GRB 070318

J. R. Cummings (NASA/UMBC), S. D. Barthelmy (GSFC),
D. N. Burrows (PSU), G. Cusumano (INAF-IASFPA),
N. Gehrels (NASA/GSFC), C. Gronwall (PSU),
V. La Parola (INAF-IASFPA), T. Mineo (INAF-IASFPA) and
D. M. Palmer (LANL) report on behalf of the Swift Team:

At 07:28:56 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 070318 (trigger=271019).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 48.540, -42.962 which is 
   RA(J2000)  =  03h 14m 10s
   Dec(J2000) = -42d 57' 42"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows a single FRED-like peak
with a duration of about 70 sec.  The peak count rate
was ~4000 counts/sec (15-350 keV), at ~2 sec after the trigger. 

The XRT began observing the field at 07:29:59 UT, 64 seconds after the
BAT trigger. XRT found a bright, fading, uncatalogued X-ray source
located at RA, Dec 48.4856, -42.9468 which is
   RA(J2000)  =  03h 13m 56.5s
   Dec(J2000) = -42d 56' 48.4"
with an uncertainty of 4.7 arcseconds (radius, 90% containment). 
This location is 153 arcseconds from the BAT on-board position,
within the BAT error circle. The initial flux in the 2.5s image
was 2.6e-09 erg/cm2/s (0.2-10 keV). 

UVOT took a finding chart exposure of 400 seconds with the V filter starting
180 seconds after the BAT trigger. There is a candidate afterglow in the
rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	03:13:56.83 =  48.4868
  DEC(J2000) = -42:56:46.3  = -42.9462
with a 1-sigma error radius of about 0.5 arc sec. This position is 3.8 arc sec. 
from the center of the XRT error circle. The estimated magnitude is 15.4 with a
1-sigma error of about 0.5 mag. No correction has been made for the expected
extinction of about 0.1 magnitudes.

J. Cummings (GSFC/UMBC), L. Barbier (GSFC), S. D. Barthelmy (GSFC),
E. Fenimore (LANL), N. Gehrels (GSFC), H. Krimm (GSFC/USRA),
C. Markwardt (GSFC/UMD), D. Palmer (LANL), A. Parsons (GSFC),
T. Sakamoto (GSFC/ORAU), G. Sato (GSFC/ISAS), M. Stamatikos (GSFC/ORAU),
J. Tueller (GSFC)
on behalf of the Swift-BAT team:
 
Using the data set from T-120 to T+302 sec from recent telemetry downlinks,
we report further analysis of BAT GRB 070318 (trigger #271019)
(Cummings, et al., GCN Circ. 6210
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 ).  The BAT ground-calculated position is
RA, Dec = 48.486, -42.950 deg  which is 
   RA(J2000)  =  03h 13m 56.6s 
   Dec(J2000) = -42d 57' 1.1" 
with an uncertainty of 0.8 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 80%.
 
The mask-weighted lightcurve is a single FRED peak starting at ~T-10sec,
peaking at ~T+2 sec, and returning to background at around T+120 sec.
There is the possibility of weak peaks at ~T+140 and T+170 sec at the 
~3-sigma level.  T90 (15-350 keV) is 63 +- 3 sec (estimated error
including systematics).
 
The time-averaged spectrum from T-1.0 to T+89.4 is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.44 +- 0.08.  The fluence in the 15-150 keV band is 2.3 +- 0.1 x 10^-6 erg/cm2.
The 1-sec peak photon flux measured from T+1.94 sec in the 15-150 keV band
is 1.6 +- 0.2 ph/cm2/sec.  All the quoted errors are at the 90% confidence
level.

F.E. Marshall (NASA/GSFC) and J.R. Cummings (NASA/UMBC) report
on behalf of the Swift team:

The bright optical afterglow of GRB 070318 (Cummings et al., GCN Circ. 6210
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 )
is following an approximately power-law decay. At 13,000 sec. after the
trigger, the magnitude in the UVOT B filter is ~19.9 with a best-fit
decay index of 1.16. If this rate of decay continues, the B magnitude
will be 22.3 at one day after the trigger and 23.2 a day later.
 
The afterglow has been detected in all 7 UVOT filters indicating that
the redshift is less than ~2.

Observations with ground-based telescopes are encouraged.

