GRB 080503

J. Mao (INAF-OAB), W. H. Baumgartner (GSFC/UMBC), D. N. Burrows (PSU),
M. M. Chester (PSU), N. Gehrels (NASA/GSFC), C. Guidorzi (INAF-OAB),
S. T. Holland (CRESST/USRA/GSFC), J. A. Kennea (PSU),
C. B. Markwardt (CRESST/GSFC/UMD), F. E. Marshall (NASA/GSFC),
P. T. O'Brien (U Leicester), D. M. Palmer (LANL),
P. Romano (INAF-IASFPA), T. N. Ukwatta (GSFC/GWU) and
D. E. Vanden Berk (PSU) report on behalf of the Swift Team:

At 12:26:13 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 080503 (trigger=310785).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 286.617, +68.778 which is 
   RA(J2000) = 19h 06m 28s
   Dec(J2000) = +68d 46' 41"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a single spike
structure with a duration of about 0.5 sec, although there may
be further activity during the slew.  The peak count rate
was ~6500 counts/sec (15-350 keV), at ~0.1 sec after the trigger. 

The XRT began observing the field at 12:27:28.1 UT, 74.9 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source with an enhanced position: RA, Dec
286.62049, 68.79379 which is equivalent to:
   RA(J2000)  = 19h 06m 28.92s
   Dec(J2000) = +68d 47' 37.6"
with an uncertainty of 2.7 arcseconds (radius, 90% containment). This
location is 57 arcseconds from the BAT onboard position, within the BAT
error circle. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of
5.56e+20 cm^-2 (Kalberla et al. 2005), so we cannot constrain the
redshift at this time using the relation from Grupe et al. (2007). A
summary of the promptly downlinked data is given at
http://www.swift.ac.uk/sper/310785/. 

The initial flux in the 2.5 s image was 7.82e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 100 seconds with the White (160-650 nm)
filter starting 84 seconds after the BAT trigger. No afterglow candidate has
been found in the initial data products. The 2.7'x2.7' sub-image covers 100% of
the XRT error circle. The 3-sigma upper limit is about 20th mag. 
The 8'x8' region for the list of sources generated on-board covers 100% of the
XRT error circle. The list of sources is typically complete to about 18 mag. No
correction has been made for the expected extinction corresponding to E(B-V) of
0.06. 

Burst Advocate for this burst is J. Mao (jirong.mao AT brera.inaf.it). 
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.)

D. A. Perley (UC Berkeley), D. Lai, C. Rockosi (UCSC/Lick), and J. S. 
Bloom (UC Berkeley) report:

After the GRB alert we slewed to the position with the Keck I telescope, 
equipped with LRIS.  We detect a possible new source at a position 
consistent with the XRT position in the LRIS guider camera near the 
detection limit of the guider (~20-21st mag).  Spectroscopic 
observations began at ~13:01 UT.  Results will be reported in 
forthcoming circulars.

J. S. Bloom and D. A. Perley (UC Berkeley) report on behalf of a larger 
collaboration:

We obtained a sequence of images of the field of GRB 080503 (Mao et al. 
GCN 7665
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 ) from Gemini North using GMOS. In a stack of five 180s 
exposures starting 2008-05-03 13:23:39 UT, we detect a faint source 
("s1") consistent with the Mao et al. XRT position at:

  19:06:28.694 +68:47:37.71 J2000

With a positional uncertainty of about 250 mas. Given the faintness of 
the source, we can claim neither variability nor that the source is 
pointlike, but it is possible that the source is the host and/or the 
afterglow. Note that s1 is not the source we originally identified as 
the possible afterglow in Perley et al. (GCN 7666
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 ); we now believe that 
that object identified in the Keck guider image was outside the XRT 
error circle and unrelated to the GRB.

This and other sources of interest near the XRT position are encircled 
in green in the finder:

       http://lyra.berkeley.edu/~jbloom/grb080503.png

We thank Rachel Mason of Gemini for her assistance.

