GRB 050315

A. Parsons, S. Barthelmy, L. Barbier (GSFC), J. Cummings (GSFC/NRC),
E. Fenimore (LANL), R. Fink, N. Gehrels (GSFC), S. Holland (GSFC/USRA),
D. Hullinger (GSFC/UMD), K. Hurley (UCB), H. Krimm (GSFC/USRA),
C. Markwardt (GSFC/UMD), D. Palmer (LANL), S. Piranomonte (ASDC),
T. Sakamoto (GSFC/NRC), G. Sato (ISAS), A. Smale (NASA HQ), M. Suzuki 
(Saitama),
J. Tueller (GSFC)
on behalf of the Swift-BAT team:

At 20:59:42 UT, the Swift Burst Alert Telescope (BAT) triggered and
located on-board GRB 050315.  The spacecraft automatically slewed
to the burst location, and UVOT began automated observations.  XRT was
not in a mode suitable for immediate position determination.

Using the time interval of the trigger of the burst, the 
ground-calculated
location is RA,Dec 306.476,-42.591 (J2000) with an uncertainty of 3 
arcmin
(radius, including a systematic uncertainty, 90% containment).
The burst was 40 degrees off the BAT boresight (60% coding).

The burst lightcurve has 2 overlapping FRED-like peaks.  The duration
is ~40 sec.  There is no further emmission out to T+180 sec, where the 
end
of the real-time TDRSS data ends.  The peak count rate is ~2500 
counts/sec.

S. Rosen, 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. Koch (PSU), M. Carter,
H. Huckle (MSSL), P. Broos (PSU), T.  Kennedy, P. Smith, B. Hancock
(MSSL), A. Parsons (GSFC), J. Nousek (PSU), N. Gehrels (GSFC), on
behalf of the Swift UVOT team.

     The Swift Ultra-Violet/Optical Telescope (UVOT) began observing
the field of GRB 050315 (GCN 3094
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 ) at 21:01:13 UT, approximately 90
seconds after the BAT trigger.  The first image was sent down via the
TDRSS link approximately 200 seconds after the trigger.  This image
was a 100 second exposure taken with the V filter.  It was centred on
the BAT position, and had a size of 5 x 5 arcminutes, which covered
88% of the BAT error circle.  The middle of the exposure was 140.19
seconds after the BAT trigger.  The image was binned 8 x 8 to reduce
the bandwidth required for transmission.  A comparison against the DSS
showed no new sources down to a 5-sigma limiting magnitude of
approximately V = 18.5.  We stress that this limiting magnitude is
based on ground calibrations and may be uncertain by approximately 0.5
mag.  The full-resolution version of this image will be transmitted to
the ground during the next Malindi pass, which will occur at
approximately 0000 UTC March 16, 2005.

D. C. Morris, D. N. Burrows, J. E. Hill, J. A. Kennea, J. A. Nousek (PSU), 
C. Pagani,  G. Chincarini  (INAF-OAB), A. F. Abbey, P. T. O'Brien (U. 
Leicester), G. Cusumano, T. Mineo (INAF-IASF/Palermo), M. Capalbi (ASDC), 
A. Parsons, S. Holland, N. White, N. Gehrels (GSFC), K. Mason (MSSL), and 
K. Hurley (UCB) report on behalf of the Swift XRT team:

The Swift BAT instrument detected GRB 050315 at 20:59:42 UT on 15 March 
2005 (Parsons et al 2005, GCN 3094
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 ).  The observatory executed an automated 
slew to the BAT position and the XRT began taking data at 21:01:05.5 
UT.  The XRT was in Manual state and made observations in Photon Counting 
mode.  Data were dumped to the ground at about 00:00 UT on 16 March 2005 
and a preliminary analysis was performed by the XRT team.  We find a bright 
uncataloged, rapidly fading X-ray source located at:

RA(J2000) = 20:25:54,
Dec(J2000) = -42:36:0.2

We estimate an uncertainty of about 6 arcseconds.   This source is 32 
arcseconds from the BAT position reported in GCN 3094
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 .

Mark H Wieringa (Australia Telescope National Facility), Brian P Schmidt 
(Australian National University)
and Alicia Soderberg (CalTech) report on behalf of a larger collaboration:

"We observed a region covering the Swift burst GRB 050315 (GCN#3094
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 )
using the Australia Telescope Compact Array for 1 hour centered on
March 16.02 UT. At frequencies of 4.8 and 8.6 GHz no emission was
detected within the 6 arcsec diameter error circle of the XRT position
(GCN#3097
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 ). The 3-sigma detection limits are 0.6 and 0.8 mJy at 4.8 and
8.6 GHz respectively. No detections above these limits are present
in a larger area (1 arcmin diameter) around the XRT position."

