GRB 050318

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

At 15:44:37 UT, the Swift Burst Alert Telescope (BAT) triggered and
located on-board GRB 050318.  The spacecraft did not automatically slew
to the burst location because of an observing constraint.  The orbit is such
that we just started the approximate 6-hour gap in telemetry downlink orbits,
so it will be ~7 hours before we get the full data set on this burst
and can say anything more than what is available from the real-time
TDRSS messages.

Using the time interval of the trigger of the burst (0.5 sec),
the ground-calculated location is RA,Dec 49.651,-46.392 (J2000)
with an uncertainty of 3 arcmin (radius, including a systematic uncertainty,
90% containment).  The burst was 22 degrees off the BAT boresight (75% coding).

The burst lightcurve has 2 peaks.  The first peak is ~3 sec long with a peak
count rate of 4000 cnts/sec.  The second peak starts at T+25 and lasts ~15 sec
with a peak rate of 4500 cnts/sec (15-350 keV) .

C. B. Markwardt (GSFC/UMD), P. Boyd (GSFC/UMBC), N. Gehrels (GSFC)
K. Hurley (UCB), F. E. Marshall (GSFC), M. Still (GSFC/USRA)

RXTE observed GRB 050318 starting at 17:47 UTC (123 minutes after the
BAT trigger), at the BAT reported position (Krimm et al, GCN Circ
#3111
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 ).  The exposure was 768 seconds.  The RXTE PCA is a non-imaging
instrument with a field of view of approximately 1 degree (FWHM).

A PCA flux of 7E-12 erg/s/cm^2 was detected (2-10 keV), which is about
four times the expected fluctuations due to the cosmic X-ray
background.  The flux is primarily below 6 keV.  No obvious temporal
variations are detected.

J. A. Nousek, D. C. Morris, D. N. Burrows, D. Grupe, M. Chester, P. 
Meszaros (PSU),
V. La Parola, V. Mangano (INAF-IASF/Palermo), S. Campana, G. 
Tagliaferri  (INAF-OAB), M. Capalbi (ASDC),  O. Godet, M. R. Goad (U. 
Leicester), L. Cominsky (Sonoma State U.), J. Greiner (MPE), H. Krimm, M. 
Still, and N. Gehrels (GSFC),  report on behalf of the Swift XRT team:

The Swift BAT instrument detected GRB 050318 at 15:44:37 UT on 18 March
2005 (Krimm et al 2005, GCN 3111
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 ).  The BAT position was within the Swift 
Earth horizon constraint.  The observatory executed an automated slew to 
the BAT position and the XRT began taking data in Auto State  at 16:39:14 
UT.  Swift did not have contact with the ground station until about 22:29 
UT.  On receiving the data at Penn State, a preliminary analysis was 
performed by the XRT team.  We find a bright uncataloged, rapidly fading 
X-ray source located at:

RA(J2000) = 03:18:50.8,
Dec(J2000) = -46:23:45.1.

We estimate an uncertainty of about 6 arcseconds.   This source is 87
arcseconds from the BAT position reported in GCN 3111
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 .

John Mulchaey and Edo Berger (Carnegie Observatories) report:

"On 2005, March 18.995 UT we imaged the BAT error circle of GRB 050318
(GCN 3111
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 ) with IMACS on the Magellan/Baade telescope in the R-band.  
Within the 6" radius XRT error circle (GCN 3113
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 ) we find a single bright
object which is not present in the DSS at coordinates (J2000):
        RA  =  03:18:51.0
        DEC = -46:23:44
with an uncertainty of about 0.5" in each coordinate.  We consider this
object to be the optical afterglow of GRB 050318."

[GCN OPS NOTE (19Mar05): Changed the Subject-line from "030518" to "050318".]

K. McGowan, M. De Pasquale (MSSL), P. Boyd (GSFC/UMBC),
S. T.  Holland, M. Still (GSFC/USRA), 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, S. Koch (PSU), M. Carter,
H. Huckle (MSSL), P. Broos (PSU), T.  Kennedy, P. Smith, B. Hancock
(MSSL), H. Krimm (GSFC/USRA), 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 050318 (Krimm
et al; GCN 3111
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 ) at 16:39:16 UT, 3,279s after the BAT
trigger.  The delay in acquiring the target was caused
by an earth limb constraint. The first data taken after
spacecraft settling was a 100s V image, sent directly to
the ground through the TDRSS system, with pixel binning
of 8x8 to reduce telemetry. Binned pixel sizes are 4x4
arcsec and the field of view is 5.3x5.3 arcmin.
Mid-exposure time was 3,329s after the trigger. A
comparison against the Digitized Sky Survey reveals a
new source inside the BAT and XRT (Nousek et al; GCN
3113
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 ) error circles at:

RA = 03 18 51.2(1) Dec = -46 23 43(2) J2000

and consistent with the R-band detection of Mulchaey &
Berger (GCN 3114
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 ) The V band magnitude is V = 18.4(3).
The magnitude is based on ground data and will require
confirmation by flight calibration.

The full-resolution version of this image will be
transmitted to the ground during the next Malindi pass
providing an improved position.

Edo Berger and John Mulchaey (Carnegie Observatories) report:

"Using IMACS on the Magellan/Baade telescope we also obtained spectra of
the afterglow of GRB 050318 (GCN 3114
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 ) on 2005, March 19 UT.  We find
several absorption features corresponding to two redshift systems at
z=1.20 and z=1.44.  We conclude that the redshift of GRB 050318 is
z=1.44."

M. De Pasquale (MSSL), P. Boyd (GSFC/UMBC), S. T.  Holland, M. Still 
(GSFC/USRA), 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 
(PSU), S. Rosen, K. McGowan (MSSL), S. Koch (PSU), M. Carter, H. Huckle 
(MSSL), P. Broos (PSU), T.  Kennedy, P. Smith, B. Hancock (MSSL), H. 
Krimm (GSFC/USRA), J. Nousek (PSU), N. Gehrels (GSFC), on behalf of the 
Swift UVOT team.

Analysis of a more complete data set from the Swift-UVOT
of GRB 050318 (Krimm, et al., GCN 3111
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 ) yields an
improved position from stacked V image exposures of

RA = 03h 18m 51.15(2)s, Dec = -46d 23m 43.7(3)s (J2000).

Images in various filters, stacked over the first 34
ksec after the trigger indicate time-averaged
instrumental magnitudes of UVW2 > 22.0, UVM2 > 22.1,
UVW1 > 21.5, U = 20.5(2), B = 20.0(1) and V = 19.7(1).  
This indicates either a highly reddened source, or an
object of redshift z ~ 2.7 in front of the burst.

Magnitudes are based on ground calibration and will
require confirmation in flight.

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

Continued analysis of the available data for the Swift-BAT GRB 050318
(Krimm, et al., GCN 3111
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 ) yields the following additional results.
Due to an error in an on-board data processing script, event data
from times after T+31.4 seconds were not recorded.  Based on analysis
of the rate data, we estimate that the lost event data represent
about 18 per cent of the total burst flux.  The fluence derived from the
available event data and scaled to cover the entire burst is
2.1 X 10^-6 erg/cm^2 in the 15-350 keV band.  The 1-s peak flux
is 3.8 ph/cm^2/s (also 15-350 keV).  The photon index of the 1-s
peak spectrum (T+28.5 s) is 1.9 +/- 0.12 (90% confidence).
The time averaged spectrum yields a photon index of 2.1 +/- 0.11
(90% confidence).  The overall measure of burst duration
is T90 = 32 +/- 2 s (including systematics).