Discovery of a High-Energy Gamma-Ray-Emitting Persistent Microquasar

Microquasars are stellar x-ray binaries that behave as a scaled down version of extragalactic quasars. The star LS 5039 is a new microquasar system with apparent persistent ejection of relativistic plasma at a 3 kiloparsec distance from the sun. It may also be associated with a gamma-ray source discovered by the Energetic Gamma Ray Experiment Telescope (EGRET) on board the COMPTON-Gamma Ray Observatory satellite. Before the discovery of LS 5039, merely a handful of microquasars had been identified in the Galaxy, and none of them was detected in high-energy gamma-rays.

The V = 11.2 magnitude star LS 5039 (1) has been recently identified as a nearby high-mass x-ray binary with spectral type O7V((f)) (2) and persistent radio emission (3,4). Here, we report high-resolution radio observations with the Very Long Baseline Array (VLBA) and the Very Large Array (VLA) that reveal that LS 5039 is resolved into bipolar radio jets emanating from a central core.
Because LS 5039 appeared unresolved (≤ 0.1 ′′ ) to the VLA alone, we proceeded to study this object with milliarc sec resolution using the VLBA at the frequency of 5 GHz (6 cm wavelength) on 8 May 1999. The VLA in its phased array mode, equivalent to a dish of 115 m diameter, also participated as an independent station, providing sensitive baselines with the VLBA antennas. The source 3C345 was used as a fringe-finder, whereas J1733−1304 was the phasing source for the VLA. The data were calibrated using standard procedures in unconnected radio interferometry. The resulting pattern of the observed visibility amplitudes, decaying as a function of baseline length, indicated that LS 5039 had structure at milliarc sec scales.
The final synthesis map ( Fig. 1) shows that bipolar jets emerge from a central core. A deconvolved angular size of about 2 milliarc sec is estimated for the core. The jets extend over 6 milliarc sec on the sky oriented along a position angle (PA) of 125 • with respect to the North, and they account for 20% of the total 16 mJy flux density. To obtain some order of magnitude estimates, we will assume that the overall size of the radio source is approximately 6 × 2 milliarc sec 2 . This implies a high brightness temperature of ∼ 9.4 × 10 7 K, indicating synchrotron radiation. The LS 5039 radio spectrum as a function of frequency ν, namely S ν ∝ ν α , often displays a negative spectral index α = −0.5 in agreement with a non-thermal optically thin emission mechanism (3,4). The detection of jets occurred at a time when the source was at its typical persistent level of radio emission, and only moderately variable, as inferred from concurrent radio monitoring by the Green Bank Interferometer (GBI) (Fig. 2).
The absence of any precursor outburst for the radio jets strongly suggests that they are always present and continuously emanating from the core. The flux density ratio between the SE and NW jet components is estimated as 2.1 ± 0.4. It seems reasonable that this brightness asymmetry reflects a relativistic Doppler boosting effect (5). If a continuous jet flow is assumed, the projected velocity required is then v cos θ = (0.15 ± 0.04)c, where c is the speed of light and θ the ejection angle with the line of sight. It is straightforward to then derive a lower and upper limit for the jet velocity [v ≥ (0.15 ± 0.04)c] and the ejection angle (θ ≤ 81 • ± 2 • ), respectively.
X-ray binaries with collimated radio jets belong to the class of galactic microquasars.
The production of jets is almost certainly related to the capture of matter from a normal star by a black hole or neutron star companion. This is a highly energetic process with observable consequences from radio to hard x-rays (6) and possibly beyond. The recent third EGRET catalog of high-energy (E γ > 100 MeV) γ-ray sources (7) contains nearly 100 unidentified emitters at low galactic latitudes. The position of LS 5039 is well inside the 95% confidence contour of the EGRET source 3EG J1824−1514, whose radius is about 0.5 • . Moreover, LS 5039 is the only x-ray emitter within 1 • of 3EG J1824−1514 listed in the ROSAT (Roentgen Satellite) All Sky Survey (8). Such a good position agreement between an EGRET source and a peculiar radio jet x-ray binary strongly implies that both objects are the same. Thus, this microquasar system is likely associated with an EGRET source.
The γ-ray emission observed reveals a rather persistent flux of > 100 MeV photons for the last 10 years (Fig. 3).
Using modern photometric data (9) and the reddening free parameter formulation (10), we obtained a distance estimate of 3.1 kpc. This value is in excellent agreement with independent results based on the star color excess (2). On the other hand, a common intrinsic radio luminosity has been recently suggested for persistent x-ray binaries (11). According to this, the LS 5039 average flux density of a few tens of mJy at cm wavelengths would imply a rough distance value not higher than 2 kpc. Thus, different distance indicators show that LS 5039 is nearby, and we adopt a distance of 3 kpc. Therefore, this star appears to be one of the closest, and optically brightest, microquasars among the persistent members of this The synchrotron radio luminosity between 0.1 and 100 GHz for this distance is L rad ∼ 7.5 × 10 30 erg s −1 . The average γ-ray flux for all EGRET viewing periods in Fig. 3 is The EGRET photon index of LS 5039 is practically identical to that of 1E 1740.7−2942, i.e., steeper than the p < 2 values usually found for pulsars (14). The corresponding integrated luminosity amounts to L γ (> 100 MeV) ∼ 3.8 × 10 35 erg s −1 , compared to an x-ray luminosity (4) of L X (1.5 − 12 keV) ∼ 5 × 10 34 erg s −1 . Additional information on the source energetics can be obtained by assuming energy equipartition between the relativistic electrons and the magnetic field (15). We are forced to use the overall source parameters observed, because not enough information is yet available for appropriate calculations in the rest frame of the ejecta. The corresponding results are nevertheless expected to be within an order of magnitude for a mildly relativistic system. Under these assumptions, the observed radio properties of LS 5039 imply a total energy content in relativistic electrons of E e ∼ 4.8 × 10 39 erg, with an equipartition magnetic field of ∼ 0.2 G.
While flowing away into opposite jets, the relativistic electrons are exposed to a huge output of ultraviolet (UV) photons from the hot optical star. Thus, it appears likely that a significant fraction of the EGRET emission arises as a result of inverse Compton (IC) scattering of these photons by the same radio-emitting electrons. The energy shift in this process is such that E γ ∼ γ 2 e E ph , where the energies of the γ-ray and the stellar photon are related through the squared Lorentz factor of the relativistic electron. For an O7 main sequence star, a UV luminosity of L * ∼ 10 38 erg s −1 is expected to be mostly emitted by photons with E ph ∼ 10 eV. In order to scatter them into γ-rays with E γ ∼ 100 MeV, electrons with Lorentz factors ∼ 10 3 , equivalent to energies of ∼ 10 −3 erg, are needed. Considering the persistent EGRET luminosity, the lifetime of such electrons against dominant IC losses will be t c ∼ E e /L γ ∼ 1.3 × 10 4 s.
The electron energy will decay with time by IC scattering according to (15) (in centimeter-gram-second units): where U rad is the UV radiation energy density. For electrons flowing away into jets, assumed perpendicular to the plane of a circular orbit with radius r, we have at a time t after injection. For an electron with initial energy E 0 , its IC lifetime can be expressed as t c = 25.2/U rad E 0 when injected into the jet basis close to the compact object. This implies then that the γ-ray-emitting electrons must be initially exposed to U rad ∼ 2.0 erg cm −3 . Such values of radiation energy density are available if the jets originate at a distance r ∼ 1.2 × 10 13 cm from the star.

