GRB 080310

R. Chornock, R. J. Foley, W. Li, and A. V. Filippenko report that:

The Katzman Automatic Imaging Telescope at Lick Observatory slewed to the 
position of GRB 080310 and found a new source not present in the DSS:

14:40:13.81  -00:10:30.6

at about magnitude 17.  The new object is detected in V, I, and unfiltered 
images starting at 14:40:20.04 UT.  We suggest it may be the afterglow of GRB 
080310.

J. R. Cummings (NASA/UMBC), W. H. Baumgartner (GSFC/UMBC),
A. P. Beardmore (U Leicester), S. Campana (INAF-OAB),
P. A. Evans (U Leicester), E. A. Hoversten (PSU),
S. D. Hunsberger (PSU), C. B. Markwardt (CRESST/GSFC/UMD),
P. T. O'Brien (U Leicester), K. L. Page (U Leicester),
D. M. Palmer (LANL), R. L. C. Starling (U Leicester),
G. Tagliaferri (INAF-OAB) and T. N. Ukwatta (GSFC/GWU) report on
behalf of the Swift Team:

At 08:37:58 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 080310 (trigger=305288).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 220.081, -0.166 which is 
   RA(J2000) = 14h 40m 19s
   Dec(J2000) = -00d 09' 55"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a precursor at
T-24 seconds and multiple peaks from T-10 to T+6 seconds. The peak count 
rate was ~2000 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 08:39:27.9 UT, 89.2 seconds after
the BAT trigger. XRT found a bright, uncatalogued X-ray source located
at RA, Dec 220.0566, -0.1756 which is equivalent to:
   RA(J2000)  = 14h 40m 13.5s
   Dec(J2000) = -00d 10' 32.1"
with an uncertainty of 5.0 arcseconds (radius, 90% containment). This
location is 94 arcseconds from the BAT onboard position, within the BAT
error circle. No event data are yet available to assess possible
redshift constraints using X-ray spectroscopy and the nH-z relation
from Grupe et al. (2007). 

UVOT took a finding chart exposure of nominal 100 seconds with the White
(160-650 nm) filter starting 99 seconds after the BAT trigger. There is a
candidate afterglow in the list of sources generated on-board at
  RA(J2000)  =	14:40:13.80 = 220.0575
  DEC(J2000) = -00:10:29.6  =  -0.1749
with a 1-sigma error radius of about 0.6 arc sec. This position is 4.1 arc sec. 
from the center of the XRT error circle. The estimated magnitude is 18.8 with a
1-sigma error of about 0.5 mag. No correction has been made for the expected
extinction corresponding to E(B-V) of 0.04. 

Burst Advocate for this burst is J. R. Cummings (jayc AT milkyway.gsfc.nasa.gov). 
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.)

[corrected UT time]

R. Chornock, R. J. Foley, W. Li, and A. V. Filippenko (UC Berkeley) report that:

The Katzman Automatic Imaging Telescope at Lick Observatory slewed to the
position of GRB 080310 and found a new source not present in the DSS:

14:40:13.81  -00:10:30.6

at about magnitude 17.  The new object is detected in V, I, and unfiltered
images starting at 08:40:01 UT.  We suggest it may be the afterglow of GRB
080310.

E. Berger (Carnegie/Princeton) and M. Rauch (Carnegie) report:

"We obtained a spectrum of the optical afterglow of GRB 080310 (GCNs 7381
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 , 
7382
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 ) using the MagE spectrograph on the Magellan/Clay 6.5-m telescope. 
From preliminary analysis we determine a redshift of about 1.7 based on 
broad Ly-alpha absorption at ~3300A.  Further observations and analysis 
are on-going."

S. Covino, P. D'Avanzo, D. Fugazza, L.A. Antonelli, L.  Calzoletti,  
S. Campana, G.  Chincarini, M.L. Conciatore, S. Cutini, V. D'Elia, F.
D'Alessio, F.  Fiore, P. Goldoni, D. Guetta,  C. Guidorzi, G.L.  
Israel, E. Maiorano, N. Masetti, A. Melandri, E. Meurs, L. Nicastro,  
E. Palazzi,
E. Pian, S. Piranomonte, L.  Stella, G.  Stratta, G. Tagliaferri, G.  
Tosti, V.Testa, S.D. Vergani,  F. Vitali report on behalf of the REM  
team:

The robotic 60-cm REM telescope located at La Silla (Chile) observed  
automatically the field of the GRB 080310  on March 10 08:40:27 UT
(about 150 seconds after the burst).

We detect the afterglow reported in GCN 7381
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  and 8383
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  (Chornock et  
al.) and GCN 7382
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  (Cummings et al.).

The object has H~14.5 at about 5 min from the burst time.

Further analyses and observations are in progress.

B. E. Cobb, 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 080310
(GCN 7382
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 , Cummings et al.) beginning 20 minutes post-burst
(2008-03-10 08:58 UT).  Total summed exposure times amounted
to 180s in BRIYJK and 120s in H and V.

