ATel #12237: A B1-2e optical classification for the optical counterpart of XTE J0052-723 (SXP 4.78; Swift J005139.2-721704)

Building upon our previous ATel #12224 we managed to obtain an optical spectrum of the counterpart. The full text follows:

ATel #12237; G. Maravelias (NOA), V. Antoniou (TTU/SAO), K. Boutsia (LCO), A. Zezas (UoC/SAO), A. Z. Bonanos (NOA), F. Haberl (MPE), D. Hatzidimitriou (UoA/NOA)

on 21 Nov 2018; 23:36 UT /

In ATel #12224 we reported the Hα emission, derived from a wide-field photometric survey of the Small Magellanic Cloud (Maravelias et al. 2017, IAUS 329, 373; Maravelias et al. 2019, in prep.), of the proposed optical counterpart source [M2002] 20671 to the X-ray transient XTE J0052-723 pulsar (SXP 4.78; Swift J005139.2-721704; ATel #12209). In addition to this clear detection, the work of Bonanos et al. (2010, AJ, 140, 416) showed that the IR colors of this source ([2dFS]0811; J=15.54 mag, J-[3.6]=0.56 mag) are indicative of a “photometric” Be star, defined as sources with an intrinsic color of JIRSF-[3.6]>0.5 mag.

However, to confirm the nature of the counterpart optical spectroscopy is needed. We obtained optical spectra using the LDSS-3 spectrograph on the 6.5m Clay/Magellan telescope (Las Campanas Observatory). The observations were performed on Nov. 20, 2018, acquiring two exposures of 600s each with a 1″ slit using the VPH-All grism (resulting in a 2630 — 10859 Å wavelength range, at a nominal dispersion of 1.9 Å/pix). The spectrum shows a clear single-peaked Hα line in emission, with a corresponding equivalent width of -10.65±0.14 Å. This is the first optical spectroscopic confirmation of the presence of Hα in emission. We note that Hβ appears in emission as well.

The spectral classification was based on the blue part of the spectrum and the classification scheme used in Maravelias et al. (2014, MNRAS, 438, 2005). The presence of the OII+CIII 4640-4650 blend and the strong HeI 4471 line point to an early-type star (spectral type up to B2), which is consistent with the absence of the MgII 4481 line and the weak SiII 4552 triplet line. The HeII 4200 and 4686 lines, which are indicative of B0.5 and earlier-spectral types, are absent, thus pointing to later types.

Combining these criteria we constrain the optical classification to a B1-2e source (with an error of 0.5 subtype). This is consistent with the early-type B star classification proposed in ATel #12229, as well as the previous classification of B0-B3 by Evans et al. (2004, MNRAS, 351, 601; source ID [2dFS]0811).

Thus, we provide the first optical spectrum with Hα emission of the optical counterpart of XTE J0052-723, further confirming its Be/X-ray binary nature.

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