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BlackCAT: A catalogue of stellar-mass black holes in X-ray transients

During the last ~50 years, the population of black hole candidates in X-ray binaries has increased considerably with 59 Galactic objects detected in transient low-mass X-ray binaries, plus a few in persistent systems (including ~5 extragalactic binaries). We collect near-infrared, optical and X-ray information spread over hundreds of references in order to study the population of black holes in X-ray transients as a whole. We present the most updated catalogue of black hole transients, which contains X-ray, optical and near-infrared observations together with their astrometric and dynamical properties. It provides new useful information in both statistical and observational parameters providing 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 already discovered less than ~5% of the total Galactic distribution. The complete version of this catalogue will be continuously updated online and in the Virtual Observatory, including finding charts and data in other wavelengths.


Introduction
X-ray binaries (XRBs) are systems formed by either a neutron star (NS) or a black hole (BH), which is accreting mass from a companion donor star. Their detection began thanks to the development of space-based instrumentation in the 1960's, with the number of detections rising substantially since the implementation of all-sky monitors on-board X-ray satellites, e.g. Ginga (1987Ginga ( -1991, RXTE (1996RXTE ( -2012 and, more recently, Swift (2004) and MAXI (2009). They are broadly divided into high-mass X-ray binaries (HMXBs) and low-mass X-ray binaries (LMXBs) according to the mass of the donor star. In the former, the early spectral type (O-B) massive star (≥ 10 M ) transfers material to the compact object mainly through strong stellar winds. On the other hand, in LMXBs the K-M spectral type star (M 2 ≤ 1 M ) fills the Roche lobe and transfers mass by Roche lobe overflow through the inner Lagrangian point (Charles & Coe 2006). In this latter case, the material forms an accretion disc around the compact object whereby material is accreted. A few XRBs are also found with intermediate mass companions of spectral types in the range B-F. These so-called intermediatemass XRBs (IMXBs) have been proposed to be the progenitors of some LMXBs through an episode of enhanced mass-transfer rate .
The mass transfer rate (Ṁ) largely determines the observational properties and gives rise to a sub-classification within the LMXBs class. Persistent sources are those with high accretion rates (Ṁ ∼ 10 −10 M yr −1 ; Tanaka & Shibazaki 1996) and X-jcorral@astro.puc.cl ray luminosities close to the Eddington limit. This high luminosity ensures that the outer parts of the accretion disc dominate the optical spectrum through reprocessing and effectively hide the companion star. On the contrary, transient sources are systems with low accretion rates (Ṁ ≤ 10 −9 M yr −1 ; Tanaka & Shibazaki 1996) which exhibit long quiescent states and sporadic outburst episodes produced by thermal-viscous instabilities in the accretion disc (see Frank et al. 2002 for further explanation). During outbursts, the brightness of the system rises to similar luminosities as those found in the persistent sources. After the outburst, transients decay back to quiescent states where they remain for most of their lifetimes. Typical recurrence times between outburst span from years to centuries depending onṀ (Ritter & King 2002;. Observations have revealed that ∼ 25% of the transient LMXBs contain bursting NS (King et al. 1996a) while the rest (∼ 75%) display X-ray spectral and/or timing properties characteristic of accreting black holes (hereafter we will refer to the transient black hole systems as black hole transients or BHTs). In this paper we focus on the properties of the Galactic BHTs as they represent the vast majority of the population of BHs. However, there are a few persistent, non-active and extragalactic XRBs that harbour or may contain BHs.
Regarding the extragalactic population of BHs, dynamical evidence has been presented for LMC X-1 (a 11 ± 1 M BH with an O7III companion; Orosz et al. 2009), LMC X-3 (a 7.0 ± 0.6 M BH with a B3-5V star; Orosz et al. 2014 andVal-Baker et al. 2007) and M33 X-7 (a 16 ± 1 M BH with a O7-8III star; Orosz et al. 2007), the first eclipsing stellar-mass BH ever  detected. On the other hand, indirect evidence for the presence of BHs in the HMXBs NGC 300 X-1 (12 − 24 M BH with a Wolf-Rayet star; Crowther et al. 2010) and IC 10 X-1 (a 23 − 34 M BH with a Wolf-Rayet star; Silverman & Filippenko 2008) has been postulated. Owing to the unique challenges of observing the winddominated Wolf-Rayet companions, the masses of the compact objects in these systems are rather uncertain and indeed a neutron star cannot be ruled out (Binder et al. 2015;Laycock et al. 2015). Finally, we should also mention ultraluminous X-ray sources (ULXs), systems with X-ray luminosities greater than the Eddington limit for a 10 M BH. The source of these luminosities is still uncertain and it has been proposed that they may be produced by intermediate-mass BHs (∼ 10 3 M . See e.g. Miller et al. 2003) or stellar-mass BHs (e.g., Poutanen et al. 2007;Kawashima et al. 2012). More recently, it has been found for one case that the compact object is a NS (M82 X-2; Bachetti et al. 2014) confounding our understanding of ULXs even further.