The afterglow of GRB 080310 (GCN 7382
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 , Cummings et al. & GCN 7383
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 ,
Chornock et al.) is detected in our images.  Preliminary comparison to
USNO-B1.0 stars in I and 2MASS stars in J indicates the following
afterglow magnitudes:

time post-burst         I mag           J mag
~25 minutes             16.3+/-0.2      15.9+/-0.2

The afterglow fades by ~0.4 magnitudes in I between 25 and 45 minutes
post-burst.  This source is therefore confirmed to
be the afterglow of GRB 080310, with a decay rate of alpha ~ -0.6.

P.A. Milne and G.G. Williams (U Arizona) report on behalf of the
Super-LOTIS team:

The 0.6m Super-LOTIS telescope began R-band observations of the error
region of GRB080310 (Swift trigger 305288) at 08:38:43 UT, 44 seconds
after the start of the burst.  The OT detected by Chornock et al.
(GCN 7381
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 ) and confirmed by Cummings et al. (GCN 7382
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 ) is not
apparent in the initial images, but is clearly visible in
subsequent images. A stack of the initial 3x10s exposures does not
detect the GRB, while subsequent single 20sec exposures do, suggesting
that the GRB brightened during the first two minutes.

We used the USNO-B star at RA=14:40:11.3, Dec==00:10:40.6 (0898-0239265) to
derive the R magnitude.

The R-band magnitude of the afterglow in the first image is:
UT Start   UT End ExpTime  R mag  error
Single
08:40:29  08:41:09 1x20s  17.97 +/- 0.18
08:41:03  08:41:23 1x20s  16.81 +/- 0.08
Stack
08:40:29  08:42:37 5x20s  16.91 +/- 0.04
08:43:24  08:54:27 10x60s 16.98 +/- 0.02

Observations are continuing.  This message can be cited.

J.X. Prochaska (UCSC), M. Murphy (Swinburne),
A.L. Malec (Swinburne), K. Miller (Lick)
report on behalf of GRAASP:

"We observed the afterglow of GRB 080310 with the Kast
dual spectrometer for a series of 900s exposures starting
at UT 09:06 under good conditions.  Analysis of the red side
reveals a series of strong absorption features (SiIV, CIV,
AlII 1670) consistent with the redshift z=2.4266.  At
present we consider this to be the redshift of GRB 080310.

Further analysis is in progress."


This GCN may be cited.

----------------------------------------------
Jason X. Prochaska
UCO/Lick Observatory
UC Santa Cruz
Santa Cruz, CA 95064

xavier@ucolick.org
http://www.ucolick.org/~xavier/
831-459-2135 (Direct)
831-459-2991 (UCO/Lick Main)
831-459-5244 (Fax)

E. Berger (Carnegie/Princeton) and M. Rauch (Carnegie) report:

"The redshift of 1.7 previously quoted in GCN 7384
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  was due to 
mis-identification of the line.  Based on the detection of Ly-alpha, SII, 
CIV, SiIV, etc.  the correct redshift is z=2.42."

P. Garnavich (Notre Dame), J. L. Prieto, R. Pogge (OSU) report:

We observed the position of GRB 080310 (Chornock et al. GCN 7381
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 ;
Cummings et al., GCN 7382
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 ) with the Large Binocular Telescope and
LBC red and blue cameras starting at 9:04 (UT) (0.44 hours
after the burst) and ending 1 hour after the burst. Observations
were obtained approximately every minute. The initial pair of
observations showed no significant decay, but then declined
rapidly into a power-law with a decay index of 0.65
(see Cobb et al., GCN 7386
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 ).

This message may be cited.

P.M. Vreeswijk (DARK), P. Jakobsson (U. Hertfordshire), A.O. Jaunsen
(U. Oslo) and C. Ledoux (ESO) report on behalf of a larger
collaboration:

We observed the optical afterglow (Chornock et al., GCN 7381
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 ) of GRB
080310 (Cummings et al., GCN 7382
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 ) with the Ultraviolet and Visual
Echelle Spectrograph (UVES) mounted at ESO's VLT Kueyen. Observations
were triggered automatically in rapid-response mode starting on 2008
March 10 at 08:51 UT (13 min after the Swift trigger). We secured two
epochs over the full UVES wavelength range before morning twilight.

Most absorption lines can be explained by three systems at redshifts
z=2.43, z=2.28, and z=1.67, confirming the probable redshift of
GRB080310 first reported by Prochaska et al. (GCN 7388
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 ).

We acknowledge excellent support from the observing staff at Paranal,
in particular Dominique Naef and Claudio Melo.

J. L. Prieto (OSU), P. Garnavich (Notre Dame), R. Pogge (OSU) report:

We continue to observe the afterglow of GRB 080310 (Garnavich et al.,
GCN 7390
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 ) with the LBT and LBC blue/red cameras. Data taken
between 1.7 and 2.1 hours after the burst show a steepening
decay rate. The power-law decay index over this period
was 1.01 suggesting a break in the light curve is underway.