V. La Parola, T. Mineo, B. Sbarufatti, V. Mangano, G. Cusumano
(INAF-IASF Pa)
report on behalf of the Swift XRT team:

We have analysed the first two orbits of Swift-XRT data on GRB 070318
(Cummings, et al., GCN Circ. 6210
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 ), with a total exposure of 427 s
seconds
in Window Timing (WT) mode and 33.4 ks seconds in Photon Counting (PC)
mode. This provides a refined XRT position at RA,Dec=48.4870,-42.9454
which is:

RA (J2000) = 03h 13m 56.9s
Dec(J2000) = -42d 56m 43.6s

with an estimated error radius of 3.5 arcseconds (90% confidence).
This
position is 16.8 arcseconds from the BAT refined position (Cummings,
et al., GCN 6212
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 ), 6.2 arcseconds from the initial XRT position, and
2.9
arcseconds from the UVOT optical candidate (Cummings, et al., GCN
Circ.
6210
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 ).

The 0.3-10 keV X-ray light curve presents a bright flare peaking at
about 280 s from the trigger and a second fainter peak
at 200 s. The underlying light curve between 70 s and 7.5 ks
can be fit with a simple power-law with a decay slope of 1.11 � 0.02.

The X-ray spectrum from the XRT/WT data covering up to the beginning
of
the bright flare is well fit by an absorbed power-law with a photon
index
of 1.4�0.1 and column density of (1.5�0.7)E21 cm**-2, higher
than the Galactic column density in the direction of the source
(2.5E20 cm**-2). The unabsorbed 0.3-10 keV flux for this spectrum is
1.3E-9 erg/cm**2/s. The XRT/WT spectrum of the bright flare and the
XRT/PC spectrum are modelled by an absorbed power law
with photon index of 1.9�0.1 and column density of (1.3�0.3)E21
cm**-2.
The unabsorbed 0.3-10 keV flux of the bright flare is 8.2E-10
erg/cm**2/s
and the one relative to the PC spectrum is 1.0E-10 erg/cm**2/s.

Assuming the X-ray emission continues to decline at the same rate, we
predict a 0.3-10 keV XRT count rate of 5E-3 count/s at T+48hr, which
corresponds to an observed 0.3-10 keV flux of 2.5e-13 erg/cm**2/s.

This circular is an official product of the Swift XRT Team.

M. Page (MSSL-UCL), M. Chester (PSU), J. R. Cummings (NASA/UMBC) report
on behalf of the Swift/UVOT team:

The Swift/UVOT began taking data in the field of GRB070318 88s after
the BAT trigger (Cummings et al., GCN 6210
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 , Marshall et al., GCN 6213
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 ).
The afterglow is detected in all UVOT filters. The photometry results 
are given for the 7 UVOT filters below. The first White exposure was 
affected by the star tracker loss of lock, so results from this exposure 
are not reported below.


Filter  T+(s)   Exp(s)  Mag
White   706     10      15.53 +/- 0.08

White  6364    197      18.14 +/- 0.07

V       214    400      15.43 +/- 0.02

B       692     10      16.25 +/- 0.12
B      6159    197      18.91 +/- 0.13

U       667     19      15.89 +/- 0.09
U      5955    197      18.23 +/- 0.11

UVW1     644     19     16.89 +/- 0.18
UVW1    5750    197     19.62 +/- 0.30

UVM2     619     19     17.97 +/- 0.37
UVM2    5545    197     > 19.87 (3-sigma UL)

UVW2     870     19     18.03 +/- 0.30
UVW2    6569    197     > 20.37 (3-sigma UL)

T+ is the start time of the exposure since the BAT trigger. These
magnitudes are not corrected for the expected extinction of
E(B-V)=0.017.

A.O. Jaunsen (Univ. Oslo), J.P.U. Fynbo (DARK/NBI), M.I. Andersen
(AIP) and P. Vreeswijk (ESO) report on behalf of a larger collaboration.

We observed the field of GRB 070318 (Cummings et al., GCN 6210
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 , La
Parola et al, GCN 6214
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  and Page et al. GCN 6215
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 ) with the ESO/VLT UT2
equipped with FORS1. Observations started on 2007 Mar 19.008 UT and
an 1800-s spectra was acquired with the 300V grism covering a
wavelength range of 3500-8500A.

