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