 We thank the ATNF Director for making this rapid response possible.

T. Sakamoto (GSFC/NRC), S. Barthelmy, L. Barbier (GSFC), 
J. Cummings (GSFC/NRC), E. Fenimore (LANL), R. Fink, 
N. Gehrels (GSFC), S. Holland (GSFC/USRA), 
D. Hullinger (GSFC/UMD), K. Hurley (UCB), H. Krimm (GSFC/USRA), 
C. Markwardt (GSFC/UMD), D. Palmer (LANL), A. Parsons (GSFC), 
S. Piranomonte (ASDC), G. Sato (ISAS), A. Smale (NASA HQ), 
M. Suzuki (Saitama), J. Tueller (GSFC) 
on behalf of the Swift-BAT team: 

Analysis of the available data for the Swift-BAT GRB050315 
(Parsons et al., GCN Circ 3094
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 ) yields a fluence of > 2.8x10^-6 
erg/cm2 in the 15-350 keV band and a peak flux of 2.5 ph/cm2/sec 
(also 15-350 keV).  The photon index of the 1-s peak spectrum (T0+25 sec.) 
is 2.5 +- 0.3 (90% confidence).  Although there is a data gap, 
the time averaged spectrum also shows a soft spectrum.  
This appears to be a soft X-ray-rich GRB.  

We are planning a further circular when more complete data 
are avaliable.

Dan Kelson and Edo Berger (Carnegie Observatories) report:

"On 2005, March 16.347 UT we imaged the XRT error circle of GRB 050315
(GCN 3097
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 ) with the newly-comissioned LDSS3 instrument on Magellan/Clay in
the r-band filter.  Within the 6" radius error circle we find a single
bright object which is not present in the DSS at coordinates (J2000):

        RA  =  20:25:54.1
        DEC = -42:36:02.2

with an uncertainty of about 0.5" in each coordinate.  We consider this
object to be the optical afterglow of GRB 050315."

Dan Kelson and Edo Berger (Carnegie Observatories) report:

"Using LDSS3 on Magellan we also obtained a 20-minute spectrum of the
afterglow candidate (GCN 3100
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 ).  We find several absorption features which
we identify as AlIII (1854.7 and 1862.8) and SiII (1808.0) at a redshift
of z=1.949.  We conclude that this is the redshift of GRB 050315.

Given a fluence of >2.8e-6 erg/cm^2 (GCN 3099
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 ), the isotropic-equivalent
gamma-ray energy is > 2.6e52 erg."

A. M. Soderberg (Caltech) and D. A. Frail (NRAO) report
on behalf of the Caltech-NRAO-Carnegie GRB collaboration:

"Using the Very Large Array at 8.5 GHz, we observed the
field of GRB050315 (GCN 3094
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 ) on 2005 Mar 16.68 UT (t=0.80 d).
Within the XRT localization (GCN 3097
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 ), we detect a radio
source at the following coordinates:

          RA: 20:25:54.17   Dec: -42:36:01.0 (J2000)

with a positional uncertainty of ~0.5 arcsec.  Noting that
this source is roughly coincident with the optical afterglow position
reported by Kelson and Berger (GCN 3100
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 ), we interpret this as the radio
afterglow of GRB050315.

Further observations are planned."

D. Bersier, J. Rhoads, A. Fruchter (STScI), D. Gilbank (Toronto)
report on behalf of the NOAO GRB collaboration:

We have observed the field of GRB 050315 (GCN 3094
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 ) with the MOSAIC II
wide-field camera on the CTIO 4m Blanco telescope, starting 0.47d after
the burst.

The afterglow seen in the optical and radio (GCNs 3100
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 , 3101
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  and 3102
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 )
is well detected in our I-band data, taken at a mean time of 0.484d
after the burst.  A preliminary reduction, using an average zero point
for this camera, yields mI=20.7.

B. E. Cobb and C. D. Bailyn (Yale), part of the larger SMARTS
consortium, report:

Using the ANDICAM instrument on the 1.3m telescope at CTIO, we
obtained optical/IR imaging of the error region of GRB 050315
(GCN 3094
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 , Parsons et al.) beginning ~11.6 hours post-burst
(2005-03-16 08:35:09 UT).  Total summed exposure times amounted
to 180s in BRIYJK and 120s in H and V.