We identify several absorption lines from MgII, MgI, FeII, MnII of which
the MgII doublet 2796,2804 is the most prominent at a redshift of
z=0.836.

We thank the excellent support from the ESO/Paranal staff.

[GCN OPS NOTE(19mar07): Per author's request, the phrase "several emission
lines" was changed to "several absorption lines" and "MI" was changed
to "Mg I".]
[GCN OPS NOTE(23may08): This time I actually changed the text!]

H.-W. Chen (U Chicago), J. X. Prochaska (UCO/Lick Observatory), S. 
Herbert-Fort (Steward Observatory), D. Christlein (Universidad de 
Chile/Yale), and S. Cortes (Steward Observatory) report on behalf of a 
larger collaboration:

"We obtained 2x1800 sec long-slit spectra of the optical transient of
GRB070318 (Cummings et al., GCN 6210
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 ), using GMOS and an the R400 grating
on the Gemini south telescope.  The observations started at March 19.001 
UT, ~ 16.5 hours after the initial Swift trigger.  The spectra covered a
wavelength range from 5930 Ang through 8750 Ang.  We also obtained 2700 
sec long-slit spectra, using IMACS and the 300 l/mm grism on the Magellan 
Baade telescope.  The observations started at March 19.011 UT, and the 
spectra cover a wavelength range from 4200 Ang through 6500 Ang.  We 
identify MgII 2796,2803, MgI 2852, FeII 2600, and TiII 3384 lines at 
z=0.840 +/- 0.002, but no other apparent MgII absorption doublet.  We 
therefore confirm the redshift reported by Jaunsen et al. (GCN 6216
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 ) as 
the location of the afterglow.

Further analysis is underway."

M.J. Page (MSSL-UCL), F.E. Marshall (NASA/GSFC) S. Immler (GSFC/USRA) S. 
T. Holland (CRESST/GSFC/USRA) and J.R. Cummings (GSFC/UMBC) report on 
behalf of the Swift/UVOT team:

The optical/UV afterglow of GRB 070318 (Cummings et al, GCN 6210
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 ) 
decayed to magnitude of 21.5 in the UVOT white filter in the first 24 
hours after the BAT trigger, and has remained at approximately constant 
brightness for at least 48 hours since that time. Examination of the 
Supercosmos scan of the UK Schmidt sky survey plate at this location 
reveals no evidence for a host galaxy (the plate limit is Bj>22 mag). 
For the flattening of the lightcurve to be due to the host galaxy, the 
host galaxy would have to be very luminous (Mb ~ -22) and have an 
unusually blue spectrum (e.g. peaking in the U band, observer frame). It 
therefore seems more likely that this is an afterglow with a very 
unusual lightcurve.

Swift continues to monitor the afterglow. Further ground-based 
observations are encouraged.

B. E. Cobb (Yale), part of the larger SMARTS consortium, reports:

Using the ANDICAM instrument on the 1.3m telescope at CTIO, we
obtained optical/IR imaging of the error region of GRB 070318
(GCN 6210
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 , Cummings et al.) with a mid-exposure time of
2007-03-19 00:09 UT (~16.7 hrs post-burst) and again
at 2007-03-21 00:07 UT (~64.6 hrs post-burst).  For each
set of observations, total summed exposure times amounted
to 36 minutes in I and 30 minutes in J.

The afterglow of GRB 070318 (GRB 6210, Cummings et al.) is detected
in the first epoch images and is observed to fade between the
first and second epochs.

time-post
bursts (hrs)    I magnitude             J magnitude
------------------------------------------------------
16.7            19.82+/-0.07            18.29+/-0.11
64.6            20.28+/-0.12            > 18.6 (3 sigma limit)

Magnitudes are calibrated using Landolt standard stars in the
optical and 2MASS stars in the IR.

The optical decay rate (afterglow flux proportional to t^-alpha)
between 16.7 and 64.6 hours post-burst is alpha ~ 0.3 (and
the IR decay is consistent with this value).  This unusually
shallow decay was also noted in the UVOT observations
of this afterglow (GCN 6220
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 , Page et al.).

A third late-time epoch was obtained at 2007-04-06 23:46 UT
(~20 days post-burst). No source or host galaxy is detected
at the position of the optical afterglow to a limiting magnitude
of I > 21.7 and J > 19.2.  This indicates that the decay
rate of the afterglow must have steepened sometime after our
second epoch observation.