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Possible Radio Counterpart of MXB 1730-335

ATel #8; R. Rutledge (UCB), C. Moore (Kapteyn Inst.) D. Fox, W. Lewin (MIT), and J. van Paradijs (UA/UAH)
on 27 Jan 1998; 19:03 UT
Credential Certification: rutledge@rosat.mpe-garching.mpg.de

Subjects: Radio, X-ray, Binary, Transient

Referred to by ATel #: 9, 32, 46, 47

Contemporaneous observations with RXTE All-Sky Monitor (2-10 keV) and the VLA at 4.9 and 8.4 GHz reveal a transient radio point-source, the intensity of which is correlated with the X-ray flux of MXB 1730-335 (The Rapid Burster; RB). The dates of observations, ASM 1-day average countrates, and flux densities are:

Date		ASM (2-10 keV)		8.44 GHz	4.9 GHz
c/s (+/-) uJy (+/-) uJy (+/-)
14 Oct 96 0.15 (0.3) 45 (31) --
6.8 Nov 96 9.8 (0.8) 370 (30) --
11.9 Nov 96 10.2 (1.5) 290 (30) 200 (30)
29.3 Jun 97 17.8 (0.6) 330 (50) 210 (70)
24.1 Jul 97 0.8 (0.7) 41 (31) --
The upper-limit on the size of the radio source is 0.26 arcsec (2-sigma). The source position is: RA=17h33m24s.61; DEC=-33d23m19s.8 (J2000), uncertainty is 0.1 arcsec.

The probability of an unrelated background radio source producing similar on/off behavior to the X-ray Rapid Burster, assuming a uniform probability of a duty cycle, is 1.6%.

The best X-ray position (Grindlay & Seaquist, 1986, ApJ, 310, 172) reported for the RB is 4.5 sigma away from this source; however the distribution of 40 X-ray source localizations which produced the X-ray error circle contained some outliers, and the Rapid Burster may be another (J. Grindlay, priv. comm).

The radio spectrum during these observations is flat or slightly inverted (0.0-1.2, 90% confidence), and is consistent with a synchrotron bubble model. In this model, emission is expected to peak at higher frequencies, early in the outburst. We estimate the following peak radio flux densities:

S(v)= 0.3 mJy (v/8 GHz)^0.4
T_peak(v) = 5-10 days (v/8 GHz)^1.3

Alternatively, the radio flux may be correlated with the X-ray flux, with the inverted spectrum observed:

 
S(v) = 27 uJy/(RXTE/ASM c/s) (v/8)^(0.6 +/- 0.3)
After the beginning of an outburst, the RXTE/ASM countrate has been observed to peak at about 25 c/s and then decay approximately exponentially with a ~15 day time constant.

The RB has outbursts separated by roughly 250 days (+/- 40 days). The next outburst is expected to occur in February-April 1998.

We are organizing a radio campaign, to begin immediately upon the start of the next outburst. Observers interested in performing observations should contact R. Rutledge . In addition, the onset of the next outburst will be announced via the Instant Email Notices for transients, within a few hours of its beginning.

These coordinates were also reported in IAUC 6812