An optical source is detected in R and I at the coordinates of the 
reported optical afterglow candidate (GCN 3100
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 , Kelson & Berger).  This 
source is, however, only slightly brighter than our detection limits.  
Preliminary comparison with Landolt Standard PG 1047+003 and USNO B1.0 
stars yields the following approximate afterglow magnitudes:

         R = 20.9
         I = 20.3

No source was detected in B or V with upper limits of B > 22.3 and V > 21.4.

Additionally, the afterglow candidate is not detected in the IR images to 
the following detection limits (obtained by comparison with 2MASS):

         J > 18.0
         H > 17.4
         K > 16.3

H. Krimm (GSFC/USRA), S. Barthelmy, L. Barbier (GSFC), J. Cummings (GSFC/NRC),
E. Fenimore (LANL), R. Fink, N. Gehrels (GSFC), S.T. Holland (GSFC/USRA),
D. Hullinger (GSFC/UMD), K. Hurley (UCB), H. Krimm (GSFC/USRA),
C. Markwardt (GSFC/UMD), D. Palmer (LANL), A. Parsons (GSFC),
S. Piranomonte (ASDC), T. Sakamoto (GSFC/NRC), G. Sato (ISAS),
A. Smale (NASA HQ), M. Suzuki (Saitama), J. Tueller (GSFC)
on behalf of the Swift-BAT team:

Continued analysis of the full data set (now with telemetry gaps 
filled in) for the Swift-BAT GRB050315 (Parsons et al., GCN Circ 
3094
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 , Sakamoto et al., GCN Circ 3099
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 ) yields a fluence of 4.2 x 10^-6 
erg/cm2 in the 15-350 keV band and a peak flux of 2.4 ph/cm2/sec 
(also 15-350 keV). The photon index of the 1-s peak spectrum (T0+24.6 
sec.) is 2.4 +- 0.3 (90% confidence).  The time averaged spectrum 
also shows a soft spectrum with a power law index of 2.3 +- 0.1 (90% 
confidence).   The Swift mission is still in its early orbit phase 
and the BAT spectral response continues to be refined.  Further 
spectral analysis is in progress.  In particular, since there is an 
uncertainty in the energy response at the lowest energies at the 
current time, it is not possible to conclude that this event is an 
X-ray flash.

The burst shows considerable structure in its light curve.  There is 
a precursor starting ~60 seconds before the trigger and continuing up 
until the main peak emission begins. The first of the two main peaks 
has a slow rise of about ten seconds and a gradual decline 
interrupted at TO ~22 seconds by a second peak with a much steeper 
rise.  The burst shows signs of spectral evolution (hard to soft) 
from the precursor through the two main peaks. There is no 
significant emission after the decay of the second peak.  The overall 
measures of burst duration give T90 = 96 +- 10 seconds and T50 = 25 
+- 5 seconds.  The errors in duration are estimated systematic errors.

B. E. Cobb and C. D. Bailyn (Yale), part of the larger SMARTS
consortium, report:

Using the ANDICAM instrument on the 1.3m telescope at CTIO, we
obtained additional optical/IR imaging of the reported optical afterglow
candidate (GCN 3100
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 , Kelson & Berger) of GRB 050315
(GCN 3094
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 , Parsons et al.) beginning ~35.6 hours post-burst.
Total summed exposure times amounted to 10 minutes R and I and
12.5 minutes in J.

The source identified as the afterglow candidate has dimmed
since our previous observations (GCN 3104
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 , Cobb & Bailyn).
Preliminary comparison with Landolt Standards PG 1047+003 and RU 149
yields the following approximate afterglow magnitudes:

        R = 21.6
        I = 21.5

The counterpart was again not detected in the IR, with a
magnitude limit of J > 18.6.

Our R and I observations are summarized below:

        time post-burst:        R magnitude:    I magnitude:
        11.6 hours              20.9            20.3
        35.6 hours              21.6            21.5

We note that the I-band observations are close to the detection
limit, and have measurement errors of >0.2 magnitudes, so the
evidence for a color change should be treated with caution.

The R-band magnitudes have errors of ~0.2, so assuming
a power-law afterglow decay (afterglow flux proportional to
t^alpha), then this corresponds to an alpha of -0.57 +\- 0.25.

GRB 050315: Variability Analysis of Swift BAT Time History

T. Q. Donaghy, T. Sakamoto, D. Q. Lamb, E. E. Fenimore, D. E. Reichart, 
on behalf of the Swift BAT Team report:

We have calculated the variability measure V (Fenimore and Ramirez-Ruiz
2000; Reichart et al. 2001) for GRB 050315 (Parsons et al., GCN Circular
3094
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 ), using the Swift BAT time history data for this burst.  We find
values V = 0.017 +/- 0.027, 0.068 +/- 0.022, 0.086 +/- 0.041, and 0.064
+/- 0.016 (68 percent confidence regions) in the 15-25 keV, 25-50 keV,
50-100 keV, and 25-100 keV energy bands, respectively.  These values of
V are the same within the statistical uncertainties; this is consistent
with the results of Reichart et al. (2001), who found no significant
trend of V with energy for BATSE bursts.