Y.A. Mori, Y. Kudou, T. Shimokawabe and N. Kawai (Tokyo Tech) report on
behalf of the MITSuME collaboration: 

We observed the error box of GRB 080503 (Mao et al. GCN 7665
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 )
with the 3-color 50cm MITSuME Telescope at Akeno, Japan starting
at 2008-05-03 12:31:53 UT, 5.4 min after the trigger.
In the co-added images of Ic, Rc, and g' bands, we did not detect
any afterglow candidate in the XRT error circle.  
The 3-sigma limiting magnitudes based on USNO-B1.0 (I-band) and NOMAD
(R-band,g'-band) stars are following.

Filter  start (UT)  end (UT)    Exposure   LimitMag
---------------------------------------------------
g'      12:31:53    12:41:03    9 x 60 s     18.8
Rc      12:31:53    12:41:03    9 x 60 s     19.2
Ic      12:31:53    12:41:03    9 x 60 s     18.1
----------------------------------------------------

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

Using 307 s of overlapping XRT Photon Counting mode and UVOT
data for GRB 080503, 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 = 286.61961, +68.79330 which is equivalent
to:

RA (J2000): 19h 06m 28.71s
Dec (J2000): +68d 47' 35.9"

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), the current algorithm is an
extension of this method.

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

T. Ukwatta (GWU), S. D. Barthelmy (GSFC), W. Baumgartner (GSFC/UMBC),
J. Cummings (GSFC/UMBC), E. Fenimore (LANL), N. Gehrels (GSFC),
H. Krimm (GSFC/USRA), C. Markwardt (GSFC/UMD), K. McLean (GSFC/UMD),
J. Mao (INAF-OAB), D. Palmer (LANL), T. Sakamoto (GSFC/UMBC),
G. Sato (GSFC/ISAS), M. Stamatikos (GSFC/ORAU), J. Tueller (GSFC)
(i.e. the Swift-BAT team):
 
Using the data set from T-239 to T+963 sec from recent telemetry downlinks,
we report further analysis of BAT GRB 080503 (trigger #310785)
(Mao, et al., GCN Circ. 7665
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 ).  The BAT ground-calculated position is
RA, Dec = 286.686, 68.803 deg, which is 
   RA(J2000)  =  19h 06m 44.7s 
   Dec(J2000) = +68d 48' 09.2" 
with an uncertainty of 3.5 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 75%.
 
The mask-weighted light curve shows an initial spike starting at ~T+0.1 sec
with a fast rise to a peak at ~T+0.2 sec, with a roughly exponential decay
down to background at ~T+0.7 sec.  The softer emission starts at about T+10 sec,
rising with two peaks at about T+26 and +37 sec, and then falling to background
levels at T+220+/-20 sec.  T90 (15-350 keV) is 170 +- 40 sec (estimated error
including systematics).
 
The time-averaged spectrum from T+0.2 to T+220.6 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
2.00 +- 0.13.  The fluence in the 15-150 keV band is 2.0 +- 0.1 x 10^-6 erg/cm2.
The 1-sec peak photon flux measured from T+26.58 sec in the 15-150 keV band
is 0.9 +- 0.1 ph/cm2/sec.  All the quoted errors are at the 90% confidence level.

As to whether this burst is a SHB with extended emission or a Long burst
with a particularly short pulse at the beginning, we can not say yet.  Looking
at the 4-band lightcurves, the initial spike is harder than the later emission.
We are waiting for (a) separate spectral analysis on the two portions
of the lightcurve, and (b) for the lag analysis.  We are currently in an 8-hour
gap in the downlink sessions, so it will be a while before we can say
with better confidence if this is truely a SHB or not.  We will issue
another circular.
 
The results of the batgrbproduct analysis are available at
http://gcn.gsfc.nasa.gov/notices_s/310785/BA/

GRB 080503: Swift-XRT refined analysis


C. Guidorzi & J. Mao (INAF-OAB) report on behalf of the Swift-XRT team:

The Swift-XRT began observing GRB 080503 (trigger=310785, Mao et
al., GCN Circ. 7665
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 ) in Window Timing mode, 81 s after the BAT
trigger. The enhanced XRT position was given in Goad et al.
(GCN Circ. 7669
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 ).

The light curve from 81 s to 13.2 ks can be modelled with
the combination of an exponential with
a power law taking over at late time, with the following
best-fitting parameters: e-folding time= (72 � 4) s, power-law
index alpha= 2.2 (-1.1, +0.6). The chisq/dof=202/151 is due to
short-term variability superposed to the model.