This message may be cited.

S. Covino, G. Tagliaferri, D. Fugazza, G. Chincarini, on behalf of  
the MISTICI collaboration report:

We observed the field of GRB 080310 (GCN 7382
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 , Cummings et al.) with  
the VLT equipped with FORS2 with the Rapid Response Mode starting  
from 2008-03-20 at 08:42:59 UT  (about 5 minutes from the burst) for  
almost one hour of continuous observations. The optical afterglow  
(GCN 7381
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 , Chornock et al.) is well detected and it was brighter than  
R~17 (compared to USNO stars) already in the first frame.

A complete analysis is in progress.

[GCN OPS NOTE(10mar08): Per author's request, the "VBLT" in the 
Subject-line was changed to "VLT".]

J.P. Osborne, A.P. Beardmore, P.A. Evans and M.R. Goad (U. Leicester) 
report on behalf of the Swift-XRT team.

Using 99 s of overlapping XRT Photon Counting mode and UVOT
data for GRB 080310, we find an astrometrically corrected X-ray
position (using the XRT-UVOT alignment and matching UVOT field sources
to the USNO-B1 catalogue): RA, Dec = 220.05801, -0.17496 which is equivalent
to:

RA (J2000): 14h 40m 13.92s
Dec (J2000): -00d 10' 29.9"

with an uncertainty of 1.8 arcsec (radius, 90% confidence).

This position may be improved as more data are received. The latest position
can be viewed at http://www.swift.ac.uk/xrt_positions. Position enhancement is
described by Goad et al. (2007, A&A, 476, 1401
http://www.swift.ac.uk/xrt_positions/Goad.pdf).

This circular was automatically generated, and is an official product of the
Swift-XRT team.

T.W. Chen, L.C. Huang, K.Y. Huang and Y. Urata 
on behalf of EAFON report:

"We have been monitoring the optical afterglow of GRB080310 
(GCN #7381
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 , 7382
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 ) using Lulin 1m telescope. The brightness 
in R-band at 15:40 (~7hrs after the burst) is ~20 mag. 
Continuous optical monitoring is encouraged."

Further observation and analysis are in progress."

Richard J. Cool (Arizona), Daniel J. Eisenstein (Arizona),
David W. Hogg (NYU), Michael R. Blanton (NYU), David J. Schlegel
(LBNL), J. Brinkmann (APO), Donald Q. Lamb (Chicago), Donald
P. Schneider (PSU), and Daniel E. Vanden Berk (PSU) report:

The Sloan Digital Sky Survey (SDSS) imaged the field of burst
GRB080310 prior to the burst.  As these data should be useful
as a pre-burst comparison and for calibrating photometry,
we are supplying the images and photometry measurements for
this GRB field to the community.

Data from the SDSS, including 5 FITS images, 3 JPGS, and
3 files of photometry and astrometry, are being placed at
http://mizar.as.arizona.edu/~grb/public/GRB080310

We supply FITS images in each of the 5 SDSS bands of a 8'x8'
region centered on the GRB position (ra=220.081 (14:40:19.4),
dec=-0.166000 (-00:09:57.6); Swift-BAT TRIGGER 305288), as well
as 3 gri color-composite JPGs (with different stretches). The
units in the FITS images are nanomaggies per pixel.  A pixel
is 0.396 arcsec on a side. A nanomaggie is a flux-density unit
equal to 10^-9 of a magnitude 0 source or, to the extent that
SDSS is an AB system, 3.631e-6 Jy.  The FITS images have WCS
astrometric information.

In the file GRB080310_sdss.calstar.dat, we report photometry
and astrometry of 486 bright stars (r<20.5) within 15' of the
burst location.  The magnitudes presented in this file are asinh
magnitudes as are standard in the SDSS (Lupton 1999, AJ, 118,
1406). Beware that some of these stars are not well-detected
in the u-band; use the errors and object flags to monitor
data quality.

In the files GRB080310_sdss.objects_flux.dat and
GRB080310_sdss.objects_magnitudes.dat, we report photometry
of 818 objects detected within 6' of the GRB position.
We have removed saturated objects and objects with model
magnitudes fainter than 23.0 in the r-band.  The fluxes listed
in GRB080310_sdss.objects_flux.dat are in nanomaggies while
the magnitudes listed in GRB080310_sdss.objects_magnitudes.dat
are asinh magnitudes.

All quantities reported are standard SDSS photometry, meaning
that they are very close to AB zeropoints and magnitudes are
quoted in asinh magnitudes.  Photometric zeropoints are known
to about 2% rms.  None of the photometry is corrected for
dust extinction.  The Schlegel, Finkbeiner, and Davis (1998)
predictions for this region are A_U=0.202 mag, A_g=0.149 mag,
A_r = 0.108 mag, A_i=0.082 mag, and A_z=0.058 mag.

The file GRB080310_sdss.spectro.dat contains a list of the
3 objects with SDSS spectroscopy within 6 arcminutes of the
GRB position.  In addition to the redshift and 1-sigma error
for each object, this file also lists the object spectroscopic
classification.


