Aims. During the last ∼50 years, the population of black hole candidates in X-ray binaries has increased considerably, with 59 Galactic objects being detected in transient low-mass X-ray binaries, as well as a few in persistent systems (including ∼5 extragalactic binaries). Methods. We collect near-infrared, optical, and X-ray information spread over hundreds of references to study the population of black holes in X-ray transients as a whole.Results. We present the most updated catalogue of black hole transients. This contains X-ray, optical, and near-infrared observations, together with their astrometric and dynamical properties. The catalogue provides new and useful information in both statistical and observational parameters and provides a thorough and complete overview of the black hole population in the Milky Way. Analysing the distances and spatial distribution of the observed systems, we estimate a total population of ∼1300 Galactic black hole transients. This means that we have only discovered less than ∼5% of the total Galactic distribution.

Aims. We present a detailed near-infrared/optical/UV study of the transient low-mass X-ray binary 1RXS J180408.9-342058 performed during its 2015 outburst, which is aimed at determining the nature of its companion star. Methods. We obtained three optical spectra (R ∼ 1000) at the 2.1 m San Pedro Mártir Observatory telescope (México). We performed optical and NIR photometric observations with both the REM telescope and the New Technology Telescope (NTT) in La Silla. We obtained optical and UV observations from the Swift archive. Finally, we performed optical polarimetry of the source using the EFOSC2 instrument mounted on the NTT. Results. The optical spectrum of the source is almost featureless since the hydrogen and He I emissions lines, typically observed in LMXBs, are not detected. Similarly, carbon and oxygen lines are not observed either. We marginally detect the He II 4686 Å emission line, suggesting the presence of helium in the accretion disc. No significant optical polarisation level was observed. Conclusions. The lack of hydrogen and He I emission lines in the spectrum implies that the companion is likely not a main-sequence star. Driven by the tentative detection of the He II 4686 Å emission line, we suggest that the system could harbour a helium white dwarf. If this is the case, 1RXS J180408.9-342058 would be an ultra-compact X-ray binary. By combining an estimate of the mass accretion rate together with evolutionary tracks for a He white dwarf, we obtain a tentative orbital period of ∼40 min. We also built the NIR-optical-UV spectral energy distribution (SED) of the source at two different epochs. One SED was gathered when the source was in the soft X-ray state and this SED is consistent with the presence of a single thermal component. The second SED, obtained when the source was in the hard X-ray state, shows a thermal component along with a tail in the NIR, which likely indicates the presence of a (transient) jet.

Aims. We obtained phase-resolved spectroscopy of the accreting millisecond X-ray pulsar SAX J1808.4-3658 during its outburst in 2008 to find a signature of the donor star, constrain its radial velocity semi-amplitude (K 2 ), and derive estimates for the pulsar mass. Methods. Using Doppler images of the Bowen region, we find a significant (≥8σ) compact spot at a position where the donor star is expected. If this is a signature of the donor star, we measure K em = 248 ± 20 km s −1 (1σ confidence), which represents a strict lower limit to K 2 . Also, the Doppler map of He II λ4686 shows the characteristic signature of the accretion disc, and there is a hint of enhanced emission that may be a result of tidal distortions in the accretion disc that are expected in very low mass-ratio interacting binaries.Results. The lower limit on K 2 leads to a lower limit on the mass function of f (M 1 ) ≥ 0.10 M . Applying the maximum K-correction gives 228 < K 2 < 322 km s −1 and a mass ratio of 0.051 < q < 0.072. Conclusions. Despite the limited S /N of the data, we were able to detect a signature of the donor star in SAX J1808.4−3658, although future observations during a new outburst are still needed to confirm this. If the derived K em is correct, the largest uncertainty in determining of the mass of the neutron star in SAX J1808.4-3658 using dynamical studies lies with the poorly known inclination.