Hereafter, we will only focus on the Galactic population of BHs. We have performed a thorough search of all X-ray systems published in literature since 1962, when the first extrasolar X-ray source was detected, (the NS system Sco X-1; Giacconi et al. 1962) up to 2015. In Sec. 2 we motivate this catalogue with a historical view of the sample of BHTs and present the catalogue itself. In Sec. 3 we study the population of BHTs, where we analyse the vertical distribution of BHTs constrain the expected number of systems in the Milky Way. In Sec. 4 we focus on the population of dynamically confirmed BHs, presenting their distributions of periods, magnitudes and masses. Since 1966Since up to 2015 BHTs have been discovered, represented in Fig. 1 as a cumulative histogram of red bars, the slope of which represents a detection rate of ∼ 1.2 targets per year. However, only 17 of these have been dynamically confirmed to harbour accreting BHs (i.e. mass function 3 M ; see, e.g., Casares & Jonker 2014) and are represented by the blue bars in Fig. 1. Here, we should also add Swift J1357.2-0933, where the dynamical confirmation is indirect because it is not based on the detection of the secondary star. However, it has robust evidence of containing a BH (see Corral-Santana et al. 2013 andMata Sánchez et al. 2015 for more details).  (Table 2) as seen from the Galactic pole. Dynamically confirmed black holes are marked in orange circles) whereas BH candidates are indicated by yellow stars. Distance ranges are represented with bars while the systems with lower limits are marked with arrows, following the same colour index. Some systems are overlapped with other. Each faint white ring in the background image represents an additional ∼ 1.5 kpc from the Sun. Objects seem to concentrate along the direction towards the Galactic centre. The dynamical BHT in the upper right of the figure is BW Cir. Background image credit: NASA/JPL-Caltech/R. Hurt (SSC/Caltech). Therefore, the 17+1 dynamical BHs represent ∼ 30% of the total number of BHTs discovered so far. This low fraction is due to the fact that most BHTs become too faint in quiescence for radial velocity studies with current instrumentation due to intrinsically faint companions, high extinction, large distance or a combination of these. This problem could be alleviated with the exquisite sensitivity of future facilities like the E-ELT, specially with better IR instrumentation. The rest of the population of BHTs are named BH candidates because they share similar X-ray characteristics in outburst to the confirmed ones, but they lack a final dynamical confirmation (see, e.g., Belloni et al. 2011 for reviews on the X-ray observational properties of the BH candidates). Indeed, NS and BH sometimes display qualitatively similar phenomenology in outburst (see, e.g., Muñoz-Darias et al. 2014) and we cannot discard that some of the objects included in this catalogue might harbour NS.

BlackCAT: The catalogue of Galactic BHs
BlackCAT is a complete catalogue containing the astrometric, photometric (near-infrared -NIR-/optical magnitudes in outburst and quiescence), number of outbursts, the peak X-ray flux, distance, finding charts and dynamical parameters of all the BHTs discovered so far. The electronic and most complete version of this catalogue is available on www.astro.puc.cl/ BlackCAT although all data will also be available through the Virtual Observatory. In this paper we present the most relevant properties divided in the tables explained below.
We divide the catalogue in 3 main types of Galactic BHs: transients, persistent and non-active, depending on their X-ray activity.

Persistent
Cyg X-1 (a 15±1 M BH with a O9.7 Iab donor star; Orosz et al. 2011a) is the only confirmed Galactic BH in a persistent X-ray binary. On the other hand, 4U 1957+11 Nowak et al. 2008Nowak et al. , 2012Hakala et al. 2014) and 1E 1740.7-2942(Sunyaev et al. 1991 are persistent BH candidates but they have not been dynamically confirmed. GRS 1758-258 (Mandrou 1990) is a quasi-persistent microquasar with a large extinction A v ∼ 8.4 (Mereghetti et al. 1997) located near the Galactic center region. Finally, SS 443 (Stephenson & Sanduleak 1977) is a non-transient source with a supercritical accretion regime onto a relativistic star (see Fabrika 2004 for a detailed review on the system). It is very likely a BH candidate with indirect arguments that support a compact object of 10 − 20 M with a high inclination (Eikenberry et al. 2001). However, despite it has been intensively studied for almost 30 years, the nature of this system is still uncertain and it could be a Galactic ULX.