SDSS astrometry is generally better than 0.1 arcsecond per
coordinate.  Users requiring high precision astrometry should
take note that the SDSS astrometric system can differ from
other systems such as those used in other notices; we have
not checked the offsets in this region.

More detailed information pertaining to our SDSS GRB releases
can be found in our initial data release paper (Cool et
al. 2006, PASP 118, 733).  See the SDSS DR4 documentation for
more details: http://www.sdss.org/dr5.

These data have been reduced using a slightly different
pipeline than that used for SDSS public data releases.
We cannot guarantee that the values here will exactly match
those in the data release in which these data are included.
In particular, we expect the photometric calibrations to differ
by of order 0.01 mag.

This note may be cited, but please also cite the SDSS data
release paper, Adelman-McCarthy et al. (2007, ApJS, 172, 634),
when using the data or referring to the technical documentation.

J.X. Prochaska (UCSC), R.J. Foley (UCB), B. Holden (UCCS),
D. Magee (UCSC),  M. Cooper (Arizona) and A. Dutton (UCSC)
report on behalf of GRAASP:

"We observed the afterglow of GRB 080310 (Chornock, #7381)
with the Keck/DEIMOS spectrometer for a series of 900s exposures
starting ~2hr after the burst under good conditions.  Analysis of the
data confirms the three absorption systems reported by
Prochaska et al. (#7388) and Vreeswijk et al. (#7391).
We identify modest strength FeII and MgII resonance lines associated
with the absorber at z=2.4266 and also weaker FeII* absorption.  The  
latter observation
establishes this gas as the ISM surrounding GRB 080310.

Further analysis is in progress."


This GCN may be cited.

E. A. Hoversten (PSU) and J. R. Cummings (NASA/UMBC) report on behalf  
of the Swift UVOT team:

    The Swift/UVOT observed the field of GRB 080310 (trigger 305288)  
starting 99 seconds after the BAT trigger (Cummings, et al., GCN  
Circ. 7382
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 ) starting with a 100s finding chart exposure in the white  
filter.  A bright afterglow was clearly detected in the initial white  
and V band exposures.

Further analysis shows that a fading source is detected at the XRT  
position in the U, B, V, and white filter images.  The source is  
still detectable in these filters around 7000 seconds after the  
burst.  The white and U band images show a brightening of the source  
reaching a peak after 1 ks after the burst before fading smoothly.   
The B and V images show a more smooth decay.

The afterglow is not detected in the UVW1 and UVM2 filters and is  
weakly detected in the UVW1 filter.  This is consistent with the  
measured redshift of 2.42 (Prochaska, et al., GCN Circ. 7388
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 ).

The detections and 3-sigma upper limits in the UVOT photometric  
system (Poole, et al. 2008, MNRAS 383, 627) are given in the  
following table.  The white and optical magnitudes are individual  
exposures while the UV magnitudes are from co-added images.

Filter T_start(s) T_stop(s) Exposure(s)  Mag

white     99        197       98         18.50 +/- 0.06
white    868        967       98         17.95 +/- 0.05
white   6788       6985      197         18.99 +/- 0.07

v        208        602      393         17.38 +/- 0.05
v       7199       7334      135         18.5  +/- 0.3

b        685        695       10         17.7  +/- 0.2
b       6584       6780      197         18.9  +/- 0.1

u        660        680       19         17.8  +/- 0.2
u       1430       1450       19         17.3  +/- 0.2
u       6379       6575      197         18.3  +/- 0.1

uvw1     638       6373      452         20.3 +/- 0.3
uvm2     613       6168      197        >19.8
uvw2     716       7194      432        >20.8


No correction has been made for the expected Galactic extinction of E 
(B-V) of 0.04 mag in the direction of the burst (Schlegel, et al. 1998).

A.P. Beardmore, J.P. Osborne, R.L.C. Starling, K.L. Page, P.A. Evans 
(U. Leicester) and J.R. Cummings (NASA/UMBC) report on behalf of 
the Swift-XRT team:

The Swift-XRT started observing GRB 080310 (trigger no. 305288,
Cummings et al. GCN 7382
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 ) at 2008-03-10 08:39:27.85 UT, 89s after the
trigger. The observation so far spans 3 snapshots, with Windowed Timing
mode data from T+95s to T+799s, and Photon Counting mode thereafter.

The best XRT position is the UVOT-enhanced position from Osborne et al.
(GCN 7394
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 ).

At first, the 0.3-10keV X-ray light curve decays from 39 count s^-1 at
T+95s to 18 count s^-1 at T+130s but then shows strong flaring
activity. An initial rebrightening occurs from about T+135s to T+420s,
reaching ~135 count s^-1 at approximate times of 200s, 250s, 280s and
350s after the trigger.  This is followed by a second flare from T+500
to T+620s, where it reaches a maximum of ~115 count s^-1 at T+565s.
Following the flaring activity, the X-ray light curve, thus far, shows
a slow decline out to the end of the third snapshot (T+1.0ks to
T+18.7ks), where it reaches a count rate of 0.1 count s^-1 with an
underlying decay slope of alpha~0.5.