Analyzing the Swift BAT spectral data for GRB 050315, we find a peak 
photon energy flux in 1 s of (9.2 +/- 0.6) x 10-8 erg cm-2 s-1 (68%
confidence region) in the 33.9 - 339.1 keV energy band [which
corresponds to the 100-1000 keV energy band in the rest frame of the
source for a redshift of z = 1.949 (Kelson and Berger, GCN Circular
3101
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 )].  This peak photon energy flux corresponds to an isotropic-
equivalent peak luminosity in the 100-1000 keV energy band in the rest
frame of the source of L_obs = (3.2 +/- 0.2) x 10^51 erg s-1.

Using V in the 25-100 keV energy band as a luminosity (and therefore a
redshift) estimator, we estimate an isotropic-equivalent luminosity in
100-1000 keV in the rest frame of the source of log L_V (erg s-1) =
51.46 +/- 0.747 and a redshift z_V = 1.94 (+2.02/-0.97) for the burst.
These values are consistent with the observed values log L_obs (erg s-1) 
= 51.51 (+0.02/-0.04) (90% confidence region) and z = 1.949 (Kelson and 
Berger, GCN Circular 3101
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 ).

These results are encouraging.  They suggest that analyses of the Swift
BAT time history data for a large sample of bursts will provide a
decisive test of the validity of the variability measure V as a
luminosity (and therefore a redshift) estimator, and if the analyses
confirm its validity, they will make possible a better calibration of
the estimator.

A. P. Beardmore, K. L. Page (U. Leicester), V. Mangano (INAF-IASF/Palermo), 
P. Romano (INAF-OAB), M. R. Goad, O. Godet, J. P. Osborne (U. Leicester), 
G. Chincarini, S. Campana (INAF-OAB), D. N. Burrows, J. L. Racusin 
(PSU),  G. Cusumano, T. Mineo (INAF-IASF/Palermo), M. Perri, F. Tamburelli 
(ASDC), L. Angelini, F. Marshall, N. Gehrels (GSFC) report on behalf of the 
Swift XRT team:

We report refined XRT source positions for GRB 050315 (GCN3094
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 , GCN3097
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 ), 
GRB 050318 (GCN3111
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 , GCN3113
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 ), and GRB 050319 (GCN3117
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 ), based on improved 
ground-based analysis of extended data sets from these three bursts.  We 
find the following afterglow positions:

GRB 050315: RA(J2000) = 20:25:53.9, Dec(J2000) = -42:36:01.4 (P. Romano), 
which is 2.3" from the optical counterpart (GCN3100
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 ).

GRB 050318: RA(J2000) = 03:18:51.1, Dec(J2000) = -46:23:44.7 (V. Mangano), 
which is 1.1" from the optical counterpart (GCN3123
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 ).

GRB 050319: RA(J2000) = 10:16:48.1, Dec(J2000) = +43:32:52.3 (K. Page), 
which is 3.1" from the optical counterpart (GCN3116
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 ).

J. Gorosabel (IAA-CSIC, Granada), P. Laursen, B.L. Jensen, J.P.U.
 Fynbo, J. Hjorth, D. Watson, H. Pedersen, P. Jakobsson, J.M. Castro
 Cerón (NBI, Copenhagen), M.I. Andersen (AIP, Potsdam), report:

  "We have obtained deep R-band imaging of the field of the GRB
   050315 optical afterglow (GCN Circ. 3100
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 ) with the 1.54m Danish
   telescope on La Silla. The observations were carried out during
   five nights (8, 9, 11, 12 & 14 April 2005) with a total exposure
   time of ~7 hours. In the stacked image, which has a 3 sigma
   limiting magnitude of 24.2, an object is detected within an arcsec
   of the afterglow position. This object, which we tentatively
   identify as the GRB 050315 host galaxy, shows an elongated shape
   in the E-W direction.

   The magnitude of the potential host galaxy is R=23.9+/-0.2. The
   photometric calibration is based on observations of Landolt fields.
   A finding chart can be found at:

   http://www.dsri.dk/~jgu/grb050315/FCs/GRB050315.R.Danish.medianw.gif

   Further sub-arsecond deep observations are necessary to clarify
   the nature of this object."

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