The Green Bank Interferometer (GBI) at present consists of two 26 meter telescopes spaced 2.4 km apart, equipped with dual-frequency (8.3 and 2.25 GHz) dual-polarization cryogenically cooled receivers. Observing since 1996 has been in support of NASA High Energy Astrophysics programs and concentrates on galactic X-ray binary stars. X-ray and gamma-ray monitoring of these sources by satellite observatories RXTE, BATSE, and OSSE, are compared with radio data.
About 50 objects are observed daily at 2.25 and 8.3 GHz. Included are many well-known objects such as Sco 1, Cyg X-1, Cyg X-2, Cyg X-3, SS433, and Algol. The GBI tries to follow promising X-ray and gamma ray burst sources, but few become detectable in the radio. One that became unusually bright in the radio was XTE J0421+560, which flared in April 1998, and was watched by the VLA and GBI. It reached a flux density of 0.8 Jy at 8 GHz about 3 days after the X-ray outburst, with a slow decline over the following weeks. VLA mapping showed development of a jet. This source was found to be a known symbiotic star, CI Cam (Hjellming and Mioduszewski, IAU Circular 6862).
Two famous galactic "microquasar" sources, GRO 1655-40 and GRS 1915+105, are monitored daily by the GBI. The third galactic source with superluminal motion, XTE J1748-288, was discovered in 1998 and placed on the GBI program quickly enough so the GBI provided the most continuous observations of a series of two major events. This source turned into a steady jet source like SS433 in 1999.
These three microquasars were joined by V4641 Sgr, whose X-ray outburst in September 1999 called attention to itself. Radio light curves based on GBI and VLA monitoring show a power law decay following the initial outburst. VLBA maps (Hjellming, Rupen, and Mioduszewski) showed a one-sided jet that died quickly only to be replaced by a weakly decaying continuous jet source. HI absorption mapping indicates a distance of less than 1 kpc, which makes this by far the closest known microquasar.
In October 1999 an X-ray transient source, XTE J1859+226, was discovered by the RXTE satellite. Initial follow-up radio observations, done with the GBI, VLA, and Ryle Telescopes, showed a radio source about of about 10 mJy, which, a few days later, flared to about 100 mJy (Pooley, Hjellming, IAUC 7278). Optical spectra (IAUC 7276) suggest it is an X-ray nova. The source continues to be monitored.
Cygnus X-3 has been monitored closely by the GBI since 1982. The major radio flares in February and June 1997 were studied in unprecedented detail by monitoring at Green Bank, Cambridge, the VLA, and the VLBA. Detailed VLBA maps by Hjellming, Mioduszewski, and Rupen showed the evolution of features in the expanding jet. Onset of features corresponded to outbursts seen in the GBI monitoring. Correlations with X-ray observations done with of Cyg X-3 during these outbursts showed a correlation between the radio flares and the hard X-ray emission. But during quiescent periods, there is an anti-correlation (McCollough et.al. NewAR 42, 629, 1998). A possible explanation is that during quiescent radio states the radio emission is generated in or near the accretion disk, while during flares, the radio emission is from the emerging jets.
An important clue to the understanding of Cyg X-3 was the discovery that the radio flux density drops to a very low level (< 30 mJy) shortly before a major flare (Waltman et.al. A.J. 108,179, 1994; A.J. 112, 2690, 1996). Increased mass transfer to an accretion disk may temporarily quench the radio emission before a flare erupts, fed by the increased transfer rate.
Cygnus X-3 is now (April 2000) in an active phase, having flared to 13.5 Jy at 15 GHz on April 1. An active phase can last a month or two with several large outbursts. The situation is being monitored with the GBI, VLA, VLBA, Ryle Telescope, MERLIN, and the RATAN 600 in the radio, and with BATSE, Beppo-SAX, and Chandra in the X-ray. The initial outburst followed a typical scenario of expansion of an initially optically thick synchrotron emitting plasma. Important results will undoubtedly emerge after the observations and analyses are complete.