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Recent studies of relativistic jet sources in the Galaxy, also known as microquasars, have been very useful in trying to understand the accretion/ejection processes that take place near compact objects. However, the number of sources involved in such studies is still small. In an attempt to increase the number of known microquasars we have carried out a search for new Radio Emitting X-ray Binaries (REXBs). These sources are the ones to be observed later with VLBI techniques to unveil their possible microquasar nature. To this end, we have performed a cross-identification between the X-ray ROSAT all sky survey Bright Source Catalog (RBSC) and the radio NRAO VLA Sky Survey (NVSS) catalogs under very restrictive selection criteria for sources with |b|<5 degrees. We have also conducted a deep observational radio and optical study for six of the selected candidates. At the end of this process two of the candidates appear to be promising, and deserve additional observations aimed to confirm their proposed microquasar nature.
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We report optical spectroscopic observations of a sample of 6 low-galactic latitude microquasar candidates selected by cross-identification of X-ray and radio point source catalogs for |b|<5 degrees. Two objects resulted to be of clear extragalactic origin, as an obvious cosmologic redshift has been measured from their emission lines. For the rest, none exhibits a clear stellar-like spectrum as would be expected for genuine Galactic microquasars. Their featureless spectra are consistent with being extragalactic in origin although two of them could be also highly reddened stars. The apparent non-confirmation of our candidates suggests that the population of persistent microquasar systems in the Galaxy is more rare than previously believed. If none of them is galactic, the upper limit to the space density of new Cygnus X-3-like microquasars within 15 kpc would be 1.1times10^{-12} per cubic pc. A similar upper limit for new LS 5039-like systems within 4 kpc is estimated to be 5.6times10^{-11} per cubic pc.
In an attempt to increase the number of known microquasars, Paredes et al. (2002) have presented a long-term project focused on the search for new objects of this type. They performed a cross-identification between X-ray and radio catalogs under very restrictive selection criteria for sources with |b|<5 degrees, and obtained a sample of 13 radio-emitting X-ray sources. Follow-up observations of 6 of these sources with the VLA provided accurate coordinates, which were used to discover optical counterparts for all of them. We have observed these six sources with the EVN and MERLIN at 5 GHz. Five of the six objects have been detected and imaged, presenting different morphologies: one source has a two-sided jet, three sources have one-sided jets, and one source is compact. With all the presently available information, we conclude that two of the sources are promising microquasar candidates in our Galaxy.
We use near-infrared (J-K)-colours of bright 2MASS galaxies, measured within a 7-radius aperture, to calibrate the Schlegel et al. (1998) DIRBE/IRAS Galactic extinction map at low Galactic latitudes ($|b| < 10^{rm o}$). Using 3460 galaxies covering a large range in extinction (up to $A_K$ = 1.15 or E(B-V) ~ 3.19), we derive a correction factor $f = 0.83 pm 0.01$ by fitting a linear regression to the colour-extinction relation, confirming that the Schlegel et al. maps overestimate the extinction. We argue that the use of only a small range in extinction (e.g., $A_K$ < 0.4) increases the uncertainty in the correction factor and may overestimate it. Our data confirms the Fitzpatrick (1999) extinction law for the J- and K-band. We also tested four all-sky extinction maps based on Planck satellite data. All maps require a correction factor as well. In three cases the application of the respective extinction correction to the galaxy colours results in a reduced scatter in the colour-extinction relation, indicating a more reliable extinction correction. Finally, the large galaxy sample allows an analysis of the calibration of the extinction maps as a function of Galactic longitude and latitude. For all but one extinction map we find a marked offset between the Galactic Centre and Anticentre region, but not with the dipole of the Cosmic Microwave Background. Based on our analysis, we recommend the use of the GNILC extinction map by Planck Collaboration (2016b) with a correction factor $f = 0.86 pm 0.01$.
We have used the field stars from the open cluster survey BOCCE, to study three low-latitude fields imaged with the Canada-France-Hawaii telescope (CFHT), with the aim of better understanding the Galactic structure in those directions. Due to the deep and accurate photometry in these fields, they provide a powerful discriminant among Galactic structure models. In the present paper we discuss if a canonical star count model, expressed in terms of thin and thick disc radial scales, thick disc normalization and reddening distribution, can explain the observed CMDs. Disc and thick disc are described with double exponentials, the spheroid is represented with a De Vaucouleurs density law. In order to assess the fit quality of a particular set of parameters, the colour distribution and luminosity function of synthetic photometry is compared to that of target stars selected from the blue sequence of the observed colour-magnitude diagrams. Through a Kolmogorov-Smirnov test we find that the classical decomposition halo-thin/thick disc is sufficient to reproduce the observations--no additional population is strictly necessary. In terms of solutions common to all three fields, we have found a thick disc scale length that is equal to (or slightly longer than) the thin disc scale.
We present an Ultra-Cool Dwarf (UCD) catalogue compiled from low southern Galactic latitudes and mid-plane, from a cross-correlation of the 2MASS and SuperCOSMOS surveys. The catalogue contains 246 members identified from 5042 sq. deg. within 220 deg. <= l <= 360 deg. and 0 deg. < l <= 30 deg., for |b| <= 15 deg. Sixteen candidates are spectroscopically confirmed in the near-IR as UCDs with spectral types from M7.5V to L9. Our catalogue selection method is presented enabling UCDs from ~M8V to the L-T transition to be selected down to a 2MASS limiting magnitude of Ks ~= 14.5 mag. This method does not require candidates to have optical detections for catalogue inclusion. An optimal set of optical/near-IR and reduced proper-motion selection criteria have been defined that includes: an Rf and Ivn photometric surface gravity test, a dual Rf-band variability check, and an additional photometric classification scheme to selectively limit contaminants. We identify four candidates as possible companions to nearby Hipparcos stars -- observations are needed to identify these as potential benchmark UCD companions. We also identify twelve UCDs within a possible distance 20 pc, three are previously unknown of which two are estimated within 10 pc, complimenting the nearby volume-limited census of UCDs. An analysis of the catalogue spatial completeness provides estimates for distance completeness over three UCD MJ ranges, while Monte-Carlo simulations provide an estimate of catalogue areal completeness at the 75 per cent level. We estimate a UCD space density of Rho (total) = (6.41+-3.01)x10^3/pc^3 over the range of 10.5 <= MJ ~< 14.9, similar to values measured at higher Galactic latitudes (|b| ~> 10 deg.) in the field population and obtained from more robust spectroscopically confirmed UCD samples.
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