The X-ray data also show strong spectral evolution during the flaring
intervals.  A 1.5-10keV/0.3-1.5keV hardness ratio reveals the X-ray
emission initially hardens from T+135s to T+200s as the light curve
rebrightens, at which point it remains approximately constant until
T+360s, before softening as the light curve decays.  Similarly, the
source hardens again during the second flare from T+500s to T+565s,
then softens as this flare declines.

A spectrum of the WT data from T+200s to T+360s, where the hardness
ratio is approximately constant, can be well fit by an absorbed
powerlaw with photon index 1.45 +/- 0.02 and column density of (7.0
+/- 1.0)e21 cm^-2 at the redshift of the burst (z=2.43, Prochaska et
al. GCN 7388
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 ; Berger and Rauch GCN 7389
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 ; Vreeswijk et al. GCN 7391
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 ),
in addition to the Galactic column density of 3.3e20 cm^2 in this
direction. The observed 0.3-10keV flux is (6.1 +/- 0.1)e-9 erg cm^-2 s^-1,
which corresponds to an unabsorbed flux of 6.8e-9 erg cm^-2 s^-1. 
The counts to observed flux conversion factor at the time of
this spectrum is 4.9e-11 erg cm^-2 count^-1.

The PC mode data from the second snapshot (T+5.1ks to T+7.3ks) show a 
softer spectrum, with a photon index of 1.9 +/- 0.2 and column density 
consistent with the value determined above. The observed 0.3-10keV flux at 
this time is (1.1 +/- 0.1) e-11 erg cm^-2 s^-1, and corresponds to a flux 
conversion factor of 4.7e-11 erg cm^-2 count^-1.

If the underlying powerlaw decay continues as is, we predict an XRT count 
rate of 0.05 count s^-1 at T+24hr, which corresponds to an observed 
0.3-10keV flux of 2.5e-12 erg cm^-2 s^-1.

This circular is an official product of the Swift-XRT team.

J. Tueller (GSFC),
S. D. Barthelmy (GSFC),
J. Cummings (GSFC/UMBC),
E. Fenimore (LANL),
N. Gehrels (GSFC),
H. Krimm (GSFC/USRA),
C. Markwardt (GSFC/UMD),
K. McLean (GSFC/UMD),
D. Palmer (LANL),
A. Parsons (GSFC),
T. Sakamoto (GSFC/UMBC),
G. Sato (GSFC/ISAS),
M. Stamatikos (GSFC/ORAU),
T. Ukwatta (GWU)
(i.e. the Swift-BAT team):
 
Using the data set from T-239 to T+963 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 080310 (trigger #305288)
(Cummings, et al., GCN Circ. 7382
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 ).  The BAT ground-calculated position is
RA, Dec = 220.040, -0.164 deg which is
   RA(J2000)  = 14h 40m 9.6s
   Dec(J2000) = -0d 9' 49"
with an uncertainty of 1.4 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 68%.
 
The mask-weighted lightcurve shows many peaks starting at ~T-60 sec.  The
main peak extends from ~T-12 to T+7 sec, then there is a period with no
detectable emission, then a broad, weaker peak from T+180 to T+360 sec.
The later peak corresponds to the first strong flare seen in the XRT (Beardmore
et al., GCN circ. 7399
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 ).  BAT sees no emission at the time of the second flare
seen in the XRT, even though in the XRT energy range it was of similar intensity
at the peak to the first flare.

The location of the burst entered the BAT field of view at about T-80 sec during
a preplanned slew.  It is possible that some very weak emission preceded that
time, but the BAT raw lightcurve, which is partially sensitive to photons from
outside the FOV rules out strong emission. T90 (15-350 keV) is 365 +- 20 sec
(estimated error including systematics).
 
The time-averaged spectrum from T-71.8 to T+318.7 sec is best fit by a simple
power-law model.  The burst does not appear to have significant emission above
about 150 keV however.  The power law index of the time-averaged spectrum is
2.32 +- 0.16.  The fluence in the 15-150 keV band is 2.3 +- 0.2 x 10^-06 erg/cm2.
The 1-sec peak photon flux measured from T+1.27 sec in the 15-150 keV band
is 1.3 +- 0.2 ph/cm2/sec.  All the quoted errors are at the 90% confidence
level.
 
The results of the batgrbproduct analysis are available at
http://gcn.gsfc.nasa.gov/notices_s/305288/BA/

[GCN OPS NOTE(10mar08): Per author's request, the missing minus sign
on the Dec specification was added (-0d 9' 49").]

P. Wozniak, W.T. Vestrand, J. Wren, and H. Davis
(Los Alamos National Laboratory)

Our RAPTOR-S and RAPTOR-T telescopes began simultaneous multicolor
(clear, V, R, I) imaging of GRB 080310 at UT 08:38:27.14, 28.5 seconds
after the trigger, 6.7 seconds after receiving the trigger.