The WT mode spectrum spanning from 81 to 282 s can be fit by a
power-law model, with a photon index of 1.27 � 0.03 and column
density consistent with the average Galactic column density in
this direction of 5.6e20 cm^-2.
The PC mode spectrum, starting from 282 s and totalling 6.3 ks
of exposure, is fit with a power law with a photon index of 2.45+/-0.25
and no evidence for any additional column density with respect
to the Galactic one.
The corresponding observed (unabsorbed) 0.3-10 keV flux is 2.9e-12
(3.9e-12) erg cm^-2 s^-1.

If the burst continues to decay at the same rate, we predict an
XRT count rate of 1.7e-6 count/s at T+24 hours, which corresponds
to an observed 0.3-10 keV flux of approximately 4e-17 erg cm^-2 s^-1.

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

P. J. Brown (PSU) and J. Mao(INAF-OAB),
on behalf of the Swift/UVOT team.

The Swift Ultra-Violet/Optical Telescope (UVOT)
began settled observations of GRB 080503 (trigger 310785)
84 seconds after the BAT trigger (Mao et al.,GCN Circ. 7639).
No afterglow is detected at the enhanced XRT position
(Goad et al., GCN Circ. 7665
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 ) in the initial white finding chart
or subsequent summed images. The limiting magnitudes
(3-sigma in 5" radius apertures) in each of the UVOT filters
are as follows:

Filter  T_start(s)  T_stop(s)  Exp(s)  Mag UL (3sig)

white       85       184        98     >20.0
white       85     12500      1314     >22.3
v          191      6749      1219     >20.5
b          671     11588      1203     >21.4
u          646      7365       471     >20.3
uvw1       622      7160       471     >20.4
uvm2       597      6954       471     >20.3
uvw2       701     13219      1133     >21.0

The values quoted above are in the UVOT photometric system
(Poole et al. 2008, MNRAS, 383, 627).  They are not corrected
for the expected Galactic extinction corresponding to a
reddening of E(B-V)=0.06 mag in the direction of the burst
(Schlegel et al. 1998).

D. Kuroda, K. Yanagisawa (NAOJ) and N. Kawai (Tokyo Tech) report on
behalf of the MITSuME collaboration: 

We observed the error box of GRB 080503 (Mao et al. GCN 7665
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 ) with the
3-color 105cm Telescope at Ishigakijima Astronomical Observatory
starting at 2008-05-03 13:49:55 UT, 1.4 hours after the trigger.
In the co-added images of Ic, Rc, and g' bands, we did not detect any
afterglow candidate.  The 3-sigma limiting magnitudes based on
USNO-B1.0 (I-band) and NOMAD (R-band,g'-band) stars are following.

Filter  start (UT)  end (UT)    Exposure   LimitMag
---------------------------------------------------
g'      13:49:55    13:59:19    9 x 60 s     20.5
Rc      13:49:55    13:59:19    9 x 60 s     20.2
Ic      13:49:55    13:59:19    9 x 60 s     20.1
----------------------------------------------------

T. Ukwatta (GWU), M. Stamatikos (GSFC/ORAU), T. Sakamoto (GSFC/UMBC),
J. Norris (U. of Denver), S. D. Barthelmy (GSFC)
(for the Swift-BAT team):
 
For GRB 080503 (Trigger 310785, GCNs 7665
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  & 7673),
the spectral lag analysis of the data from T-1 sec to T+1 sec
(the initial spike), yields a lag of -13 +/- 9 msec for the
15-25 to 50-100 keV bands using lightcurve binnings of 4, 8 and 16 msec.
For the on-going emission (T+10 to T+220 sec), the signal is too weak
and lacking sufficient structure to yield a significant lag measurement.

Expanding on the total-burst spectral analysis in GCN 7673
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 ,
we have divided the burst into an initial phase (T+0.0 to T+0.7 sec)
and the extended phase (T+10 to T+170 sec).  The simple power law fits are:
  Phase     PLI            Norm(@50keV)         Chi2/DoF
  Initial   1.59 +/-0.28  (8.15 +- 1.5)E-3      69/59
  Extended  1.91 +/-0.12  (1.19 +- 0.08)E-3     52/59
The units of Normalization are ph/cm2/s/keV.

