a larger distance from the X-ray source (~6″).

Observations with the IMACS f/2 camera on the 6.5m Magellan Telescope (4 October 2004) give B=15.652±0.009 mag and I=15.524±0.017 mag (Strantzalis et al. 2018, IAUS 344, 453; Strantzalis et al. 2019, in prep.), compatible with an OB star.

All the identifications described above secure the nature of Swift J005139.2-721704 = XTE J0052-723 = SXP 4.78 as Be-XRB pulsar in the SMC.

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