Non-active BHs
Further, MWC 656 has been recently proposed as the first BH HMXB with a Be-type companion star (3.8 − 6.9 M BH with a B1.5-2 III star; . This is based on radial velocity curves of gas encircling the companion star and the spectroscopic mass of the Be star. This system has not shown any type of outbursting activity, so we label it as a non-active BH. There are other 3 black hole candidates discovered in globular clusters: the flat-spectrum radio sources M22-VLA1 and M22-VLA2 (Strader et al. 2012) and M62-VLA1 (Chomiuk et al. 2013).

Transients
The rest of systems are transients, although some of them could be considered as semi-persistents (e.g. GRS 1915+105 or 4U 1630 because of their long stays in the outburst state. Below, we detail the content of the tables presented in this paper. The systems included are either dynamically confirmed BHs or have shown spectra and/or timing features typically found in BHs . We also note the existence of Cyg X-3 (Giacconi et al. 1967), a transient source showing strong radio outbursts. However, the nature of the primary accompanying the Wolf-Rayet donor is unclear and it could be either a NS or a BH. Thus, we have excluded Cyg X-3 from our list of BHTs. Table 2: Astrometric properties presents the basic astrometric properties of the BHTs sorted chronologically by year of detection in X-rays. The dynamically confirmed BHs are highlighted in grey. The columns distribution is: (1) Year of discovery of the BHT; (2) Name of the system and optical counterpart when known; (3-5) Right ascension (RA) and declination (DEC) coordinates in equinox J2000. The accuracy in the astrometry and the source of the coordinates are also shown; (6-7) Galactic longitude ( ) and latitude (b) in degrees; (8-9) Estimated distance (d) and height above the Galactic plane (z) in kpc; (10) Number of outbursts detected after discovery in X-rays; (11) References for the detection, best coordinates and distance determinations.  3: Peak X-ray flux and optical/NIR photometric parameters shows the main properties both in outburst and in quiescence for all the BHTs presented in Table 2. We also list the measured or predicted orbital period. Again, the dynamically confirmed BHs are highlighted in grey. The column distribution is: (1) ID number used for cross-reference with the web version of this catalogue; (2) Name of the system and optical counterpart when known; (3) Peak X-ray flux in erg s −1 cm 2 , standardized to the 2-10 keV band. To do so, we begin with the X-ray flux published in the literature (or in archive). We assume a power-law spectrum with a photon index Γ = 2 ) and the total neutral Galactic Hydrogen column density (N H ) published by Kalberla et al. (2005). If there is a measured N H published in literature derived from direct X-ray spectral analysis, we use this instead of the radio-derived interstellar one; (4-5) Optical or IR magnitude in the peak of the outburst and quiescence, respectively, in the AB system. In order to document the original observed band, we provide the name of the band in its original system; (6) Optical Galactic extinction [E(B − V)] reported in the literature. If unknown, we list the total Galactic line-of-sight absorption given by the Schlafly & Finkbeiner (2011) dust maps.
(7) Reported or estimated orbital period of the binary in hours; (8) References for all the parameters above. (1) Preferred name of the dynamically confirmed BHT; (2-9) Quiescent magnitudes of the dynamically confirmed BHTs. All the magnitudes were transformed to the AB system using the transformation coefficients taken from table 2 in Frei & Gunn (1994) and equations 5 in Blanton et al. (2005). In order to document the original observed band, we provide the name in its original system.
(10) References of the magnitudes. (2) Spectral type of the companion star; (3) The orbital period of the binary in hours; (4) The radial velocity of the companion star (K 2 ) in km s −1 ; (5) The mass function of the BH f (M 1 ) in M ; (6) The mass of the BH (M 1 ) in M . If there is an uncertain value, we preferred to show a range of masses; (7) The binary mass ratio q = M 2 /M 1 ; (8) The inclination of the system (i) in degrees; (9) The rotational broadening (v rot sin i) in km s −1 ; (10) References of all the parameters.
