//////////////////////////////////////////////////////////////////////// TITLE: GCN GRB OBSERVATION REPORT NUMBER: 3180 SUBJECT: Swift-BAT detection of GRB 050406 DATE: 05/04/06 17:04:17 GMT FROM: Ann M. Parsons at NASA/GSFC/Swift A. Parsons (GSFC), S. Barthelmy (GSFC), J. Cummings (GSFC/NRC), E. Fenimore (LANL), N. Gehrels (GSFC), D. Hullinger (UMD), S. Hunsberger (PSU), H. Krimm (GSFC/USRA), C. Markwardt (GSFC/UMD), D. Palmer (LANL), T. Sakamoto (GSFC/NRC), G. Sato (ISAS), M. Suzuki (Saitama), J. Tueller (GSFC) D. Burrows, J. Kennea, J. Nousek (PSU), G. Chincarini, P. Romano (INAF-OAB) on behalf of the Swift team: At 15:58:48.40 UT Swift-BAT triggered on burst GRB 050406 (trigger 113872). The BAT-derived position is RA,Dec= 34.428, -50.178 (J2000). Significant emission has been detected for at least 3 sec with possible low significance peaks over an additional 30 sec. The peak count rate was 800 cnts/sec. The source was detected at 15 degrees off the bore-site and thus is in the fully-coded field-of-view. Swift slewed promptly and the UVOT imaged the field at the end of the slew and did not find a bright optical source in the 8 arcmin square binned field of view centered on the BAT position. The XRT also imaged the field promptly and did not find a bright X-ray source within the field of view. Based on previous experience, this lack of prompt x-ray afterglow suggests either that this trigger is not a GRB, or that it is a very interesting and unusual GRB. The BAT rate trigger and image are significant enough to lean toward an interpretation of an unusual GRB. //////////////////////////////////////////////////////////////////////// TITLE: GCN GRB OBSERVATION REPORT NUMBER: 3181 SUBJECT: GRB 050406: Swift XRT Position DATE: 05/04/06 17:32:02 GMT FROM: Jamie A. Kennea at PSU/Swift-XRT G. Cusomano (IASF/Palermo), J. Kennea, D. N. Burrows, J. E. Hill, D. Grupe, J. A. Nousek (PSU), J. P. Osborne, K. Page, M. Goad, A. Beardmore, A. F. Abbey, A. A. Wells (U. Leicester), S. Campana, A. Moretti, C. Pagani, P. Romano, G. Tagliaferri, G. Chincarini (OAB), V. Mangano (IASF/Palermo), P. Giommi, M. Capalbi, M. Perri (ASDC), L. Angelini, F. Marshall, N. White, N. Gehrels (NASA/GSFC), P. Roming, P. Meszaros (PSU), P. Schady (MSSL), report on behalf of the Swift XRT team: The Swift BAT instrument detected a GRB at 15:58:48 UT on 6th April 2005 (GCN Circ 3161). The observatory executed an automated slew to the BAT position and the XRT began taking data at 16:00:15 UT. The XRT was in Auto state but was not able to centroid on the afterglow due to low source brightness. From downlinked data we find a uncataloged, fading X-ray source located at: RA(J2000) = 2:17:52.4, Dec(J2000) = -50:11:18.9. We estimate an uncertainty of about 5 arcseconds. [GCN_OPS_NOTE(12apr05): Per author's request, "Cusomano" was changed to "Cusumano".] //////////////////////////////////////////////////////////////////////// TITLE: GCN GRB OBSERVATION REPORT NUMBER: 3182 SUBJECT: GRB 050406: Early observations with Swift-UVOT DATE: 05/04/06 19:07:04 GMT FROM: Martin Still at NASA/GSFC Swift SSC W. Landsman (GSFC), S. Hunsberger (PSU), A. Breeveld (MSSL), P. Roming (PSU), K. Mason, P. Schady (MSSL), M. Ivanushkina (PSU), T. Poole (MSSL), C. Gronwall (PSU), A. Blustin (MSSL), P. Brown, S. Rosen, K. McGowan, M. De Pasquale (MSSL), P. Boyd (GSFC/UMBC), S. T. Holland, M. Still (GSFC/USRA), S. Koch (PSU), M. Carter, H. Huckle (MSSL), P. Broos (PSU), T. Kennedy, P. Smith, B. Hancock (MSSL), J. Nousek (PSU), N. Gehrels (GSFC), on behalf of the Swift UVOT team. The Swift Ultra-Violet/Optical Telescope (UVOT) began settled observations of the field of GRB 050406 (Parsons et al; GCN 3180) at 16:00:16 UT, 88s after the BAT trigger. The first data taken after spacecraft settling was a 100s V image with pixel size 0.5x0.5 arcsec and a field of view of 17x17 arcmin. Mid-exposure time was 138s after the trigger. A comparison against the Digitized Sky Survey reveals no new sources in the field down to a 3-sigma background limit of V = 18.8. This limit is based upon preliminary flight calibrations. //////////////////////////////////////////////////////////////////////// TITLE: GCN GRB OBSERVATION REPORT NUMBER: 3183 SUBJECT: GRB 050406 BAT refined analysis DATE: 05/04/06 19:57:04 GMT FROM: Hans Krimm at NASA-GSFC H. Krimm (GSFC/USRA), L. Barbier, S. Barthelmy (GSFC), J. Cummings (GSFC/NRC), E. Fenimore (LANL), N. Gehrels (GSFC), D. Hullinger (UMD), D.Q. Lamb (U. Chicago), C. Markwardt (GSFC/UMD), D. Palmer (LANL), A. Parsons (GSFC), T. Sakamoto (GSFC/NRC), G. Sato (ISAS), M. Suzuki (Saitama), M. Tashiro (Saitama U.), J. Tueller (GSFC), on behalf of the Swift/BAT team: At 15:58:48.40 UT Swift-BAT detected GRB 050406 (trigger=113872) (GCN Circ 3180, Parsons et al.). The refined BAT ground position is (RA,Dec) = 34.471 -50.181, [deg; J2000] +- 3 arcmin, (95% containment). This is 28 arc seconds from the position determined on-board and originally reported. Examination of the mask weighted light curve confirms that only the initial peak is associated with the burst. The other possible emission reported in GCN Circ 3180 is attributed to background fluctuations. The shape of the peak is fast-rise, exponential decay in the 15-25 keV band. In the 25-50 keV band, the peak starts ~2 secs earlier and the shape is more symmetric. We derive T90 (15-25 keV) = 3 seconds +/- 1 s, and T90 (15-350 keV) = 5 seconds +/- 1 s. Errors on T90 include systematics. Analysis of the event data shows that this is a very soft burst with no significant flux above 50 keV. Plotting GRB 050406 on a color-color diagram indicates that this burst may have the characteristics of an X-ray Flash. The fluence derived from the event data is 9.0 X 10^-8 erg/cm^2 in the 15-350 keV band, and 4.8 X 10^-8 erg/cm^2 in the 15-50 keV band. The 1-s peak flux (T+0.6 s) is 3.2 ph/cm^2/s (15-25 keV). The photon index of the 1-s peak spectrum (T+0 s) is 2.32 +/- 0.53 (90% confidence). The time-averaged spectrum yields a photon index of 2.38 +/- 0.34 (90% confidence). Both the 1-s and time-averaged spectra are well fit by a simple power-law. //////////////////////////////////////////////////////////////////////// TITLE: GCN GRB OBSERVATION REPORT NUMBER: 3184 SUBJECT: GRB 050406: early Swift XRT analysis results DATE: 05/04/06 21:45:48 GMT FROM: David Burrows at PSU/Swift M. Capalbi, M. Perri (ASDC), P. Romano, A. Moretti, C. Pagani, G. Tagliaferri (INAF-OAB), D. Malesani (SISSA), D. N. Burrows, J. Kennea, D. Grupe (PSU), K. Page, M. Goad, A. Beardmore (U. Leicester), V. La Parola, T. Mineo (IASF/Palermo), N. Gehrels (NASA/GSFC), and S. Kobayashi (PSU) report on behalf of the Swift XRT team: We have analyzed the Swift XRT data from the first orbit observation of GRB 050406 (Parsons et al., GCN3180; Cusumano et al., GCN 3186). The new refined coordinates are: RA(J2000) = 2h 17m 52.64s Dec(J2000) = -50:11:18.80 This position is 28 arcseconds from the refined BAT position given in GCN 3183 (Krimm et al. 2005). We estimate an uncertainty of 5 arcseconds radius (90% containment). This is 30 arcsec from the refined BAT position (Krimm et al., GCN 3183) and 3.2 arcsec from the preliminary XRT position (GCN 3186). The [0.2-10] keV light curve in Windowed Timing (WT) and Photon Counting (PC) mode starts 106 seconds from the BAT trigger (T0). At the beginning, the count rate is rapidly rising, peaking at about 218 seconds from T0. Then it decays very fast, possibly flattening at T-T0=300 seconds. A preliminary spectral fit to the WT data gives a spectral power law photon index of 2.1 ± 0.3 in the [0.5-10] keV band, assuming Galactic absorption (3.3E20cm^-2). The average (in the time range 100-700 seconds from trigger) estimated unabsorbed 0.5-10 keV flux is then about 2.8E-11 erg cm^-2 s^-1. //////////////////////////////////////////////////////////////////////// TITLE: GCN GRB OBSERVATION REPORT NUMBER: 3185 SUBJECT: GRB 050406: Magellan optical observations DATE: 05/04/07 00:36:33 GMT FROM: Edo Berger at Carnegie Obs Edo Berger, Gus Oemler, and Mike Gladders (Carnegie Observatories) report: "On 2005, April 6.99 UT we imaged the position of GRB 050406 (GCN 3180) with LDSS-3 on the Magellan/Clay telescope in the r-band and i-band. Within the 5" radius XRT error circle (GCN 3184) we find a single faint source (r~22 mag) close to the detection threshold of both images. The position of this object is (J2000): RA = 02:17:52.3 DEC= -50:11:15 with an uncertainty of about 0.5" in each coordinate. At the present we cannot confirm whether the object has faded." //////////////////////////////////////////////////////////////////////// TITLE: GCN GRB OBSERVATION REPORT NUMBER: 3186 SUBJECT: Swift-UVOT Optical Counterpart Candidate to XRF 050406 DATE: 05/04/07 01:19:53 GMT FROM: Pete Roming at PSU E. Rol (University Leicester), P. Schady (MSSL/PSU), S. Hunsberger (PSU), A. Breeveld (MSSL), P. Boyd (GSFC/UMBC), C. Kouveliotou (MSFC), P. Roming (PSU), J. Nousek (PSU), N. Gehrels (GSFC), on behalf of the Swift UVOT team. Starting with data taken 88 s after the BAT trigger (Parsons et al; GCN 3180), we have co-added the early time data of GRB 050406 in each of the Swift Ultra-Violet/Optical Telescope (UVOT) U, B, and V bands. We detect a source within the XRT error circle (M. Capalbi et al; GCN 3184) at the 4.3 (19.0 magnitude), 3.0, and 2.5-sigma detection levels in the U, B, and V respectively. We do not detect a source in a second deeper U band observation taken 1.3 hours after the burst. The source is located at RA = 02:17:52.2, DEC = -50:11:15.8. The UVOT source location and fading is consistent with the Magellan candidate by Berger et al (GCN 3185) strongly suggesting that this is the optical counterpart of XRF 050406. More detailed analysis of the UVOT data is underway.