TITLE: GCN GRB OBSERVATION REPORT NUMBER: 1138 SUBJECT: The X-ray Flash 011030 ('GRB 011030') DATE: 01/11/09 16:25:10 GMT FROM: Jean int Zand at SRON J. Heise (SRON, Utrecht), J.J.M. in 't Zand (Utrecht Univ. and SRON), S.R. Kulkarni (Caltech), and E. Costa (CNR, IAS, Rome) report: "We have carried out further investigations of SAX J2043.6+7717, the X-ray rich transient of 30 October, 2001 discovered by BeppoSAX-WFC (GCN 1118). The flash peaks at 0.26 Crab units (2-28 keV), while activity is detected for about 1400 s (but we note that the observations started about 100 s before the first activity from this source was detected). The spectrum is characterized by a power law with a photon index of -1.9 +/- 0.1. A black body fit is inconsistent with the data at 99.97% confidence. The 2-28 keV fluence is estimated at 9E-7 erg/cm2. For X-ray light curve and spectrum, we refer to http://wfc.sron.nl/flash/ What could SAX J2043.6+7717 be? By several accounts, SAX J2043.6+7717 does not appear to be a traditional GRB. First, the absence of detection of the X-ray transient in the Gamma-Ray Burst Monitor on BeppoSAX means that the peak energy of the transient must be lower than ~40 keV. In contrast, GRBs are distinguished by peaking in the 50 to 500 keV range. Second, GRBs with such long duration are rare, e.g. the longest T90 listed in the 4th BATSE catalog (Paciesas et al. 2000, ApJS, 122, 465) out of 1234 bursts is 674 s (trigger 3458) and in general have strong pulses. In contrast, SAX J2043.6+7717 lasted more than a thousand seconds long. The second possibility, especially given the galactic latitude of SAX J2043.6+7717 (b=+20.7 degrees), may argue for this source to be a Galactic transient. However, the X-ray spectrum rules out SAX J2043.6+7717 being a type I X-ray burst. This then leaves us with the possibility that SAX J2043.6+7717 is a Fast X-ray Transient (FXT; e.g., Grindlay 1999, ApJ, 510, 710). However, FXTs usually last longer than an hour. More importantly, FXTs have been identified with relatively bright quiescent optical/IR counterparts e.g. RS CVns, nearby dMe stars, super flares from pre-main sequence stars and black hole transients. However, as noted in GCN 1137, there is no visible optical counterpart to the radio source nor was a bright optical or IR transient (GCN 1127) seen following the detection of the X-ray transient. We conclude that most likely SAX J2043.6+7717 is a member of the newly recognized class of X-ray flashes (Heise et al. 2001, in Proc. 'Gamma-Ray Burst in the Afterglow Era', Rome, Oct. 17-20, 2000, in press). Given that we know little about such events (with the 011030 flash possibly being the first event localized to arcsecond accuracy; see GCN 1136) it is not appropriate to call such non-triggered transients as "X-ray rich, gamma-ray poor" GRBs (or other equally oxymoronic names). Recognizing that astronomical research is largely based on empiricism, we suggest that events such as the one on 011030 characterized by the following criterion be termed as "X-ray flashes" (XRF): (1) Strong non-thermal emission in the X-ray (2-20 keV band; this criterion distinguishes them from type I bursts); (2) Weak in the traditional GRB gamma-ray band, 50-250 keV; this explains why typical XRFs do not trigger gamma-ray burst monitors. (3) Durations less than a few thousand seconds; this differentiates XRFs from the so-called fast X-ray transients which have durations of several hours (many of which are stars with intense coronal activity and some are binaries containing a compact object such as XTE J1819-254). (4) Display no strong quiescent optical/IR counterpart; this criterion distinguishes XRFs from stars with strong coronal activity (e.g. strong flares from pre-main sequence stars, dMe stars, RSCVn stars, Be stars etc). The accurate position of this X-ray flash presents an excellent opportunity to understand this new phenomenon which previously has never been detected outside the X/gamma-ray regime. We strongly encourage multi-wavelength observations." This message may be cited.