At the end of each table, we present a detailed list with the particularities marked in Table 2 and Table 3.
This census is the most updated available and, compared to previous catalogues (e.g. Gottwald et al. 1991;Chen et al. 1997;Ritter & Kolb 2003;Liu et al. 2006Liu et al. , 2007, it represents a substantial improvement in both statistics and observational parameters. It is focused only in the population of BHs but providing a thorough and more complete coverage of optical and NIR data and dynamical parameters. It is online and will be continuously updated with more systems and information in other spectral bands. Table 2 allows us to perform a statistical study of the distribution of BHTs in the Galaxy. In Fig. 2, we plot the 35 objects with estimated distances as viewed from the pole of the Milky Way. Filled orange circles represent the dynamically confirmed BHs while yellow stars mark the BH candidates. About 50% of the confirmed BHs are located within 4.5 kpc of the Sun. This is a clear indication that interstellar extinction is a severe limitation to dynamical mass determinations. Moreover, from Fig. 2 it seems that almost all BHs lie within a spiral arm. Thus, for BHTs with uncertain distances, likely values could be estimated or constrained using the distance to the spiral arm. Fig. 3 shows the position of all the BHTs in Galactic coordinates overlaid on a projected image of the Milky Way, as well as histograms of their distributions in Galactic longitude and latitude. There is a clear concentration of objects towards the direction of the bulge (340 • < < 20 • and |b| < 10 • , Dwek et al. 1995) and disc. There are only two objects located at high Galactic latitudes (XTE J1118+480 at b = +62 • and Swift J1357.2-0933 at b = +50 • ), while three BHTs lie between 150 • < < 210 • . 31 out of the 59 BHTs have no reported qui-escent optical counterparts. In addition, eighteen of them were detected in outburst, but they are either too faint in quiescence or the crowding of the field has prevented the detection of a quiescent counterpart. On the other hand, 28 radio counterparts have been found (mostly during outburst). In column 5 of Table 2 we indicate the range of the source of the coordinates together with the accuracy. Stellar evolution predicts a population of 10 8 −10 9 BHs in our Galaxy (van den Heuvel 2001; . However, the total number of Galactic BHTs is very uncertain and depends on several considerations. van den Heuvel (1992van den Heuvel ( , 2001) estimated a population between of 500 and 1000 BHTs based on only 3-4 BHTs known at the time. Current estimates predict 10 3 − 10 4 Galactic BHTs (Romani 1998;Kiel & Hurley 2006;Yungelson et al. 2006) based on models, although these numbers are likely underestimated because of the existence of systems with extremely long outburst duty cycles (Ritter & King 2002) or very faint peak X-ray luminosities (King & Wijnands 2006).

Analysis of the spatial distribution of BHTs
Here we want to revisit the problem of the size of the Galactic population of BHs, based on the currently observed sample, which is heavily affected by extinction. Duerbeck (1984) modelled the density distribution of several interacting binaries in the Galaxy. In a similar way, we want to obtain the density of Galactic BHTs ρ(z) and analyse the height distribution (z; col-umn 8 in Table 2). Only 35 out of the 59 detected BHTs have estimated distances. Due to their transient nature, we have only detected those systems which went into outburst. Therefore, in a given period of time, we have discovered only a fraction of them (e.g., the brightest and/or closest BHTs and with the shortest outburst cycles). Thus, in order to derive the true z distribution we first need to find the maximum radial distance out to which completeness of the sample is guaranteed, i.e., such that none of the X-ray outbursts were missed. Fig. 4. Variation of the density of BHTs (Σ) in a cylindrical volume of infinite height h with increasing radii r. Above r > 4 kpc the density decreases and therefore we infer the sample of BHTs is complete out to r ∼ 4 kpc (grey bar).
The completeness of the sample can be obtained from the analysis of the density of objects as a function of the radial distance projected on the Galactic plane (r). The latter is derived from the Galactic latitude and distance listed in columns 5 and 6 of Table 2.