This lag value plus the spectral results puts this burst
in the short hard burst category.

D. A. Perley, J. S. Bloom (UC Berkeley), and J. X. Prochaska (UCO/Lick) 
report:

Further to Bloom et al. (GCN 7667
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 ), we acquired additional imaging of 
the probable short-hard (Ukwatta et al., GCN 7677
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 ) GRB 080503 (Mao et 
al., GCN 7665
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 ) with extended emission the night after the burst.   A 
total of 10 exposures of 180 seconds each were taken in r-band between 
UT 14:08 and 14:46 (2008-05-04), 26 hours after the BAT trigger, using 
GMOS on Gemini-North.  In addition, we report analysis of additional 
imaging taken between 1.0-2.5 hours after the burst in g, r, i, and z 
filters.

The source noted in our previous circular, s1, does not appear to be 
variable between images, and no other sources are detected in the XRT 
error circle to a very deep limit of r >~ 26.5 during the first night. 
However, a new source ("s2") appears in the second night's observation 
that was not detected in our r-band imaging during the first night.  If 
this source is associated with GRB 080319B this would be extremely 
unusual behavior for a short gamma-ray burst.  It is possible that the 
rebrightening is the first example of the detection of a Li-Paczynski 
"mini"-supernova (1998ApJ...507L..59L; see also Kulkarni 
astro-ph/0510256). More tentatively, we note that s1 may the host galaxy 
(we note that all previous SHB's with extended emission have been seen 
at small offset from their host galaxies: Troja et al., astro-ph/0711.3034).

The source is located near the center of the enhanced XRT error circle 
(Goad et al, GCN 7669
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 ) at:

RA=19:06:28.766  Dec=+68:47:35.53 (J2000)

(uncertainty is 250 mas in both coordinates relative to the ICRS)

And can be seen at the following links:
http://lyra.berkeley.edu/~jbloom/grb080503-comp.png
http://lyra.berkeley.edu/~jbloom/grb080503-rebrightening.mpeg
http://lyra.berkeley.edu/~dperley/080503/080503_s2b.png

We report the following photometry, calibrated relative to USNO-B1.0:

Acquisition r-band image (t=0.98 hr)
s1: R = 25.56 +/- 0.33
s2: R > 25.6

First night r-band stack (t=1.5 hr)
s1: R = 26.00 +/- 0.18
s2: R >~ 26.5

Second night r-band stack (t=26.0 hr)
s1: R = 25.70 +/- 0.12
s2: R = 25.28 +/- 0.09

UT_start exposure   filter magnitude
13:23    1x180      r      R = 24.8 +/- 0.3
13:38    1x30+2x300 R      R > 25.5 (2-sigma)
13:47    5x180      r      R = 25.4 +/- 0.2

Photometric uncertainties do not include calibration uncertainties 
relative to USNO-B1.0, which are at the level of 0.1 magnitude.  (This 
does not affect the relative magnitudes between the images).

In our images in other filters, s2 is not detected in i-band or z-band 
during the first night.  It may be marginally detected in g-band imaging 
near the limiting magnitude of that observation.

Further observations of this source are planned and strongly encouraged.

D. A. Perley, J. S. Bloom (UC Berkeley), and J. X. Prochaska (UCO/Lick)
report:

The line that states:

"If this source is associated with GRB 080319B this would be extremely 
unusual behavior for a short gamma-ray burst."

Should of course read:

"If this source is associated with GRB 080503 this would be extremely 
unusual behavior for a short gamma-ray burst."

We apologize for any confusion, and thank R. Chornock for pointing out 
the error.

D. A. Perley and J. S. Bloom (UC Berkeley) report on behalf of a larger 
collaboration:

Starting at UT 11:32 on the night of 2008-05-05, we began a series of 
additional r-band imaging follow-up observations of the optical 
counterpart associated with GRB 080503 (GCN 7665
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 , Mao et al.), using 
Gemini-North (+GMOS).

The new source (s2) reported in our previous circular (GCN 7678
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 ) is 
still present at about the same magnitude.  Using a stack of 9x180s 
images, we present the following measurement of the magnitudes of s1 and s2:

s1: R = 26.33 +/- 0.20
s2: R = 25.46 +/- 0.09

This is consistent (within about 2-sigma) with the previous night's 
measurement and suggests neither additional rebrightening nor dramatic 
fading.  We continue to strongly encourage follow-up with large-aperture 
instruments.