The counterpart identified by Chornok et al. (GCN 7381
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 ) brightened
from below 17 mag to about 16.8 mag at ~200 seconds (clear band
calibrated to USNO B1.0 R2 magnitudes) and dimmed to about 17.3 mag at
400 seconds. We clearly detect the OT simultaneously in all filters
and follow its flux evolution for 1.5 hours.
We observe a broad plateau between 400 and 1100 seconds
with slight rebrightening towards the end of this time interval,
folowed by rapid decay.

D. A. Perley, J. S. Bloom, and W. Li (UC Berkeley) report:

PAIRITEL responded to GRB 080310 and began taking data at UT 2008-03-10 
  09:04:58 in J, H, and K filters simultaneously.  The afterglow 
(Chornock et al., GCN 7381
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 ) is well-detected in all three filters.  We 
report the following magnitudes:

t_mid(s)  exp(s)  magnitude
1833      259     J = 15.63 +/- 0.03
1833      259     H = 14.94 +/- 0.03
1833      259     K = 14.29 +/- 0.03

Combining PAIRITEL data with 347 KAIT observations (calibrated to SDSS 
DR6) and the UVOT magnitudes reported by Hoversten et al. (GCN 7398
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 ), we 
generate a UV-through-IR spectral energy distribution interpolated to 
the midpoint of the PAIRITEL observations.  The SED is well-fit by an 
intrinsic power-law spectral index of beta ~ 0.6 with a small amount 
(A_V = 0.10 +/- 0.05) of SMC-like host-galaxy extinction:

http://lyra.berkeley.edu/~dperley/080310/080310sed.png

Additional, late-time follow-up is planned.

P. Garnavich (Notre Dame), J. L. Prieto, R. Pogge (OSU) report:

We observed the position of GRB 080310 (Chornock et al. GCN 7381
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 ;
Cummings et al., GCN 7382
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 ) with the Large Binocular Telescope and
LBC red and blue cameras on March 11 starting at 11:30 (UT) or 26.8
hours after the burst. On the red camera the images were taken
through a Sloan-r filter and calibrating with the SDSS catalog
(Cool et al.; GCN 7396
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 ) we find the brightness to be r=21.38+/-0.05 mag.

Extrapolating the decay from LBT observations of March 10 (Prieto
et al., GCN 7392
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 ) with an index of 1.15, the afterglow is about
0.2 mag brighter than expected. This could be due to host galaxy
light or deviations from a single power-law decay.

The LBT is an international collaboration among institutions in the
United States, Italy and Germany. The LBT Corporation partners are:
*  The University of Arizona on behalf of the Arizona university system
*  Istituto Nazionale di Astrofisica, Italy
*  LBT Beteiligungsgesellschaft, Germany, representing the Max Planck
Society, the Astrophysical Institute Potsdam, and Heidelberg University
*  The Ohio State University
*  The Research Corporation, on behalf of The University of Notre Dame,
University of Minnesota and University of Virginia

This message may be cited.

M. Yoshida, K. Yanagisawa, Y. Shimizu, S. Nagayama, H. Toda (OAO,
NAOJ) and N. Kawai (Tokyo Tech) report on behalf of the MITSuME
collaboration:

We detected the optical afterglow of GRB 080310 (Cummings, et al.,
GCN 7382
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 ) in the error circle of the XRT position (Osborne et al.
GCN 7394
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 ) with the 50cm MITSuME Telescope at Okayama Astrophysical
Observatory on March 10 UT. Photometric calibration was done using
USNO-B1.0 catalog. The light curve shows a plateau at around 19:00 -
20:00 UT (about 11 hours after the trigger).
The photometric results are summarized below.

Observation date: 2008-03-10 UT

mid-UT     exp-T         g'            Rc            Ic
------------------------------------------------------------
15:09:46   60min     20.0+-0.2     19.7+-0.3     19.1+-0.2
16:24:47   60min     20.4+-0.2     20.2+-0.2     19.5+-0.2
17:39:46   60min     20.6+-0.1     20.4+-0.2     19.8+-0.2
18:54:44   60min     20.6+-0.2     20.5+-0.1     20.2+-0.3
19:57:05   42min     20.6+-0.2     20.3+-0.1     20.0+-0.2
------------------------------------------------------------

F. Yuan (U Mich), R. Quimby (Caltech), H. Swan (U Mich), C. Akerlof (U 
Mich), report on behalf of the ROTSE collaboration:

ROTSE-IIIb, located at McDonald Observatory, Texas, responded to GRB 
080310 (Swift trigger 305288; Cummings et al., GCN 7382
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 ) and began 
imaging at UT 08:38:25.7 (5.7 sec after the GCN notice time) under 
cloudy conditions. The first usable image with a detection of the OT 
(Chornock et al. GCN 7381
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 ) starts at UT 09:03:03.4. Observations 
continued in fluctuating weather conditions until about 3 hours after 
the trigger.
 
