Long‐Term Flux Monitoring of LSI +61o303 at 2.25 and 8.3 GHz

Abstract

LSI +61 • 303 is an exotic binary system consisting of a ∼ 10 M ⊙ B star and a compact object which is probably a neutron star. The system is associated with the interesting radio source GT 0236+610 that exhibits bright radio outbursts with a period of 26.5 days. We report the results of continuous daily radio interferometric observations of GT 0236+610 at 2.25 and 8.3 GHz from 1994 January to 1996 February. The observations cover 25 complete (and 3 partial) cycles with multiple observations each day. We detect substantial cycle-to-cycle variability of the radio emission characterized by a rapid onset of the radio flares followed by a more gradual decrease of the emission. We detect a systematic change of the radio spectral index α (defined as S ν ∝ ν α ) which typically becomes larger than zero at the onset of the radio outbursts. This behavior is suggestive of expansion of material initially optically thick to radio frequencies, indicating either that synchrotron or inverse Compton cooling are important or that the free-free optical depth to the source is rapidly changing. After two years of observations, we see only weak evidence for the proposed 4-year periodic modulation in the peak flux of the outbursts. We observe a secular trend in the outburst phases according the the best published ephemeris. This trend indicates either orbital period evolution, or a drift in outburst orbital phase in response to some other change in the system.

The radio to TeV orbital variability of the microquasar LS I +61 303

“…It is seen in Fig. 2 that the computed radio flux is not particularly high during periastron, although it strongly increases later on, as it is observed (see Ray et al 1997 for example). The computed X-ray luminosity is the highest around phase 0.5, like for TeV radiation, and is similar to what is observed in both energy bands (Harrison et al 2000 andAlbert et al 2006, respectively).…”

Hard X-rays from Be star LSI +61$\degr $303

“…Apparao (1999) has suggested that the plasmon formed near the periastron, floats to the top of the Be star gas disk due to buoyancy and subsequently expands in the wind of the Be star; the peak of the radio emission is reached when the bubble becomes optically thin to the radio emission. This picture accounts for the observed (see Paredes et al 1991 and references therein;Tavani et al 1996;Ray et al 1997) radio intensity dependent delay in the radio maximum, compared to that of the X-ray maximum. We shall adopt the picture suggested by Paredes et al (1991) and Apparao (1999) in this paper.…”

“…Using the radio flux at 2.25 GHz as ∼230 mJy (Ray et al 1997), p = 1.6, B = 1 G (Paredes et al 1991), R = 4 10 11 cm (Straizys & Kuriliene 1981) and r = 3 10 12 cm (the periastron distance), the inverse Compton spectrum is calculated using the Eqs. (1-4).…”

“…The radio emission occurs after every periastron passage of the compact object, when the Be star has a gas disk, and each flare lasts several days. The radio emission has been measured at several frequencies (Taylor & Gregory 1984;Ray et al 1997 and references therein) and the spectrum can be fitted with a power law of the form S ∝ ν −α where ν is the radio frequency (Ray et al 1997); the spectral index α varies during an outburst, with a negative value near the peak of radio emission and with positive values subsequently. The spectral index reaches an average value of ∼0.3 (Taylor et al 1996;Ray et al 1996) during the later stages of the flare.…”

Bayesian revolution in spectral analysis

“…Very sparsely sampled measurements were obtained from the initial discovery in 1977 until 1992. However, beginning in January 1994 Ray et al [22], detailed monitoring was performed (several times a day) with the National Radio Astronomy Observatory Green Bank Interferometer. With such sparsely sampled data the eye is unable to pick out any obvious periodicity.…”