Gamma-ray Burst 070721B

(All information courtesy of the instrument teams.)

Previous IAU Circulars

• none

• GCN Circular #6640
  H. Ziaeepour (UCL-MSSL), S. D. Barthelmy (GSFC), A. P. Beardmore (U Leicester), D. N. Burrows (PSU), P. A. Evans (U Leicester), K. L. Page (U Leicester) and P. Schady (MSSL-UCL) report on behalf of the Swift Team:
  At 10:33:48 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 070721B (trigger=285654). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 33.131, -2.184 (J2000) with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). Like the previous trigger, there continues be a problem with the messages, so we are not able to say anything about the lightcurve at this time.
  The XRT found a bright, uncatalogued source at

  RA(J2000) = 33.13506
  DEC(J2000) = -2.19602
  

  which is 45.7 arcseconds from the BAT position. We do not have an error estimate, but the typical uncertainty for on-board centroids is 6 arcseconds. The source flux was about 1.296e-09 cgs. While we don't have the usual GCN notices for this trigger, the realtime telemetry from the spacecraft indicates that the source is fading.
  At the moment no UVOT data are available. We are waiting for the next Malindi pass to download more data.
  Burst Advocate for this burst is H. Ziaeepour (hz AT mssl.ucl.ac.uk). Please contact the BA by email if you require additional information regarding Swift followup of this burst. In extremely urgent cases, after trying the Burst Advocate, you can contact the Swift PI by phone (see Swift TOO web site for information: http://www.swift.psu.edu/too.html.)
• GCN Circular #6641
  P. Schady (MSSL-UCL) reports on behalf of the Swift team
  The Swift/UVOT observed the field of GRB 070721B and took a finding chart exposure of 100 seconds with the White (160-650 nm) filter starting ~50 seconds after the BAT trigger. A candidate afterglow was found on-board in the rapidly available 2.7'x2.7' sub-image, consistent with the XRT position (Ziaeepour et al., GCN 6640) at

     RA(J2000)  =  02:12:32.95 =  33.1373 degrees
     DEC(J2000) = -02:11:40.6  =  -2.1946 degreesa
  

  with a 1-sigma error radius of about 0.9 arc sec. This position is 9.7 arcsec. from the center of the BAT error circle. The estimated magnitude is 15.9 with a 1-sigma error of about 0.5 mag. No correction has been made for the estimated Galactic reddenning of E(B-V)=0.02 mag (Schlegel, Finkbeiner & Davic, 1998)
• GCN Circular (without number)
  

  TO:  GCN Customers
  DT:  21 Jul 07
  RE:  Outage of Swift-to-GCN Operations
  

  This morning (10:01:08 and 10:33:48 UT) there were two Swift-BAT triggers (285653 and 285654) which at first glance appear to be real GRBs. However, there was a problem in the connection between the front-end part of GCN (swift_tdrss_receiver) and the main GCN program (gcn_main) which prevented the Swift GCN Notices from being distributed to the regular customer list.
  This problem has been elimited. Normally, the parts of the GCN system that monitor this (and other) connections was not able to clear the problem and re-establish the connection. This loss of connection occurs at the 2 or 3 times a year level, and the automated reconnection monitor works within 4 minutes after the start of the loss of connection. However, this time it did not succeed. And to further compound the problem, the automated notification (to me) of this connection problem did not work. Hense the outage was allowed to continue without human intervention.
  Peliminary investegations show that the connection was in a partial state. The connection state was sufficient so as to not trip the connection monitor, but was not fully connected to allow the passage of data from the front-end program to the main program. This new scenario will be futher investigated and a solution will be found to prevent this new type of loss-of-service problem.
  This problem affected only the Swift-related Notices. All the other GCN Notice types were and continue to function normally. Nor did it affect the Circulars.
  I apologize for the loss of these two bursts.
  The Swift team is waiting for the downlink of the full data sets and will have the usual "refined analysis" circulars on these two triggers.
  Again, I apologize. GRBs are a rare commodity,oand to loose two of them is serious and intolerable loss to the scientific community.
  Sincerely, Scott Barthelmy
• GCN Circular #6643
  D. Palmer (LANL), L. Barbier (GSFC), S. D. Barthelmy (GSFC), J. Cummings (GSFC/UMBC), E. Fenimore (LANL), N. Gehrels (GSFC), H. Krimm (GSFC/USRA), C. Markwardt (GSFC/UMD), A. Parsons (GSFC), T. Sakamoto (GSFC/ORAU), G. Sato (GSFC/ISAS), M. Stamatikos (GSFC/ORAU), J. Tueller (GSFC), T. Ukwatta (GWU), H. Ziaeepour (UCL-MSSL) (i.e. the Swift-BAT team):
  Using the data set from T-119 to T+183 sec from the recent telemetry downlink, we report further analysis of BAT GRB 070721B (trigger #285654) (Ziaeepout, et al., GCN Circ. 6640). The BAT ground-calculated position is RA, Dec = 33.128, -2.198 deg which is