We also issue the following correction to GCN 7678
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 : the data table at 
the bottom (with heading "UT_start exposure filter magnitude") is not 
valid.  This is photometry of s1 based on an older calibration, and 
including a Keck R-band limiting magnitude that will be refined in a 
future circular.  The preceding photometry of s1 and s2 in that circular 
is still valid.

To reiterate, the r-band evolution of the optical counterpart (s2) to 
date, calibrated to USNO B1.0 R-band, is given by:

t(hr)  magnitude
0.98   R > 25.6
1.5    R > 26.5
26.0   R = 25.28 +/- 0.09
47.4   R = 25.46 +/- 0.09

Further observations are ongoing.

Many long GRBs have optical light-curves which are initially slowly 
rising. The optical light curve of the short hard burst (SHB) 080503 (Mao 
et al. GCN 7665
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 ; Ukwatta et al. GCN 7673
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 ; Guidorzi et al. GCN 7674
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 ) 
discovered by Perley et al. (GCNs 7678
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 , 7680
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 ) may be their analog 
slow-rise optical afterglow (AG) changing to a plateau phase and entering 
the gradual transition to an asymptotic decline, which, in the CB model, 
has roughly the form,

            Fnu ~ [1/(t+t_b}^{\beta_O+1/2}] * nu^{-beta_O}

for a constant low-density SHB environment, or

            Fnu  ~ [1/ (t+t_b)^{beta_O+1}] * nu^{-beta_O}

for an isothermal sphere environment with a density propto 1/r^2.
t_b is the bending time of its canonical AG, and beta_O = Gamma - 1 ~ 1.1 is 
the spectral index above the bend frequency. In the CB model, the initial 
increase of the CB's radius, R^2 ~ t^2/(t^2+t_exp^2), where t_exp is the time 
beyond which the CB's radius settles to its approximate constant value, 
produces the initial rise of the synchrotron AG.  The low density environments 
of SHBs yields typically, t_b~ 10^5 s, and a long expansion time, t_exp, of the 
CB's radius relative to its values in slow-rise optical AGs of long GRB such as 
071010A (Covino et el. arXiv:0804.4367).

A. de Ugarte Postigo, (ESO), C. C. Th�ne, D. Malesani,
J. P. U. Fynbo (DARK) and T. Purismo (NOT) on behalf
of a larger collaboration report:

We have observed the field of GRB 080503 (Mao et al.,
GCN 7665
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 ) from the 2.5m NOT (+MOSCA) under bad
atmospheric conditions. We do not detect the object reported
by Perley et al. (GCN 7678
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 ) down to a  3-sigma limiting
magnitude of R~21.7 at a mean time of 6.091 May 2008 UT
(2.573 days after the burst).

Dale A. Frail (NRAO) and Poonam Chandra (NRAO/UVA) report on behalf
of the Caltech-NRAO-Carnegie GRB Collaboration:

"We used the Very Large Array to observe the field of view toward GRB
080503 (GCN 7665
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 ) at a frequency of 8.46 GHz at 2008 May 6.57 UT.
We do not detect the GRB afterglow in 2.5 hours of VLA observations.
The 3-sigma upper limit on the afterglow flux is 54 uJy (rms 18 uJy).

The National Radio Astronomy Observatory is a facility of the National
Science Foundation operated under cooperative agreement by Associated
Universities, Inc."

H. Tanaka, N. Ohmori, H. Hayasi, R. Hara, K. Kono, M. Yamauchi
(University of Miyazaki)

We have observed the field covering the error circle of
GRB080503 (GCN 7669
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 , M.R. Goad et al.) with
the unfiltered CCD camera on the 30-cm telescope at
University of Miyazaki.
The observation was started 15:25:55 UT; about 180 minutes
after the Swift trigger time.

