GRB 050813

Derek B. Fox (Penn State) and S. Bradley Cenko (Caltech) report on
behalf of a larger collaboration:  

"The Robotic 60-inch Telescope at Palomar Observatory (P60) responded
automatically to Swift Trigger 150139, recording a series of images
beginning at 06:50:30 UT, approximately 6 minutes after the burst.

Visual inspection of these images does not reveal any new objects by
comparison to the second-generation DSS images (XDSS).  Times and
approximate limits (calibrated against USNO-B1.0 catalog photometry)
for new point sources within the BAT error circle are as follows:

    Band    Exp     Epoch       Limit
   ==========================================
      R     60s     06:51:00   R > 18.3 mag
      i     60s     06:53:22   I > 17.0 mag
      R     3x60s   06:58:18   R > 18.9 mag
   ==========================================

Observations will continue as long as the source is visible."

A. Retter (PSU), L. Barbier (GSFC), S. Barthelmy (GSFC), A. Blustin (MSSL),
D. Burrows (PSU), A. Cucchiara (PSU), N. Gehrels (GSFC), 
O. Godet (U. Leicester), J. Kennea (PSU), C. Markwardt (GSFC/UMD),
D. Morris (PSU), D. Palmer (LANL), J. Racusin (PSU), P. Roming (PSU)
on behalf of the Swift team, report:
 
At 06:45:09.8 UT, Swift-BAT triggered and located GRB050813
(trigger=150139).  The spacecraft slewed immediately.  The BAT
on-board calculated location is RA,Dec 242.010, +11.252 {+16h 08m 02s, 
+11d 15' 07"} (J2000), with an uncertainty of 3 arcmin (radius,
90% containment, stat+sys).  There is a single peak with an
approximate duration of 0.7 sec FWHM.  The peak rate is approx 2000
ct/s in the 15-350 keV band.

At 06:46:23 UT XRT attempted to centroid on the BAT position, 73 secs
after the BAT trigger. No position was acquired by the onboard
software and the TDRSS lightcurve shows no evidence for a decaying
x-ray source.  We can not rule out the presence of a weak x-ray source
like that seen for the early afterglow of short GRB 050509b.  We await
ground analysis of the early XRT data following the first Malindi
pass.

The UVOT began observing this trigger at 06:46:52.0 UT, 102 seconds
after the BAT trigger. No new source (compared to the DSS image) is
observed in the 100 second V-band UVOT GeNI image down to a 5-sigma
upper limit of 18.15 mag.

This trigger (=150139) appears to be a real GRB, as opposed to the
recently retracted trigger (=150131).

D. C. Morris, D. N. Burrows, J. A. Kennea, J. L. Racusin, N. Cucchiara, A. 
Retter (PSU), and N. Gehrels (GSFC) report on behalf of the Swift XRT team:

The Swift XRT began observing the short burst GRB 050813 (Retter et al., 
GCN 3788
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 ) at 06:46:23, but was unable to obtain an on-board centroid due to 
the lack of any bright source in the field of view.  Analysis of the 
ground-processed data finds a very faint (~0.01 cps), uncataloged source in 
the first orbit of data at coordinates

RA(J2000) = 16:07:57
Dec(J2000) = +11:14:52

Because of the faintness of the source, we estimate an uncertainty of 10 
arcseconds radius in this preliminary position.

This object is not visible in the second orbit.  We therefore consider it 
to be the afterglow of the GRB 050813.  We note the similarity (in terms of 
the extreme weakness of the X-ray counterpart) with short burst GRB 050509B.

A. J. Blustin (UCL-MSSL), A. Retter (PSU), M. J. Page (UCL-MSSL),
J. Nousek (PSU), W. Voges (MPE), N. Gehrels (GSFC) on behalf of
the Swift UVOT team

Swift/UVOT began observing the field of GRB050813 at 06:46:52.0 UT,
102 s after the BAT trigger (Retter et al., GCN 3788
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 ). We detect no
new source with respect to the DSS within the XRT error circle
(Morris et al., GCN 3790
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 ) down to the following 3-sigma upper
limits:

Filter   T_range (s)   Exp (s)  3sig UL (mag)

V        103-963       188      19.1

B        249-1200      188      20.2

U        235-1096      188      19.9

UVW1     221-991       98       19.7

UVM2     207-977       98       19.3

UVW2     264-1305      188      20.6

Where T_range is the time in seconds since the BAT trigger.

