Optical spectroscopy of the microquasar GRS 1758−258: a possible intermediate mass system?

Abstract

Context. GRS 1758−258 is one of two prototypical microquasars towards the Galactic Center direction discovered almost a quarter of a century ago. The system remains poorly studied in the optical domain due to its counterpart being a very faint and absorbed target in a crowded region of the sky. Aims. Our aim is to investigate GRS 1758−258 in order to shed light on the nature of the stellar binary components. In particular, the main physical parameters of the donor star, such as the mass or the spectral type, are not yet well constrained. Methods. GRS 1758−258 has remained so far elusive to optical spectroscopy owing to its observational difficulties. Here, we use this traditional tool of stellar astronomy at low spectral resolution with a 10 m class telescope and a long slit spectrograph.Results. An improved spectrum is obtained as compared to previous work. The quality of the data does not allow the detection of emission or absorption features but, nevertheless, we manage to partially achieve our aims comparing the de-reddened continuum with the spectral energy distribution expected from an irradiated disc model and different donor star templates. Conclusions. We tentatively propose that GRS 1758−258 does not host a giant star companion. Instead, a main sequence star with mid-A spectral type appears to better agree with our data. The main impacts of this finding are the possibility that we are dealing with an intermediate mass system and, in this case, the prediction of an orbital period significantly shorter than previously proposed.

Are jets in GRS 1758-258 precessing?

“…If the classical companion KIV spectral type is assumed, with P o = 18.45 days, then the P p ∼ 3 years of our fit would lead to P o /P p = 0.017, which is half of the theoretical value obtained from Equation 6.2. However, the scenario proposed by the authors with an A5V companion and P o ∼ 0.6 day [12,13,14] together with the assumption of P p = 18.45 days would lead to a P o /P p very much closer to the one derived from Equation 6.2. In addition, the same theory [28] leads to a too large accretion disc for being compatible with observed emission if the classical KIV scenario is assumed.…”

“…This mechanism has been proposed for X-ray binaries, and it could be very well at work for the case of GRS 1758-258. 48 days precession period proposed in [14], both in the comoving frame of the source.…”

“…Let assume again ξ in = ξ ms and that Σ(ξ ) is a potential function with exponent s ∈ {−2, −1, 0} to model different decreasing (or constant) behaviors. We will take ξ out = 10 3 ξ ms as in [14] and numerically integrate the previous equation to obtain the precession period as a function of a and s as may be seen in Fig. 2.…”

“…However, the optical/infrared counterpart remained elusive for decades because of the high extinction, the crowded field and some astrometric uncertainties [7,8,9,10,11]. Finally, a clear candidate was proposed [12,13,14], which seems to be compatible with an A5V star orbiting the black hole in less than a day, clearly in contradiction with the commonly accepted companion spectral type and orbital period. Unfortunately, this counterpart is so difficult to observe that no clear spectrum has been obtained up to now.…”

Effects of precession versus instabilities on the jets of GRS 1758–258

“…Although a weak stellar counterpart of GRS 1758-258 was finally identified (Muñoz-Arjonilla et al 2010;Luque-Escamilla et al 2014), it was not possible to obtain any clear spectral signature even using the 10 m Gran Telescopio Canarias (Muñoz-Arjonilla et al 2010;Martí et al 2016). Nevertheless, the system was definitively associated to the Milky Way when large-scale morphological changes were noticed by Martí et al (2015).…”