Optical observations of the field around the X-ray pulsars XTE J1858+034 and GRO J2058+42

ATel #308; Pablo Reig (University of Crete), Tasos Kougentakis (University of Crete), Giannis Papamastorakis (University of Crete)
on 15 Jul 2004; 13:26 UT
Credential Certification: Pablo Reig (pablo.reig@uv.es)

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We report photometric and spectroscopic optical observations of the likely optical counterparts to the 221-s XTE J1858+034 (Remillard & Levine and Takeshima & Corbet, 1998, IAUCirc 6826) and the 196-s high-mass X-ray binary (HMXB) pulsars GRO J2058+42 (Wilson et al. 1996, IAUCirc 6514, Wilson et al. 1998, ApJ, 499, 820).

Optical photometric observations of the field around the best-fit ISGRI/IBIS INTEGRAL position R.A.=18h58m43s, Decl.=03d26m06s (Molkov et al, 2004, ATel 274) of XTE J1858+034 using Johnson BVR and a H alpha filter were carried out on the night 20 May 2004 (JD 2,453,146.45) with the 1.3-m telescope of the Skinakas Observatory (Crete, Greece). Based on the strength of the Halpha line several candidates were identified. Subsequent spectroscopic observations revealed that only one exhibits Halpha emission. The position of the proposed candidate (from Aladin Sky Atlas) is R.A.= 18h58m36s, Decl.=03d26m09s consistent with the ISGRI/IBIS uncertainty, although outside the JEMX error circle as given by Molkov et al. Measured photometric magnitudes are B=19.63 V=18.01 R=16.94 with an uncertainty of 0.03 mag. The mean equivalent width of the H alpha line is -6.5 Angstroms.

We also performed optical photometric and spectroscopic observations of the best-fit GRO position of the X-ray source GRO J2058+42. These observations points to the star located at R.A.= 20h58m47s, Decl.=41d46m36s as the likely optical counterpart of this X-ray pulsar. The photometric data were obtained on three nights: 7 June 2003 (JD 2,452,798.494), 8 June 2003 (JD 2,452,799.442) and August 24, 2003 (JD 2,452,876.380) and resulted in the following mean magnitudes B=16.04, V=14.92, R=14.23 and I=13.49, with an error of 0.03 mag. Spectra obtained on 25 June 2004 (JD 2,453,182.444) and 6 July 2004 (JD 2,453,193.491) of the proposed candidate show a double-peak Halpha profile with a mean equivalent width of -4.5 Angstroms.

Emission lines together with IR excess and V/R variability (changes in the relative strength of the blue and red peaks of a double peaked line) are the signatures of Be stars. The origin of these two observational properties resides in the presence of a circumstellar disk formed with matter expelled from the photosphere of the Be star in a way that it is still not fully understood. The double-peak profile of the H alpha line seen in the low resolution spectra of GRO J2058+43 and its transient nature give support to its classification as a Be/X-ray binary. Supergiant X-ray binaries (the other type of HMXB) may also show H alpha in emission. The equivalent width and shape (a single and narrow peak) of the H alpha line in XTE J1858+034 are not incompatible with the system containing an evolved companion. Further higher resolution spectra are needed to find out its spectral type.