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X-ray activity in GX 339-4

ATel #707; N. Bezayiff and D. M. Smith (U. C. Santa Cruz)
on 23 Jan 2006; 02:01 UT
Distributed as an Instant Email Notice Request For Observations
Credential Certification: David M. Smith (dsmith@ssl.berkeley.edu)

Subjects: X-ray, Request for Observations, Binary, Black Hole, Transient

Referred to by ATel #: 968

Observations with the Rossi X-ray Timing Explorer (RXTE) show that the suspected black-hole binary GX 339-4 has become active after 8 months in quiescence. Three consecutive monitoring observations taken with the Proportional Counter Array (PCA) on 15 January (20:52 UT), 18 January (18:54 UT), and 21 January (07:38 UT) are all significantly above background, and show an increase with time in count rate from 1.0 to 3.8 to 5.8 counts/s/PCU. These rates are less than 1% of peak outburst levels, and it is not yet clear whether this represents a new outburst or variable low-level activity as was seen throughout 1996 and 1997 (see, e.g., Smith et al. 2002, ApJ 569, 362, Figure 4). The unabsorbed fluxes for these three observations rise from approximately 10^-12 to 10^-11 erg/cm2/s.

The spectra are shown in the figure linked below (white = Jan. 15, red = Jan. 18, green = Jan. 21). The two earlier spectra require a disk blackbody component in addition to a hard power law, while the latest is consistent with the canonical hard state, showing only a small improvement in the fit by the addition of a disk blackbody.

The iron line in the first observation is striking, and is fit with an equivalent width of (3.2 +/- 1.6) keV and center of (6.57 +/- 0.14) keV. The equivalent width declined to (790 +/- 460) eV by 18 January and (270 +/- 320) eV by 21 January. It is interesting that these data appear consistent with a simple power law being gradually added to the initial spectrum of 15 January.

Feng et al. (2001, ApJ 553, 394) reported that at very low flux, the iron line from GX 339-4 approached the He-like line energy of 6.7 keV and had a large equivalent width of up to 500 eV. The measurement of an equivalent width of over 3 keV, although not of high statistical significance, is the highest we have measured in any black hole system during our monitoring campaigns with the PCA, and suggests a continuation of the trend reported by Feng et al. We are exploring a pure reflection spectrum as a possible explanation of the 15 January data. No iron line is seen in the many observations we have made in quiescence, so Galactic diffuse emission is not responsible for this feature.

As always in what may be the early stages of a LMXRB transient outburst, multiwavelength observations are strongly encouraged.

Erratum: In ATEL #322, we reported that during the transition from the hard to soft state in last summer's outburst of GX 339-4, the observation made at 14h on 17 August, 2005 showed a soft energy spectrum but had an active power spectrum, with band-limited noise and a QPO, and therefore we described the system as being in the Very High State at that time. The power spectrum we reported as belonging to this date was an earlier one. The system was actually in the High/Soft state at this time, with little variability, as has been reported in a thorough paper on this outburst by Belloni et al. (astro-ph/0601031, accepted by MNRAS).

PCA spectra of GX 339-4: white, 1/15; red, 1/18; green, 1/21