     RA(J2000)  =  2h 12m 30.8s 
     Dec(J2000) = -2d 11' 54" 
  

  with an uncertainty of 1.2 arcmin, (radius, sys+stat, 90% containment). The partial coding was 23%.
  The mask-weighted light curve shows a large main peak starting at ~T-20 sec, peaking at ~T+0 sec, and ending at ~T+70 sec. We caution that currently we only have downlinked data out to T+183 sec, and that there is a hint of activity past T+70 sec. But we wanted to get this circular out without waiting for further downlinks due to the uncertainites given the GCN Notices problems earlier. T90 (15-350 keV) is 31.5 +- TTT sec (estimated error including systematics).
  The time-averaged spectrum from T-6.7 to T+33.7 sec is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.31 +- 0.11. The fluence in the 15-150 keV band is 2.1 +- 0.1 x 10^-6 erg/cm2. The 1-sec peak photon flux measured from T-0.14 sec in the 15-150 keV band is 1.5 +- 0.3 ph/cm2/sec. All the quoted errors are at the 90% confidence level.
• GCN Circular #6646
  A.P. Beardmore, K.L. Page, P.A. Evans (U. Leicester) and H. Ziaeepour (MSSL-UCL) report on behalf of the Swift-XRT team :
  We have analysed the first three orbits of Swift-XRT data on GRB 070721B (Ziaeepour et al., GCN 6640). Using 5.78ks of Photon Counting mode data, we find a refined XRT position of RA, Dec = 33.13710, -2.19462 which is