We have compared our data of 30 sec exposures
with the USNO-A2.0 catalog,there is no new source
at the reported position

the upper limits are as follows:
--------------------------------------------------------------
Start(UT) End(UT)	Num. of frames 	Limit (mag.)
--------------------------------------------------------------
15:26:57 15:27:27 	1 		~16.82
15:26:57 16:24:33 	57 		~17.43
---------------------------------------------------------------

D. A. Perley, J. S. Bloom, and W. Li (UC Berkeley), on behalf of a 
larger collaboration, report:

We have continued to monitor the optical transient (Perley et al., GCN 
7678
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 ) associated with GRB 080503 (Mao et al., GCN 7665
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 ) using Gemini-N 
(+GMOS) with additional exposures in r-band nightly since the event.

The optical counterpart remains detected in all images to date (through 
2008-05-07 UT) and is significantly, but slowly, fading.  This confirms 
the lack of further rebrightening (see also Perley et al., GCN 7680
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 ) and 
further distinguishes the behavior of the transient from ordinary long- 
and short-burst afterglows.  Alternatively, a faint host galaxy could be 
contributing to the later exposures somewhat; further follow-up may help 
resolve this ambiguity.

J. S. Bloom (UC Berkeley), W. Li (UC Berkeley), A. J. Levan (U. 
Warwick), D. A. Perley (UCB),  N. R. Tanvir (U. Leicester), N. R. Butler 
(UCB),  H-W. Chen (U. Chicago), R. Chornock (UCB), J. X. Prochaska 
(UCO/Lick), C. D. Bailyn (Yale), A. Bunker (U. Exeter), R. Chornock 
(UCB), B. Cobb (Yale), P. B. Hall (York U.), A. V. Filippenko (UCB), N. 
Gehrels (NASA/GSFC), K. Glazebrook (Swinburne), J. Granot (U. 
Hertfordshire), K. C. Hurley (UCB), D. Kocevski (UCB), B. Metzger (UCB), 
M. Modjaz (UCB), A. Miller (UCB), W. Lee (I. de Astronomia), S. Lopez 
(U. Chile), J. Norris (NASA/Ames), P. E. Nugent (LBNL), M. Pettini (U. 
Cambridge), D. Poznanski (UCB), A. L. Piro (UCB), E. Quataert (UCB), E. 
Ramirez-Ruiz (UCSC), J. Shiode (UCB), and T. Sakamoto (GSFC/UMBC) report:

Under Director's Discretionary Proposal (#11551), we acquired 
observations of GRB 080503 (Mao et al., GCN 7665
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 ) with the Hubble Space 
Telescope (HST+WFPC2), starting at 15:08 and lasting until 20:45 (UT 
2008-05-08), approximately 5.2 days after the GRB.  The total 
integration time was 2x1200s + 2x1100s (2 orbits) in F606W, 3x700s (1 
orbit) in F814W, and 3x700s (1 orbit) in F450W. The data were collected 
in a small dither pattern around the WFALL position to mitigate against 
CTE losses.

We clearly detect a point source at a location consistent with the 
optical transient (Perley et al., GCN 7678
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 ) in F606W, and marginally 
detect a source at this position in 450W.  The transient is not detected 
in F814W.  We report the following preliminary magnitudes:

F450W_AB = 26.95 +/- 0.27
F606W_AB = 26.86 +/- 0.15
F814W_AB > 26.6

The nearby source s1 (see Perley et al., GCN 7667
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 ) is also detected, and 
appears visibly extended, though this extension does not reach s2.  Two 
other faint, extended sources ("s3" and "s4") are detected about 1 
arcsecond to the northwest and southeast of the transient, respectively. 
  There is no obviously extended source detected at the position of s2.

The source is significantly fainter than observed in our Gemini 
measurements, which may suggest a break (as expected from an off-axis 
jet) or an exponential decline (as expected from an LP mini-supernova). 
  However, LP mini-SN models also predict a red late-time color, which 
is not observed.  We note also the detection of an X-ray afterglow in 
late-time Chandra observations (Butler et al., GCN in prep.)

An additional epoch of HST imaging will be obtained starting 15:05 
12-May-2008 UT.

An preliminary image with less-than-optimal cosmic ray reduction is 
posted to:
http://lyra.berkeley.edu/~dperley/080503/080503_hst660w_ep1.png

We are grateful to all the people STSCI that made such a rapid ToO possible.