Here, we have used 31 out of the 35 BHTs with a reported distance (we exclude the 4 systems with lower or upper limits). In addition, we consider a conservative 50% uncertainty in systems with only a rough estimate of distance. To derive the density of objects (Σ) up to a radii r, we count the number of BHTs lying in cylinders or radius r centred at the Sun and infinite height (h). However, given the large uncertainties with distance, in some cases an object could be placed in more than a single cylinder within errors. To account for this effect, for each object we randomly generate 10000 values assuming a Gaussian distribution. Here, we naively assume the distance errors follow this sort of distribution, under the assumption that all of the errors that went into the distance errors were Gaussian. In reality, we have no easy way to know this, as the values are from the literature, determined by many groups and in many cases not fully documented. Thus, the final density of objects obtained in a cylinder is given by the median value of all the 10000 Σ obtained in the process, with the error given by the standard deviation. The result is shown in Fig. 4 where the density is represented as a function of the radial distance and normalized to its maximum value. In this figure we find a plateau in Σ up to r = 4 kpc (within errors) and a decrease for r > 4 kpc. We interpret this as an indication that the density of objects is approximately constant with increasing radii up to ∼ 4 kpc. We should note here that it is  (2004)  7 A0620-003 1.1 ± 0.1 −0.11 ± 0.01 Cantrell et al. (2010) difficult to establish a limit, but the density decreases clearly at r > 4 kpc whereas it is not clear before that. Moreover, using the counting method proposed by Duerbeck (1984), we obtain a more abrupt decrease in Σ for r > 4 kpc. However, we believe this technique is not suitable for systems with large uncertainties in the distance. We also perform a complementary analysis based on the X-ray luminosity. The least luminous BHT detected is XTE J1118+480 which reached an X-ray luminosity of 3.6 × 10 35 erg s −1 in the 2-10 keV range (Dunn et al. 2010). Therefore, we can assume that no BHT peaked at a luminosity lower than that. The sensitivity limits by the all-sky monitor (ASM) on RXTE in the 2-10 keV range have been above 2.4 × 10 −10 erg s −1 cm −2 since 1996, which means that any BHT that reached 3.6×10 35 erg s −1 within 3.5 kpc would have been detected (without considering absorption). This is consistent with the method described above and, therefore, we assume that the sample of BHTs is complete out to ∼ 4 kpc. There are 10 objects in this volume (listed in Table 1). However, for the sake of the analysis we should only consider those BHTs detected since the time when the rate of discoveries has become more or less constant. We assume this has occurred since 1988 (see Fig. 1), when the sky started to be intensively scrutinized by all-sky monitors on-board X-ray satellites. Hence, we eliminate A0620-003 from this analysis because it was discovered in 1975, before the beginning of the assumed constant rate of discoveries. This yields 9 BHTs in the solar neighbourhood (r ≤ 4 kpc), discovered at a rate of ∼ 0.3 BHTs yr −1 since 1988. For comparison, in the same time interval, ∼ 46 BHTs have been discovered in the whole Galaxy (which implies a rate of 1.7 BHTs yr −1 ). It is expected that the vertical distribution of BHTs follows the same exponential function as the stellar distribution (Duerbeck 1984). In addition, it is reasonable to assume that the population of BHTs is representative of the parent Galactic population of BHTs since there is no strong bias against their detection in X-rays (Özel et al. 2010).
Therefore, the space/time density distribution can be approximated by: where ρ * 0 is the space/time density of objects in the Galactic plane and z 0 the scale height of the distribution perpendicular to the plane of the Galaxy, both in the solar neighbourhood.