Equation 1
can be solved to give: By imposing the condition that electrons with E 0 ∼ 10 −3 erg are able to abandon the region of heavy IC emission in the star vicinity, the condition πr/2vt c < 1 must be fulfilled so that they still retain enough energy to power the extended radio jets. This requirement allows us to constrain the jet velocity to values v > 0.05c, in agreement with the previous discussion of Doppler boosting. The true jet velocity is not likely to exceed a mildly relativistic value v ∼ 0.4c, which we crudely estimate assuming that the 6 milliarc sec LS 5039 is one of the nearest microquasars to be discovered. It has strong high-energy γ-ray emission, which sets limits on the likely velocity of its jets via inverse Compton energy losses. Most of known microquasars were discovered only after undergoing a noticeable outburst that triggered detection by the battery of satellites and ground-based observatories.
Some recent examples include CI Camelopardalis (17) and the nearby transient V4641 Sagittarii (18). The microquasar nature of these two objects is tantalizing in that both are bright optical stars. CI Camelopardalis was even catalogued as a variable star before its outburst. Therefore, a careful examination of modern archive databases may reveal a previously unnoticed population of microquasars. Indeed, our identification of LS 5039 as a potential candidate resulted from a systematic cross-correlation between public archives of astrophysical data in the x-ray, radio and optical domains (8,19,20). The success of this approach for systematic identification opens the possibility of new findings which may confirm that the microquasar phenomenon is not as rare as it seems.