  RA(J2000):   02:12:32.90
  Dec(J2000): -02:11:40.6
  

  with an uncertainty of 3.5 arcseconds (90% containment radius). This is 35 arcseconds from the refined BAT position reported by Palmer et al. (GCN 6643), 8.9 arcsec from the original XRT position reported in Ziaeepour et al. (GCN 6640), and 0.7 arcseconds from the UVOT position reported by Schady (GCN 6641).
  The X-ray light curve shows an initial steep decay from T+100s to T+144s, followed by a number of flares from T+255s to T+800s which reached a maximum count rate of 50 count/s at T+315s. The underlying decay is a powerlaw with a slope of approximately -0.9, though it is difficult to be precise with the data available so far.
  The X-ray spectrum from the Windowed Timing mode data obtained during the non-flare intervals from the 1st orbit (T+144s to T+220s and T+400s to T+475s) can be fit with an absorbed powerlaw to give a photon index of 1.48 +0.18-0.16 and a column density of 1.9 (+1.8-1.9) e20 cm^-2, consistent with the Galactic value of 2.3e20 cm^-2 in this direction (Kalberla et al. 2005). The observed 0.3-10 keV flux is 2.37+/-0.25 e-10 ergs cm^-2 s^-1, which corresponds to an unabsorbed flux of 2.45+/-0.25 e-10 ergs cm^-2 s^-1.
  If the underlying decay continues at the same rate, we predict an XRT count rate of 0.025 count/s at T+24hr, which corresponds to an observed 0.3-10keV flux of 1.4e-12 ergs cm^-2 s^-1.
  This circular is an official product of the Swift-XRT team.
• GCN Circular #6647
  A. Melandri, (Liverpool JMU), C. Guidorzi (Uni-Bicocca/INAF-OAB), I.A. Steele, C.G. Mundell, D. Carter, R.J. Smith, C.J. Mottram, D.F. Bersier, S. Kobayashi, M.J. Burgdorf, M.F. Bode (Liverpool JMU), A. Gomboc (Ljubljana), P. O'Brien, E. Rol, N. Bannister (Leicester) report
  We observed the field of GRB 070721B (trigger=285654, Ziaeepour et al., GCN 6640) with the Faulkes Telescope South on 2007 Jul 21 starting at 16:01:21 UT (~5.45 hours after the trigger).
  We do not detect any new source inside the refined XRT error circle (Beardmore et al., GCN 6646) and we do not detect the UVOT candidate reported by Schady et al. (GCN 6641) in our optical images.
  We can set an upper limit for this burst of R>19.3 and I>17.8 @ 6.4 and 6.2 hours after the trigger, respectively.
• GCN Circular #6649
  S. D. Barthelmy (GSFC), J. Cummings (GSFC/UMBC), C. Markwardt (GSFC/UMD), D. Palmer (LANL), T. Sakamoto (GSFC/ORAU), H. Ziaeepour (UCL-MSSL) (on behalf of the Swift-BAT team):
  Using the data set from T-239 to T+903 sec from recent telemetry downlinks, we report further analysis of BAT GRB 070721B (trigger #285654) (Ziaeepour, et al., GCN Circ. 6640; and Palmer, et al., GCN Circ. 6643).
  Beyond the previously reported description of the mask-weighted lightcurve (Palmer, 6643), the burst had a second episode of emission with multiple peaks from T+230 sec to T+380. We note that there are still gaps in the downlinked lightcurve data (1: T+530 to T+670, and 2: T+720 to T+830 sec) and as such, we can not say anything about burst emission in these two intervals. T90 (15-350 keV) is 340 +- 10 sec (estimated error including systematics).
  The time-averaged spectrum from T-6.7 to T+359.9 sec is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.34 +- 0.11. The fluence in the 15-150 keV band is 3.6 +- 0.2 x 10^-06 erg/cm2. The 1-sec peak photon flux measured from T-0.19 sec in the 15-150 keV band is 1.5 +- 0.3 ph/cm2/sec. All the quoted errors are at the 90% confidence level.
• GCN Circular #6650
  M. De Pasquale and H. Ziaeepour (MSSL/UCL) report on the behalf of the Swift/UVOT:
  The optical afterglow of GRB070721B (Ziaeepour et al. GCN 6640, Schady et al. 6641) is detected by Swift/UVOT in White filter and V filter finding chart exposures, taken 100s-200s and 207-607s after the BAT trigger. It is not detected at 3sigma level in other exposures, either single or summed ones in all filters, till ~7000s after the trigger.
  In the following table, we report the magnitudes of the afterglow for the exposures where the source is detected with at least 3sigma level, and 3sigma upper limits otherwise.
  Filter Time(s) Exp(s) Magnitude
  White 100-200 99 16.82 +/- 0.1
  White 701-711 10 >18.8
  White 855-955 99 >19.9
  White 5089-6716 393 >20.9
  V 207-453 399 16.7 +/- 0.1
  V 621-641 19 >17.5
  V 961-1116 154 >18.3
  V 5500-5699 199 >18.7
  B 687-850 19 >18.4
  B 4884-6519 393 >20.4
  U 662-835 39 >18.5
  U 4679-6314 393 >20.1
  UW1 638-658 39 >18.8
  UW1 638-6109 432 >20.2
  UM2 613-786 39 >18.9
  UM2 613-5904 432 >20.6
  UW2 780-800 19 >18.8
  UW2 780-6902 385 >20.3
  These values are not corrected for the Galactic reddening towards the bursts E(B-V)=0.02 mag (Schlegel, Finkbeiner & Davic, 1998)
• GCN Circular #6651
  D. Malesani (DARK), P. Jakobsson (Univ. Hertfordshire), J.P.U. Fynbo, J. Hjorth and P.M. Vreeswijk (DARK) report:
  We observed the optical counterpart of GRB 070721B (Ziaeepour et al., GCN 6640; Schady, GCN 6641) with the ESO-VLT equipped with FORS2.
  Close to the UVOT position (Schady, GCN 6641), we detect two objects:

  1: RA = 02:12:32.97, Dec = -02:11:41.0
  2: RA = 02:12:33.03, Dec = -02:11:41.6
  

  We note that object #1 is consistent with the UVOT position, while #2 is 1.6" away. We estimate R~23.8 and R~24.3 on Jul 22.319 UT (21.6 hr after the GRB) for objects #1 and #2, respectively.
  We acquired spectra for both objects. Thanks to the detection of a DLA and several metallic lines, we infer for object #2 a redshift z=3.626. The spectrum is typical of a GRB afterglow, and we thus suggest that object #2 is the optical counterpart of GRB 070721B. This redshift is also consistent with the nondetection of the object in filters bluer than V (De Pasquale & Ziaeepour, GCN 6650). Further observations are planned to test the variability of both objects.
  A finding chart and a plot of the 2D spectrum is available at:
  http://www.astro.ku.dk/~malesani/GRB/070721B
  We acknowledge excellent support from the ESO staff, in particular Angela Cortes, Dominique Naef and Yuri Beletsky. We thank Massimiliano De Pasquale for useful discussion.
• GCN Report 73.1
  GCN_Report 73.1 has been posted: http://gcn.gsfc.nasa.gov/reports/report_73_1.pdf by H. Ziaeepour at UCL/MSSL titled: "Swift observations of GRB 070721B"
• GCN Circular #6655
  J.P.U. Fynbo (DARK), P. Jakobsson (Univ. Hertfordshire), D. Malesani, J. Hjorth (DARK), I. Ilyn (AIP), report:
  We have obtained further observations of GRB 070721B (Ziaeepour et al., GCN Report 73). We also performed further analysis on our images of the optical afterglow of GRB 070721B (Malesani et al., GCN 6651).
  Our first observation was carried out with the Nordic Optical Telescope (mean time July 22.173 UT, i.e. 17.6 hr after the trigger). Close to the UVOT position, we detect a single source at the following coordinates (J2000, against USNO-B1):

  RA = 02:12:33.00
  Dec = -02:11:41.4
  

  This position is consistent with source #2 reported by Malesani et al. (GCN 6651).
  We have also obtained further imaging with the ESO-VLT on July 23.409 UT (47.3 hr after the trigger). We find that source #2 has faded compared to the two previous epochs. We hence confirm that object #2 is the optical afterglow of GRB 070721B. Object #1, reported by Malesani et al. (GCN 6651), is most likely a spurious source caused by a reflection from the bright star 0877-0042643 in the field (R~6.5).
  In our 2-dimensional spectrum we also note the presence of bright high-redshift galaxy with a redshift z=3.62, very close to the redshift of GRB 070721B (Malesani et al., GCN 6651). This object is 21" away of the afterglow, which corresponds to ~150 kpc at z=3.62.
  We acknowledge the ESO staff for excellent support.
• GCN Circular #6705
  Poonam Chandra (NRAO/UVA) and Dale A. Frail (NRAO) report on behalf of the Caltech-NRAO-Carnegie GRB Collaboration:
  "We used the Very Large Array to observe the field of view toward GRB 070721B (GCN 6640) at a frequency of 8.46 GHz on 2007 Aug 3rd at 10.42 UT. The radio afterglow of the GRB is undetected either at Swift-XRT position (GCN 6646) or at ESO-VLT position (GCN 6651). The map rms is 29 uJy.
  The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc."
• GCN Report 73.2
  GCN_Report 73.2 has been posted: http://gcn.gsfc.nasa.gov/reports/report_73_2.pdf by H. Ziaeepour at UCL/MSSL titled: "Final Swift observations of GRB 070721B"
• GCN Circular #6713
  D. Malesani (DARK) reports:
  I have looked again at the NOT and VLT images of GRB 070721B (Ziaeepour et al., GCN 6640), and realized there has been some confusion in what reported in our GCNs 6651 and 6655.
  The correct afterglow position is (J2000):

  RA = 02:12:32.97
  Dec = -02:11:40.4
  

  with an estimated error of 0.5". This position is within 0.4" of and consistent with the UVOT afterglow position as reported by Schady (GCN 6641). In the finding chart (Malesani et al., GCN 6651) posted at
  http://www.astro.ku.dk/~malesani/GRB/070721B/
  the afterglow corresponds to object #1, and not #2 as written in GCN 6655 (Fynbo et al.). Object #2 (RA = 02:12:33.03, Dec = -02:11:40.6) is a closeby, real object, likely unrelated to the GRB. The redshift z=3.626 reported in GCN 6651 correctly refers to the afterglow.
  I apologize for any confusion this may have created.