The OT is slightly blended with the two nearby stars in ROTSE images. To 
reduce the contamination, we tried two different methods to obtain the 
OT magnitudes, first by subtracting the scaled PSF of the two nearby 
stars, and, second, by subtracting a reference image constructed from 
images taken on Mar. 11  between UT 05:26:28.5 and 06:00:53.6 (when the 
OT has dropped below our detection threshold). The two methods yield 
similar results. From 0.42 to 1.72 hour after the burst, the OT is 
observed to decay with a power-law index 0.6+/-0.1, consistent with the 
observation by the LBT (Garnavich et al., GCN 7390
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 ). The later images 
have degraded seeing and don't constrain the time dependence of the OT 
very well. The magnitudes reported below are unfiltered calibrated to 
SDSS r using standard stars in the pre-burst SDSS observations (Cool et 
al., GCN 7396
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 ).

Start_UT     End_UT      mag    magerror    mlim(of image)
---------------------------------------------------------------
09:03:03.4  09:07:31.2   17.2    0.1         18.4
09:56:17.5  10:09:49.8   18.0    0.1         19.4

Y. Urata, M. Im. I. Lee, K.Y. Huang, T.W. Chen, L.C. Huang
on behalf of EAFON report:

"We have monitored the optical afterglow of GRB080310 (Cummings et al
GCN 7382
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 ) on the night of March 10th using the Mt. Lemmon 1m and the
Lulin 1m telescopes. These time coverage are 09:26 -- 12:54 UT
(Mt. Lemmon) and 15:52--21:16 UT (Lulin), respectively. All of images
taken with 300 sec exposure in V and R band show the afterglow clearly.

Based on our preliminary results calibrated using USNO-B1.0 catalog,
we confirmed the decaying light curve with a power-law index 0.7+/0.1
between 0.8 and 2.1 hours after the burst. This result is consistent
with the observations by the LBT (Garnavich et al., GCN 7390
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 ) and the
ROTSE-III (Yuan et al., GCN 7411
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 ).  The light curve between 7.3 and
12.2 hrs after the burst is well described by single power-law with
index 0.8+/-0.1 in R-band. The extrapolation of this power-law is also
consistent with the observation by the LBT (Garnavich et al. 7409).
We could not confirm the plateau phase reported by the MITSuME (Yoshida
et al., GCN 7410
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 )."

This message may be cited.

K. Kinugasa (Gunma Astronomical Observatory)  report:

 The position of GRB 080310 (Chornock et al. GCN 7381
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 ;
Cummings et al., GCN 7382
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 )  was observed with the 150 cm
telescope of Gunma Astronomical Observatory.
Starting at 15:09 and 18:01 UT (6.5 and 9.4 hours after
the burst), Rc and Ic frames were acquired for sets of
5 x 3-min and  5 x 3-min exposures.

 We clearly detected the optical counterpart in all frames.
We estimated the R magnitute below and confirmed the fading
trend. The decay index of single power-law is consistent
with the result reported by Urata et al. (GCN 7412
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 ).


mid-UT     exp           Rc
-----------------------------------
15:18:50   5 x 3-min     20.1+-0.1
18:09:42   5 x 3-min     20.5+-0.1
-----------------------------------

P. Garnavich (Notre Dame), J. L. Prieto, R. Pogge (OSU) report:

Beginning March 12.48 (UT), we again observed the position of
GRB 080310 (Chornock et al. GCN 7381
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 ; Cummings et al., GCN 7382
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 )
with the Large Binocular Telescope and LBC red and blue cameras.
The seeing was 0.7 arcsec in the r-Sloan filter.

The brightness of the afterglow 2.12 days after the burst is measured
to be r=21.67+/-0.05 mag, or only 0.3 mag fainter than the previous
night. This corresponds to a power-law decay index of only 0.42
and confirms the slowing in the decay rate noted in GCN 7409
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 .

It is unlikely that the slow fade is due to contamination from
a host galaxy or a galaxy along the line of sight. To explain the
observed decay rate, a galaxy would have to contribute at a
magnitude near r=22.5, but nothing at this brightness is detected
in the SDSS at the position of the afterglow. Also, the afterglow is
consistent with a point source in the LBT images obtained in good seeing.

We conclude that the shallow decay rate is intrinsic to the afterglow
and this suggests either a continuing energy input or that the shock has
encountered a change in density of the ambient medium. (We must note
another possibility: the afterglow is being gravitationally
microlensed.)

The LBT is an international collaboration among institutions in the
United States, Italy and Germany. The LBT Corporation partners are:
*  The University of Arizona on behalf of the Arizona university system
*  Istituto Nazionale di Astrofisica, Italy
*  LBT Beteiligungsgesellschaft, Germany, representing the Max Planck
Society, the Astrophysical Institute Potsdam, and Heidelberg University
*  The Ohio State University
*  The Research Corporation, on behalf of The University of Notre Dame,
University of Minnesota and University of Virginia

This message may be cited.

