SWIFT J195509.6+261406 / GRB 070610: A Potential Galactic Transient

ATel #1102; C. B. Markwardt (CRESST/U. Md./GSFC), C. Pagani (PSU), P. Evans (U. Leicester), F. P. Gavriil (NPP/GSFC), J. A. Kennea (PSU), H. A. Krimm (CRESST/USRA/GSFC), W. Landsman (NASA/GSFC), F. E. Marshall (NASA/GSFC), and the Swift Team
on 12 Jun 2007; 16:11 UT
Distributed as an Instant Email Notice Request For Observations
Credential Certification: Craig B. Markwardt (craigm@lheamail.gsfc.nasa.gov)

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On 06 June 2007 20:52:26 UTC, the Swift BAT instrument detected a transient outburst of gamma-rays (originally designated GRB 070610; Pagani et al. GCN #6489), which may in fact be a galactic X-ray transient. The source, now also designated SWIFT J195509.6+261406, has the potential to be similar to the supergiant fast X-ray transient V4641 Sgr.

The gamma-ray light curve had a single "FRED"-like profile, lasting about 8 seconds total, which is common for gamma-ray bursts (Tueller et al. GCN #6491). The spectrum was consistent with a power law with photon index 1.76 and total fluence 2.4 ± 0.4 x 10^{-7} erg/cm2 (there is no detectable spectral break out to E > 200 keV). The source has not been detected previously by BAT, either as a flaring source since Jan 2005, or as a persistent source in BAT daily averages since Apr 2007.

The Swift XRT detected a counterpart at position R.A. = 19h55m09.6s, Dec = +26d14'06.7" (J2000) with 90% uncertainty radius of 4.3 arcsec (Pagani & Kennea GCN #6506). However, the counterpart did not decline, but rather exhibited rapid X-ray variations, some as bright as ~25 mCrab. The X-ray spectrum is consistent with a power law with photon index 1.8 ± 0.2 and neutral absorption 0.7 x 10^{21} cm^{-2}.

Ground-based optical and infrared follow-up observations revealed a flaring source (GCN #6492, #6501, #6505, #6508), with some flares as short as a few tens of seconds (Stefanescu et al. GCN #6492). Kann et al. suggested the source is a galactic transient (GCN #6505).

The source is most reminiscent of the fast X-ray transient V4641 Sgr ( = SAX J1819.3-2525 ) which also had brief spikey flares and a hard spectrum. V4641 Sgr is regarded as a black hole system with a high mass companion (Orosz et al. 2001, ApJ, 555, 489). Rapid variability was present in the optical (Uemura et al. 2003 PASJ, 56, 823). This source is associated with radio ejecta (Hjellming et al. 2000 ApJ, 544, 977) and rapid outflows were also detected in optical (Lindstrom et al. 2005, MNRAS 363, 882). The X-ray spectrum of V4641 Sgr had a strong and broad iron line at ~6.4 keV (Revnivtsev et al. 2002, 391, 1013). To date, however, SWIFT J195509.6+261406 has not shown a similar spectral feature.

It is also possible the Swift source is an anomalous X-ray pulsar or soft gamma repeater. Both the fluence and the duration are large compared to typical AXP/SGR bursts, but not exceptionally so. However, no strong pulsations for periods longer than 5.1 sec have been detected in the XRT data to date (at a typical 2-10 keV flux of ~0.1 mCrab).

Given the power law spectrum and rapid flaring of SWIFT J195509.6+261406, it is unlikely to be a thermonuclear X-ray burster (thermonuclear bursts typically have ~hour recurrence times and a black body spectrum with kT ~ 2-3 keV).

Since this source is most similar to V4641 Sgr, it may have relativistic jet-like outflows. Radio and spectroscopic observations are encouraged.

In this circular, we have assumed that the only XRT source within the BAT error circle is indeed the counterpart (the BAT error circle has a 1.8 arcmin radius, 90% confidence; GCN #6491). However, we admit the possibility that the BAT and XRT have detected two unrelated sources. If the GRB were considered a "short burst" (the T90 duration is ~4.6 sec) then the X-ray counterpart might have faded quickly before XRT observations began. The XRT 95% upper limit to any other source in the BAT error circle is about 2 x 10^{-14} erg/cm2/s (0.3-10 keV). Given this limit, the small size of the BAT error circle, and the proximity of the source to the galactic plane, we consider this possibility unlikely. There is one other Swift XRT source 4.3 arcmin from the BAT position which will be reported in a separate ATEL.