To obtain the final space density of BHTs ρ(z), we must assume a mean outburst recurrence period (ORP). We have only  (Markwardt 2009) to the exponential function shown in eq. 1. observed transients for ∼ 50 years and the observed ORPs are thus biased by this limited time-frame for scrutinizing the X-ray sky. In Fig. 6 we show the frequency of outbursts of all the BHTs over the past 50 years. The majority of systems have shown only one outburst, with only a small fraction showing multiple outbursts. The most extreme cases are H 1743-322, GX 339-4 and 4U 1630-472, which have triggered 10, ∼ 19 and ∼ 20 outbursts in 50 years (some of them were not considered "full outbursts"), respectively. In particular, 4U 1630-472 shows an ORP of 600-700 d lasting from 100-200 d up to 2.4 years (see, e.g., Kuulkers et al. 1997). There is also evidence for additional outbursts produced by A 0620+003 (in 1917; Eachus et al. 1976) and V404 Cyg (in 1938and 1956Wachmann 1948;Richter 1989) detected by analysing photographic plates previously to their discoveries. Nevertheless, in order to be consistent we included only those outbursts discovered by X-ray satellites (i.e. since 1966) and consequently, the observed ORP is biased by the 50 yr time lapse of X-ray astronomy observations. On the other hand, based on an analysis of the mass transfer rate needed to reach the critical surface density that produces instabilities in the accretion disc, White & van Paradijs (1996) consider unlikely that BHTs have average ORPs above 100 yr. Therefore, the local density at the Galactic plane becomes ρ 0 = ORP ρ * 0 = 8 ± 1 ORP 100 yr kpc −3 . A rough extrapolation of this local density distribution to the entire Galaxy, assuming no radial dependence, allows us to obtain the total number of BHTs by integrating eq. 1 in cylindrical coordinates. Using R = 14 kpc as the truncation radius for the Galactic disc and H = 2.5 kpc as the maximum height above the Galactic plane given by MAXI J1659-152 (Kuulkers et al. 2013), we find a total number of N = 1280 ± 120 ORP 100 yr BHTs in the Galaxy with comparable properties to the systems detected so far, i.e. with peak X-ray luminosities a few tenths of the Eddington luminosity. This is consistent with the results obtained using other techniques (van den Heuvel 1992; Tanaka 1992; White & van Paradijs 1996;Romani 1998) and implies that only ∼ 4% of Galactic BHTs have been discovered. Our empirical estimate is an order of magnitude lower than the 10 4 BHTs predicted by Kiel & Hurley (2006) or Yungelson et al. (2006) using population synthesis models. However, we note that our analysis is based on the study of observed systems but limited only to the 9 BHTs with reliable distance estimates, located in a cylinder of 4 kpc radius centred on the Sun. In addition, we have assumed that the solar vertical distribution (ρ 0 , z 0 ) can be extrapolated to other regions of the Galaxy. However, the bulge contains ∼ 30% of the stellar mass of the Galaxy confined in a reduced spheroid and it is expected to host a higher concentration of BHTs (Muno et al. 2005). Furthermore, we have considered a cylinder with a height defined by MAXI J1659-152 (the object with the highest h) but there could be objects located at higher distances over the plane. Finally, we have normalized our estimated value to an average recurrence period of 100 yr, which does not consider those systems with smaller accretion rates and longer recurrence periods as well as a likely population of intrinsically faint X-ray BHTs. With all in hand, we conclude that our crude calculation of the number of BHTs expected in the Galaxy is very conservative and sets a lower limit to the hidden population.

Physical properties of dynamical BHTs
With the information on the apparent quiescent magnitudes, distances and reddening listed in Table 2-5, we can recover the Rband absolute magnitudes (corrected for extinction and in the AB system) of the BHTs in quiescence. The magnitude distribution (Fig. 7, left) strongly peaks at M R 4 − 6, where ∼ 40% of the systems lie. This result is expected since the quiescent spectra are mainly dominated by the light from the donor (mostly K-type stars) with some contribution from the accretion flow (typically ≤ 50% in this waveband). From Table 5, we note a small fraction of IMXBs with A-F spectral type companions (V4641 Sgr, 4U J1543-475 and GRO J1655-40), which have been marked in black in Fig. 7.
In addition, from Table 3 we can revisit the observed distribution of orbital periods, which shows a bimodal shape with a gap at ∼ 15 h known as the bifurcation period (Fig. 7, right). This is driven by different evolutionary paths, with systems above the gap evolving towards longer orbital periods through nuclear evolution of the donor stars, while angular momentum loses shrink the orbit of the binaries below the gap (Pylyser & Savonije 1988;Menou et al. 1999). In this case, the IMXBs are located at long orbital periods, consistent with the presence of giant/subgiant donor stars. The majority of systems have periods between 6-10 h, which corresponds to main-sequence or slightly evolved K-type donors filling their Roche lobes.
It is noticeable that none of the 59 BHTs listed in Table 2-3 present X-ray or optical eclipses (the extragalactic BH-HMXB M33 X-7 is the only one known to show eclipses). In fact, all the dynamical BHs transients found so far have binary inclinations 75 • (see Table 5). However, it is expected that at least 20% of the 17 dynamically confirmed BHs should have i > 75 • considering an isotropic distribution of inclinations. Therefore, our estimate of the expected distribution of BHTs would be underestimated by 20% and it should be N ∼ 1600 ORP 100 yr , although this percentage is probably smaller than the uncertainty associated to our systematic errors. Narayan & McClintock (2005) have proposed that the lack of high inclination systems is due to a selection effect produced by the fact that the inner accretion disc hides or obscures the central BH, making these systems very faint in X-rays and preventing their detection during outbursts. For typical disc flaring angles of ∼ 12 • (de Jong et al. 1996), the outer disc rim will obscure the central X-ray source permanently when viewed at inclinations 78 • .