N. R. Butler (UC Berkeley), J. S. Bloom (UCB), W. Li (UCB), A. J. Levan (U.
Warwick), D. A. Perley (UCB),  N. R. Tanvir (U. Leicester), H-W. Chen (U.
Chicago), R. Chornock (UCB), J. X. Prochaska (UCO/Lick), C. D. Bailyn (Yale),
A. Bunker (U. Exeter), R. Chornock (UCB), B. Cobb (Yale), P. B. Hall
(York U.), A. V. Filippenko (UCB), N. Gehrels (NASA/GSFC), D. Burrows (PSU),
K. Glazebrook (Swinburne), J. Granot (U. Hertfordshire), K. C. Hurley (UCB),
D. Kocevski (UCB), B. Metzger (UCB), M. Modjaz (UCB), A. Miller (UCB), W.
Lee (I. de Astronomia), S. Lopez (U. Chile), J. Norris (NASA/Ames), P. E.
Nugent (LBNL), M. Pettini (U. Cambridge), D. Poznanski (UCB), A. L. Piro
(UCB), E. Quataert (UCB), E. Ramirez-Ruiz (UCSC), J. Shiode (UCB), and T.
Sakamoto (GSFC/UMBC) report:

From 2008/05/07 19:18:23 UT to 2008/05/08 04:09:59 UT (4.29 - 4.66 days post
burst), Chandra Director's Discretionary Time observations with ACIS-S were
conducted of the field of GRB 080503 (Mao et al. 2008; GCN7665
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 ) for a
total exposure of 29.8 ksec (livetime).  We find a highly significant
detection of one X-ray point source in the XRT error circle (Goad et al.
2008; GCN7669
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 ) at:

RA, Dec = 19:06:28.76, +68:47:35.3 +/- 0.5"  (J2000).

The source is 0.2" away from the Gemini optical source (S2) in Perley et al.
(2008; GCN7678
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 ), and is 0.7" away from the center of the XRT error circle.
The X-ray source is therefore highly likely to be associated with S2, which
is in turn highly likely to be associated with GRB 080503.  Astrometry is
established using Chandra field sources also detected in the Gemini images.

We detect 40+/-6 counts in the 0.5-8.0 keV band.  There is weak evidence
in favor of variability.  During the first half of the exposure, we measure
21+/-5 cts, as compared to 19+/-4 counts in the second half.  Testing
against the NULL hypothesis of a steady source, we find KS-prob=0.23.

The Chandra flux implies a flattening of the X-ray flux relative to
the rapid decay observed prior to ~13 ksec by the Swift XRT (Guidorzi et al.
2008; GCN7674
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 ).  We note also that XRT observations at late time (2.4-3.2
days post burst) exhibit a flux consistent with that of Chandra, yielding a
marginal (~1-sigma) XRT detection.

Minimizing the Cash (1976) statistic, we find the Chandra spectrum to
be acceptably fit by an absorbed powerlaw with photon index Gamma=1.5+/-0.5
and unabsorbed flux Fx(0.3-10 keV) = (1.5+/-0.7) x 10^(-14) erg/cm^2/s.  We
assume Galactic absorption only, as observed for the Swift XRT spectrum
(Guidorzi et al. 2008; GCN7674
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 ).

Roughly, the X-ray flux is consistent with an extrapolation of the optical
flux as observed by HST (Bloom et al. 2008; GCN7703
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 ), assuming the X-ray
spectral index.  If the optical flux is due to a a Li-Paczynski
"mini"-supernova (1998ApJ...507L..59L) as suggested by Perley et al. (2008;
GCN7678
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 ), then the X-ray light curve may be more easily explained by
invoking a separate emission mechanism.  Alternatively, both the optical
and X-ray fluxes may be due to shock energy re-injection or flaring 
occuring on timescales ~1-4 days post burst.

More detailed analyses are ongoing.

We thank Harvey Tananbaum, Patrick Slane, Andrea Prestwich, and the
rest of the Chandra observatory staff for their impressive rapid scheduling
and execution of this observation.

This message can be cited.