Y. Urata, T.W. Chen, K.Y. Huang, L.C. Huang, M. Im, I. Lee
on behalf of EAFON report:

"We have made R-band images for the optical afterglow of GRB 080310
(Chornock et al. GCN 7381
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 ; Cummings et al., GCN 7382
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 ) using the Lulin
1m telescope. According to USNO-B1.0 calibration, the afterglow at
1.44 days after the burst is R=21.3+/-0.1. This observation confirm
the plateau or bump reported by Garnavich et al., (GCN 7414
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 ). The
R-band lightcuve made from our observations (T.W. Chen et al., GCN
7395
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 ; Urata et al., GCN 7412
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 ) and the LBT points (Garnavich et al.,
GCN 7409
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 , 7414
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 ) is resemble to that of GRB030329 (e.g. Lipkin et al
2004)."

This message may be cited.

G. Wegner (Dartmouth), P. Garnavich (Notre Dame), J. L. Prieto,
and K. Z. Stanek (OSU) report:

We observed the position of GRB 080310 (Chornock et al. GCN 7381
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 ;
Cummings et al., GCN 7382
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 ) with the Large Binocular Telescope
(LBC red and blue cameras) on March 13.40 (UT) and with the
MDM 2.4-m Hiltner telescope (Retrocam) on March 14.39 (UT) through
Sloan-r filters.

Using stars calibrated by the SDSS, we estimate the following
r-band magnitudes:

age (days)  r mag   error   telescope
3.04        22.45    0.05     LBT
4.03        23.10    0.10     MDM

The photometry indicates that the afterglow has resumed a steep
decline. Including LBT photometry from March 12 (Garnavich et al.,
GCN 7414
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 ), we estimate a power-law decline index of 2.1 between
2 and 4 days after the burst.

A plot of the light curve is available at:
http://www.nd.edu/~pgarnavi/LBT/grb080310/LBT_lightcurve.jpg

The LBT is an international collaboration among institutions in the
United States, Italy and Germany. The LBT Corporation partners are:
*  The University of Arizona on behalf of the Arizona university system
*  Istituto Nazionale di Astrofisica, Italy
*  LBT Beteiligungsgesellschaft, Germany, representing the Max Planck
Society, the Astrophysical Institute Potsdam, and Heidelberg University
*  The Ohio State University
*  The Research Corporation, on behalf of The University of Notre Dame,
University of Minnesota and University of Virginia

This message may be cited.

J. Hill (LBTO/UAz), R. Ragazzoni, A. Baruffolo (Padova), and
P. Garnavich (Notre Dame) report:

The Large Binocular Telescope (LBT) observed the position of GRB 080310
(Chornock et al. GCN 7381
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 ; Cummings et al., GCN 7382
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 ) with the
LBC red and blue cameras between March 19.3 and 19.5 (UT). Twenty
200 sec exposures in the Sloan-r filter were obtained in 1.3"
seeing and with a significant sky background from the Moon.

The combined image shows a faint source present at the position
of the afterglow. From stars calibrated in the SDSS we estimate
the brightness of the source to be r=25.4+/-0.2 mag. This is
consistent with an afterglow  power-law decay index of 2.4 between
2 and 9 days after the burst (Wegner et al., GCN 7423
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 ). The
true afterglow decay rate could be steeper if host galaxy light
is contributing to the source flux.

A faint galaxy (r~24) is detected 2.1" southeast of the afterglow and
another is 3.0" to the southwest. For a redshift of 2.43 and a standard
cosmology, the projected distances from the afterglow are 17 kpc and
24 kpc. These galaxies may be the source of the two foreground absorption
line systems observed in the afterglow spectrum (Vreeswijk et al., GCN
7391
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 ).


The LBT is an international collaboration among institutions in the
United States, Italy and Germany. The LBT Corporation partners are:
*  The University of Arizona on behalf of the Arizona university system
*  Istituto Nazionale di Astrofisica, Italy
*  LBT Beteiligungsgesellschaft, Germany, representing the Max Planck
Society, the Astrophysical Institute Potsdam, and Heidelberg University
*  The Ohio State University
*  The Research Corporation, on behalf of The University of Notre Dame,
University of Minnesota and University of Virginia

This message may be cited.

A. Henden (AAVSO) reports:

While the field of GRB080310 has been observed by SDSS, we have also
obtained a four-night BVRcIc field calibration using the 35cm robotic
telescope at Sonoita Research Observatory.  The calibration file
has a limiting magnitude around V=17, with good standards brighter
than V=11 or so.  The file is available at
ftp://ftp.aavso.org/public/calib/grb/grb080310.dat

This calibration is based on numerous Landolt standards, and has
an external zeropoint error of about 0.02mag. Our system is available
for any other bright BVRI calibrations (4<V<19) for this field or any
other field; contact the author for such requests.

The AAVSO thanks the Curry Foundation for their continued support of the
AAVSO International High Energy Network, and to John Gross of SRO for
providing setup assistance for this field.