Swift J1357.2-0933 may be the first object seen edge-on according to the optical properties displayed during the decay from its 2011 outburst (Corral-Santana et al. 2013), but it did not show X-ray nor optical eclipses. Armas Padilla et al. (2014) and Torres et al. (2015) presented alternative explanations to the high inclination based on the quiescent X-ray properties and the Hydrogen column density obtained from the Na doublet in outburst, respectively. However, Mata Sánchez et al. (2015) presented new evidences supporting the edge-on configuration.
An accurate determination of the binary inclination is crucial for obtaining the mass of the compact object. The mass distribution of black holes has a large impact on the physics of su- pernova explosions, the survival of interacting close binaries and the equation of state of nuclear matter. The distribution of masses of compact objects is expected to be smooth because of its correlation with the distribution of their progenitor masses (Fryer & Kalogera 2001). However, the observed distribution shows a gap between neutron stars and BHs between 2-5 M (Fig. 8). Unlike Özel et al. (2010, 2012), here we only mark with solid circles those systems with the most reliable mass determinations following Casares & Jonker (2014). On the contrary, for those systems with inaccurate masses, we prefer to display large bars which encompass both the minimum and maximum published masses.
The existence of this mass gap is under debate. Özel et al. (2010) argue that selection effects may bias the observed distribution at high BH masses (≥10 M ) but can not explain the gap, which may be related to the physics of supernova explosions. Farr et al. (2011) perform a Bayesian analysis of the BH mass distribution and conclude that their larger sample provides strong evidence for the existence of the gap, reinforcing the results by Bailyn et al. (1998) and Özel et al. (2010). However, Kreidberg et al. (2012) warn about an important source of systematic errors arising from a possible underestimate of the inclination angle i. This is produced by the assumption that the emission from the accretion disc (i.e., the disc veiling) is negligible in the infrared, when performing ellipsoidal modelling. They found that in the case of A0620-003, this would lead to an underestimate of the inclination angle of 10 • at least. Because of the cubic dependence of the mass function with sin i, this could lead to a considerable overestimate of the BH mass. Correcting the BH mass in GRO J0422+32 from the estimated bias in the inclination, Kreidberg et al. (2012) find that the BH would be lie in the gap. However, they also note that if this object is excluded from the analysis, previous conclusions would remain intact. On the other hand, Belczynski et al. (2012) and Fryer et al. (2012) state that the mass gap may be real and it could reveal new insights on the supernovae explosion models. More recently, Kochanek (2014) suggested an alternative explanation based on the absence of red supergiants in the range 16.5 − 25 M as progenitors of type IIp supernova. Due to the weakly bound hydrogen envelopes on these massive stars, they eject the outer layers leaving a BH with the mass of the star's helium core (5 − 8 M ). This would explain (i) the lack of supernova progenitors in the 16 − 25 M mass range and (ii) the existence of the mass gap and the typical masses of BHs.

Conclusions
We have presented the main properties of a large catalogue of BHTs available on-line. Of the 59 BHTs detected in outburst so far, only ∼ 30% have been confirmed as dynamical BHTs. A fraction of them (35) have distance estimates which allow us to study the Galactic distribution. This in turn results in a population of ∼ 1280 ORP 100 yr such systems in the Milky Way. This value is in agreement with previous estimates using other techniques but an order of magnitude lower than theoretical predictions based on population synthesis models. We argue that this value must be considered as a lower limit since it is based on the extrapolation of a small number of systems (9 out of the 59 systems detected so far). We provide several tables listing the astrometric parameters, distances, number of eruptions, X-ray fluxes at the peak of the outburst and an outburst and a quiescence magnitude, reddening and orbital periods. For dynamically confirmed systems, we have also detailed the magnitudes in quiescence in every available optical and NIR band, together with their dynamical parameters. In the online version of the catalogue, we have also added finding charts, links to the references and relevant information for forthcoming observations of these systems. We plan to include more information in other wavelengths in the near future (e.g. radio or X-ray states) and update it with new targets once discovered.
