GRB 050916

GRB050916: Swift/UVOT Upper Limits

P. J. Brown (PSU), D. Morris (PSU),  H. Huckle (UCL-MSSL),
B. Hancock (UCL-MSSL), and N. Gehrels (GSFC)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began observing the field of GRB 050916
(Swift Trigger #155408) on 2005/09/16 at 16:39:20 UT,
207 seconds after the BAT trigger (Morris et al. GCN 3991
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 ).
No source is detected within the XRT error circle
(Morris et al. GCN 3996
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 ) in the initial 100 second V-band image
down to a 5-sigma upper limit of 17.9, or in summed exposures
in any of the UVOT filters down to the following 5-sigma upper limits:

Filter  T_range (s)     Exp (s)  5sigma UL

V       207 - 74,597     4455     19.9
B       353 - 65,371     4374     20.7
U       339 - 63,016     4207     20.4
UVW1    325 - 58,536     4152     20.1
UVM2    311 - 53,803     4223     20.0
UVW2    368 - 69,038     3706     20.2

The magnitudes are uncorrected for extinction,
which is extremely high as this direction lies in the galactic plane.

E. Fenimore (LANL), L. Barbier (GSFC), S. Barthelmy (GSFC),
J. Cummings (GSFC/NRC), N. Gehrels (GSFC), D. Hullinger (UMD),
H. Krimm (GSFC/USRA), C. Markwardt (GSFC/UMD), J. Nousek (PSU),
D. Palmer (LANL), A. Parsons (GSFC), T. Sakamoto (GSFC/NRC),
G. Sato (ISAS), M. Tripicco (GSFC-SSAI), J. Tueller (GSFC),
on behalf of the Swift-BAT team:

Using the complete data set from T-300 to T+300 sec, we report further
analysis of Swift-BAT GRB 050916 (trigger #155408)  (Morris, et al.,
GCN 3991
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 ).  The refined BAT ground position is (RA,Dec) = 136.005, -51.411
{09h 04m 01.1s,-51d 24' 39"} [deg; J2000] +- 1.8 arcmin, (radius, sys+stat,
90% containment).  This is 1.4 arcmin from the XRT refined position reported
by Morris et al. (GCN 3996
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 ), The partial coding was 41%. 

The light curve shows a broad peak from T+0 to T+66 and a second weaker
peak from T+80 to T+100.  T90 (15-150 keV) is (90 +- 10) sec (estimated
error including systematics).

The power law index of the time-averaged spectrum is 1.83 +- 0.32.  The
fluence in the 15-150 keV band is (1.1 +- 0.4) x 10^-6 erg/cm^2.  The
1-sec peak photon flux measured from T+44 sec in the 15-150 keV band is
(0.66 +- 0.23) ph/cm2/sec.  All the quoted errors are at the 90%
confidence level.

D. C. Morris (PSU), D. N. Burrows (PSU), and N. Gehrels (GSFC) report on 
behalf of the Swift XRT team:

We have analysed the first six orbits of data for GRB050916 (GCN 3991
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 , 
Morris et al., 2005). Using xrtcentoid, the refined position is:

RA(J2000) = 09h 03m 56.8s
Dec(J2000) = -51d 25' 50.3"

with an uncertainty of 8 arcsec. This is 3.5 arcsec from the original 
XRT position (GCN 3995
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 , Morris et al., 2005) and 23 arcsecs from the 
initial BAT position..

The XRT began taking data at 16:39:22, 210 seconds after the BAT 
trigger. The early data (<T+1 ks) show a fading lightcurve with decay 
index ~-1.5. Data from T+5ks to T+15ks also show a fading lightcurve 
consistent with decay index=-1.5 but at a flux level ~x3 greater than 
would be expected by extrapolating the decay index from the earlier 
data. A bright flare (peaking at x100 above the expected level) is seen 
at T+18ks and flux levels remain elevated for 5-7ks thereafter.

The overall spectrum of all the data is well fit (reduced chi-sq of 
1.04) by a power law with neutral hydrogen equal to the galactic column 
density of 1.1e22 :

NH=1.1e22(frozen)
gamma=1.77 + / - 0.09

Due to the low overall number of events, separate fits to the flaring 
and non-flaring portions of the lightcurve are poorly contrained and do 
not show any significant evidence of spectral evolution:

flaring:
NH=0.89e22 + / - 0.16e22
gamma= 1.54 + / - 0.19

non-flaring:
NH=1.12e22 + / - 0.34e22
gamma=1.85 + / - 0.37

Assuming that the decay slope has returned to ~-1.5 after the flare, the 
expected count rate at T+1e5s is ~2e-3 cts/s which converts to an 
unabsorbed flux of  2.4e-13 ergs cm^-2s^-1. The peak flux (seen at the 
start of the XRT observation and at the peak of the flare) is 0.5 cts/s, 
for a peak unabsorbed flux of 6e-11 ergs cm^-2s^-1.

D. Morris (PSU), D.N. Burrows (PSU) report on behalf of the Swift/XRT team:

The Swift XRT began observing the long weak burst GRB 050916 (trigger 
#155408, Morris et al., GCN 3991
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 ) at 16:35:52 UT, but was unable to 
obtain an on-board centroid. Analysis of the initial ground-processed 
data finds a faint, fading, and uncataloged source at coordinates:

RA(J2000) = 09 03 56.8
Dec(J2000) = -51 25 46.8

We estimate an uncertainty of 8 arcseconds (90% containment). This 
position lies 20 arcseconds from the BAT position reported in GCN 3991
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 .

D. Morris (PSU), D. Burrows (PSU), J. Cummings (GSFC/NRC), N. Gehrels (GSFC),
C. Gronwall (PSU), S. T. Holland (GSFC/UMBC), J. Kennea (PSU),
K. Page (U. Leicester), D. Palmer (LANL)
on behalf of the Swift team:

At 16:35:52 UT, Swift-BAT triggered and located GRB050916 (trigger=155408).
The spacecraft slewed immediately after processing the 120-second long
image.  The BAT on-board calculated location is RA,Dec 135.992d, -51.425d 
{09h 03m 58s, -51d 25' 31"} (J2000), with an uncertainty of 3 arcmin (radius,
90% containment, stat+sys).  The BAT raw light curve showed one very broad
peak from T+0 to T+180 sec, with a spike at about T+40 to T+45 sec.  The peak
count rate was ~500 counts/sec (15-350 keV), at T+40 sec.  However, Swift was
in the sunlit part of its orbit so the current heightened solar activity can
produce variation in the BAT raw detector count rates, and a trustworthy
light curve will require analysis of the full data set after the next ground
contact.

XRT began observing at 16:39:22 UT, 210 seconds after the BAT trigger.  The
count rate was too low for an on-board centroid. The spectrum is consistent
with a weak source. The XRT position will be available following the next 
ground station contact.

The UVOT began observing at 16:39:20 UT, 208 seconds after the BAT trigger.
No UVOT image was sent through TDRSS, however, the processed source list
shows no new source.  The BAT source is at low galactic latitude and has
high extinction.

We are currently in the portion of the orbits where the spacecraft does
not pass over the Malindi downlink station. Therefore, it will be ~6
hours before we have access to the full data set for the refined analyses.