High energy processes in microquasars

Microquasars are X-ray binary stars with the capability to generate relativisticjets. It is expected that microquasars are gamma-ray sources, because of the analogy with quasars and because the theoretical models predict emission at such energy range. In addition, from observational arguments, there are two microquasars that appear as the possible counterparts for two unidentified high-energy gamma-ray sources.Comment: Universitat de Barcelona, Departament d'Astronomia i Meteorologia, 12 pages, 5 figures. Invited talk presented at the International Symposium "High-Energy Gamma-Ray Astronomy", 26-30 July 2004, Heidelberg (Germany). To be published by AIP Proceedings Serie

A possible black hole in the gamma-ray microquasar LS 5039

The population of high energy and very high energy gamma-ray sources, detected with EGRET and the new generation of ground-based Cherenkov telescopes, conforms a reduced but physically important sample. Most of these sources are extragalactic (e.g., blazars), while among the galactic ones there are pulsars and SN remnants. The microquasar LS 5039, previously proposed to be associated with an EGRET source by Paredes et al. (2000), has recently been detected at TeV energies, confirming that microquasars should be regarded as a class of high energy gamma-ray sources. To model and understand how the energetic photons are produced and escape from LS 5039 it is crucial to unveil the nature of the compact object, which remains unknown. Here we present new intermediate-dispersion spectroscopy of this source which, combined with values reported in the literature, provides an orbital period of 3.90603+/-0.00017 d, a mass function f(M)=0.0053+/-0.0009 M_sun, and an eccentricity e=0.35+/-0.04. Atmosphere model fitting to the spectrum of the optical companion, together with our new distance estimate of d=2.5+/-0.1 kpc, yields R_opt=9.3+0.7-0.6 R_sun, log (L_opt/L_sun)=5.26+/-0.06, and M_opt=22.9+3.4-2.9 M_sun. These, combined with our dynamical solution and the assumption of pseudo-synchronization, yield an inclination i=24.9+/-2.8 degree and a compact object mass M_X=3.7+1.3-1.0 M_sun. This is above neutron star masses for most of the standard equations of state and, therefore, we propose that the compact object in LS 5039 is a black hole. We finally discuss about the implications of our orbital solution and new parameters of the binary system on the CNO products, the accretion/ejection energetic balance, the SN explosion scenario, and the behaviour of the TeV emission with the new orbital period.Comment: 10 pages, 8 figures. Accepted for publication in MNRAS. Minor changes according to referee repor

Periodic morphological changes in gamma-ray binaries

Gamma-ray binaries allow us to study physical processes such as particle acceleration up to very-high energies and gamma-ray emission and absorption with changing geometrical configurations on a periodic basis. They produce outflows of radio-emitting particles whose structure can be imaged with Very Long Baseline Interferometry (VLBI). We present recent and new VLBI observations of PSR B1259-63, LS 5039, LS I +61 303, and HESS J0632+057. For the first three cases the results show the repeatability of their radio structures with the orbit of the binary system.Comment: 4 pages. Proceedings of the "5th International Symposium on High-Energy Gamma-Ray Astronomy", Heidelberg (Germany), 9-13 July 201

Periodic morphological changes in the radio structure of the gamma-ray binary LS 5039

Gamma-ray binaries allow us to study physical processes such as particle acceleration up to TeV energies and VHE gamma-ray emission and absorption with changing geometrical configurations on a periodic basis. These sources produce outflows of radio-emitting particles whose structure can be imaged with VLBI. LS 5039 is a gamma-ray binary that has shown variable VLBI structures in the past. We aim to characterise the radio morphological changes of LS 5039 and discriminate if they are either repeatable or erratic. We observed LS 5039 with the VLBA at 5 GHz during five consecutive days to cover the 3.9-day orbit and an extra day to disentangle between orbital or secular variability. We also compiled the available high-resolution radio observations of the source to study its morphological variability at different orbital phases. We used a simple model to interpret the obtained images. The new observations show that the morphology of LS 5039 up to projected distances of 10 milliarcseconds changes in 24 h. The observed radio morphological changes display a periodic orbital modulation. Multifrequency and multiepoch VLBI observations confirm that the morphological periodicity is stable on timescales of years. Using a simple model we show that the observed behaviour is compatible with the presence of a young non-accreting pulsar with an outflow behind it. The morphology is reproduced for inclinations of the orbit of 60-75 deg. For masses of the companion star in the range 20-50 Msun, this range of inclinations implies a mass of the compact object of 1.3-2.7 Msun. The periodic orbital modulation of the radio morphology of LS 5039 suggests that all gamma-ray binaries are expected to show a similar behaviour. The changes in the radio structure of LS 5039 are compatible with the presence of a young non-accreting neutron star, which suggests that the known gamma-ray binaries contain young pulsars.Comment: 15 pages, 7 figures. Accepted for publication in Astronomy and Astrophysic

Microquasars as high-energy gamma-ray sources

Galactic microquasars are certainly one of the most recent additions to the field of high energy astrophysics and have attracted increasing interest over the last decade. However, the high energy part of the spectrum of microquasars is the most poorly known, mainly due the lack of sensitive instrumentation in the past. Microquasars are now primary targets for all of the observatories working in the X-ray and gamma-ray domains. They also appear as the possible counterparts for some of the unidentified sources of high-energy gamma-rays detected by the experiment EGRET on board the satellite COMPTON-GRO. This paper provides a general review of the main observational results obtained up to now as well as a summary of the scenarios for production of high-energy gamma-rays at the present moment.Comment: Invited talk presented at the V Microquasar Workshop, Beijing, June 2004. Accepted for publication in the Chinese Journal of Astronomy and Astrophysics. 14 pages, 9 figure