Acknowledgements. We thank the anonymous referee for useful comments. We acknowledge financial support from CONICYT-Chile grants Group and LCOGT archives which are maintained as part of the CASU Astronomical Data Centre at the Institute of Astronomy, Cambridge. We are thankful to Danny Steeghs and Manuel A. P. Torres for gently provide us some of the finding charts and Jorge Andrés Perez Prieto for his help with the creation of the web. We have used the web applications of the FTOOLS (Blackburn 1995) and PIMMS (Mukai 1993) software to make the transformation of the X-ray fluxes. Some peak X-ray fluxes were provided by the ASM/RXTE teams at MIT and at the RXTE SOF and GOF at NASA's GSFC. This research has made use of the MAXI data provided by RIKEN, JAXA and the MAXI team (Matsuoka et al. 2009).
In this catalogue we have included all the BH candidates detected so far. Dynamical BHs are highlighted in grey boxes.
† In column 5, the letter preceding the error value indicate the source of the coordinates: radio (r), interferometry in radio (e), optical (o), infrared (i), X-rays (x) or a combination of them. In case of different errors in RA and DEC (respectively), both are shown in parenthesis. * These values are uncertain estimates.
(1) IGR J17454-2919: the nature of this transient source is still unclear. Paizis et al. (2015) has proposed that 2MASS J17452768-2919534 is the IR counterpart although it is 2.4" away from the X-ray position. (3) MAXI J1836-194: There is a field star 0.37 away from the position of the BH candidate (Russell et al. 2014).
Swift J1357.2-0933 in rigour is not a dynamical BHT because it lacks a detection of the secondary star but it has robust mass function determination (Corral-Santana et al. 2013;Mata Sánchez et al. 2015). The distance is still uncertain (see Rau et al. 2011b;Shahbaz et al. 2013;Mata Sánchez et al. 2015 . (6) XTE J1652-453 has no clear counterpart in IR and there are doubts between close objects.
Swift J174540.2-290005 is 1 pc from Sgr A* assuming an 8.5 kpc distance (Kennea et al. 2006b). If this transient is an X-ray binary it would be a LMXB (Wang et al. 2006).
IGR J17497-2821 seems to be blended with a bright star SE of the target (Torres et al. 2006a). Probably located in the Galactic bulge.
Swift J1753.5-0127 remains in a low/hard X-ray state since its outburst in 2005.
XTE J1748-288 showed hard spectrum. No counterpart has been found. An unresolved radio source was detected (Hjellming et al. 1998) but it was located 1' away from the center of the 1' uncertainty RXTE position (Strohmayer & Marshall 1998).
GRS 1737-31 is supposed to be close to the Galactic center (Ueda et al. 1997).
GRS J1915+105 has remained in outburst since its discovery in 1992 (Castro-Tirado et al. 1992a). It is included here because it was detected as a transient source.
EXO 1846-031 does not have a known counterpart. The error radius in the coordinates is 11" (Parmar et al. 1993).
SLX 1746-331 is a galactic centre source and therefore suffer of large extinction. It showed outbursts in 2003 (Markwardt 2003) and 2007 (Markwardt & Swank 2007). It also showed some activity in 2011 (Ozawa et al. 2011).
3A 0620-003 showed an outburst in 1917 detected in plate scales prior to its detection in X-rays in 1975 (Eachus et al. 1976).
Cen X-2 is the first transient XRB ever discovered. It has an uncertain position. Kitamoto et al. (1990) proposed that it might be GS 1354-64 (BW Cir), rediscovered in 1987. However, if this were true, it would imply an extreme X-ray flux at its distance of 25 kpc (Casares et al. 2009).
SAX J1711.6-3808: The infrared magnitudes are from a star within the X-ray error circle in a crowded field, but it is not confirmed as the true counterpart Wang & Wang (2014).
GRS 1739-278: The peak X-ray flux is taken from the MAXI SSC light curve of the 2014 outburst.
GRS J1915+105: The peak X-ray flux was obtained from the maximum value in the XTE/ASM light curves (MJD=50317.68034).
4U 1630-472 (Nor X-1): The peak X-ray flux is taken from the 2003 outburst (Tomsick et al. 2005), the brightest one detected so far. Notes. † Strictly speaking, Swift J1357.2-0933 is not a dynamical BHTs because it lacks of detection of the secondary star but it has a robust mass function determination. * These magnitudes were not transformed into the AB system. (1) Since this object has not reached the quiescent level again, the IR magnitudes are values in outburst. The I-band magnitude reported by Boeer et al. (1996) is estimated from a tentative optical counterpart in the field.