D. A. Perley (UC Berkeley), J. S. Bloom (UCB), A. J. Levan (U. Warwick), 
W. Li (UCB), N. R. Tanvir (U. Leicester), N. R. Butler (UCB),  H-W. Chen 
(U. Chicago), R. Chornock (UCB), J. X. Prochaska (UCO/Lick), C. D. 
Bailyn (Yale), A. Bunker (U. Exeter), R. Chornock (UCB), B. Cobb (Yale), 
P. B. Hall (York U.), A. V. Filippenko (UCB), N. Gehrels (NASA/GSFC), K. 
Glazebrook (Swinburne), J. Granot (U.  Hertfordshire), K. C. Hurley 
(UCB), D. Kocevski (UCB), B. Metzger (UCB),  M. Modjaz (UCB), A. Miller 
(UCB), W. Lee (I. de Astronomia), S. Lopez (U. Chile), J. Norris 
(NASA/Ames), P. E. Nugent (LBNL), M. Pettini (U. Cambridge), D. 
Poznanski (UCB), A. L. Piro (UCB), E. Quataert (UCB), E. Ramirez-Ruiz 
(UCSC), J. Shiode (UCB), and T. Sakamoto (GSFC/UMBC) report:

Further to GCN 7703
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  (Bloom et al.), we obtained additional imaging using 
the Hubble Space Telescope (+WFPC2) of GRB 080503 (Mao et al., GCN 7665
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 ) 
starting at 15:05:05 on UT 2008-05-12, about 9.2 days after the trigger. 
Four exposures of 1000s each were acquired in F606W and in F814W.

The afterglow has faded significantly since the previous HST epoch, and 
is not detected in either filter, or in a combined stack of the imaging 
from both filters.  The approximate limiting magnitude is F606W(AB) > 
28.0, confirming the rapid fading observed previously.  No sign of 
host-galaxy emission is present in either filter.

N. R. Butler (UC Berkeley), J. S. Bloom (UCB), W. Li (UCB), A. J. Levan (U.
Warwick), D. A. Perley (UCB),  N. R. Tanvir (U. Leicester), H-W. Chen (U.
Chicago), R. Chornock (UCB), J. X. Prochaska (UCO/Lick), C. D. Bailyn (Yale),
A. Bunker (U. Exeter), B. Cobb (Yale), P. B. Hall (York U.), A. V. Filippenko
(UCB), N. Gehrels (NASA/GSFC), D. Burrows (PSU), K. Glazebrook (Swinburne),
J. Granot (U. Hertfordshire), K. C. Hurley (UCB), D. Kocevski (UCB), B.
Metzger (UCB), M. Modjaz (UCB), A. Miller (UCB), W.  Lee (I. de Astronomia),
S. Lopez (U. Chile), J. Norris (NASA/Ames), P. E.  Nugent (LBNL), M. Pettini
(U. Cambridge), D. Poznanski (UCB), A. L. Piro (UCB), E. Quataert (UCB), E.
Ramirez-Ruiz (UCSC), J. Shiode (UCB), and T.  Sakamoto (GSFC/UMBC) report:

From 2008/05/25 18:11:36 UT to 2008/05/26 03:04:28 UT (22.24 - 22.61 days post
burst), Chandra Director's Discretionary Time observations with ACIS-S were
conducted of the field of GRB 080503 (e.g., Mao et al. 2008; GCN 7665
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 ) for a
total exposure of 29.8 ksec (livetime).  In a 3-pixel radius source extraction
region centered around the first epoch Chandra source (Butler et al. 2008;
GCN 7704
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 ), we find no source counts in the 0.5-8.0 keV band.

We derive a 2-sigma upper limit to the source flux of Fx(0.3-10 keV) < 9.9 x
10^(-16) erg/cm^2/s.  Relative to the first epoch flux ~18 days earlier,
the 2nd epoch flux implies a fade steeper in time than t^(-1.6).  A similarly
steep or steeper fade is implied in the optical band from Gemini and HST
observations of this source (Perley et al. 2008; GCNs 7667
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 , 7678
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 , 7679
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 , 7680
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 ,
7695
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 , 7703
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 , 7749
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 ).  Combined with a rough agreement in the extrapolated flux
levels assuming a GRB afterglow external shock model (e.g., GCN 7704
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 ), the
lightcurve similarity supports an interpretation where the external shock is
responsible for both the X-ray and optical emission.

More detailed analyses are ongoing.

We thank Harvey Tananbaum and the Chandra observatory staff for their
impressive rapid scheduling and execution of this observation.

This message can be cited.