There is a known star at the edge of the XRT error circle at the
J2000 coordinates 16:07:57.41 +11:14:42.9 with a V magnitude of
15.9 measured by the UVOT.

These magnitudes are based on preliminary zero-points, measured in
orbit, and will require refinement with further calibration.

E. Berger (Carnegie Observatories) reports:

"Inspection of DSS and 2MASS images reveals a single bright source on the
SE edge of the XRT error circle (GCN 3790
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 ) of GRB 050813 (GCN 3788
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 ) 
located at:
        RA = 16:07:57.39
        DEC = 11:14:42.95

Photometry of this source obtained from the 2MASS and USNO-B catalogs
suggests that this object is most likely a star, and not a bright
elliptical galaxy.  In particular, J-K = 0.4 mag and B-K = 2.3 mag, as
opposed to J-K ~ 1-1.3 mag and B-K ~ 4.3-5.4 mag for a typical elliptical
galaxy at redshifts of 0.1-0.3 (similar to the redshifts of previous short
bursts).

The photometry is as follows:
B = 16.61 mag
R = 15.67 mag
I = 14.72 mag
J = 14.68 mag
H = 14.37 mag
K = 14.28 mag

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

At 06:45:09.76 UT Swift-BAT detected GRB 050813 (trigger=150139) (GCN
Circ 3788
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 , Retter et al.).  The refined BAT ground position is
(RA,Dec) = 241.988, +11.246, [deg; J2000] +- 3 arcmin, (95%
containment radius).  The partial coding was 57%.  This is 7 arcsec from the
XRT position (Morris et al., GCN 3790
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 ).

The burst has a short spike lasting ~32 msec, overlayed on broader
peak of duration ~0.6 sec.  There is a hint of a second peak at T+1.3
seconds.  T90 (15-350 keV) is (0.6 +- .1) seconds (estimated error
including systematics).

The photon index of the time-averaged spectrum is 1.19 +- 0.33.  
The fluence in the 15-350 keV band is (1.24 +- 0.46) x 10^-7 erg/cm2.  
The 1-s peak photon flux measured from T+0.1 second in the 15-350 
band is (1.22 +- 0.26) ph/cm2/s.  All the quoted errors are at the 90% 
confidence level.

W. Li, University of California, Berkeley, on behalf of the 
KAIT GRB team, report: 

"The robotic 0.76-m Katzman Automatic Imaging Telescope (KAIT)
at Lick Observatory observed GRB050813 detected with Swift
(Trigger 150139; Retter et al. GCN 3788
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 , Sato et al. GCN 3793
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 ).
A series of images was automatically obtained from 06:45:49 UT
(39s after the burst) to 07:20:36 UT (2126s after the burst).
The sequence includes a combination of images taken with the V
and I filters, as well as some that are unfiltered. We did not
detect a new object in our images when compared to the DSS II
images. In particular, no new object was found within the error
circle of the Swift XRT position as reported by Morris et al.
(GCN 3790
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 ). Our first 20s unfiltered image started at 39.2s after
the burst has a limiting magnitude of about 18.6 when compared to
the USNO-B1.0 catalog."

This message may be cited.

Daniel J. Eisenstein (Arizona) and Richard Cool (Arizona)
report on behalf of the Sloan Digital Sky Survey (SDSS) Collaboration:

The Sloan Digital Sky Survey imaged the region of GRB 050813 on May 
8 & 10, 2005 (MJD 53499.42 and 53501.25, roughly 90 days before the burst
trigger; the XRT position itself was imaged on May 10).  These data will
be part of a future SDSS data release, but as they should be useful as
a pre-burst comparison image and for calibrating photometry, we are
supplying the images and photometry measurements for this GRB field.
SDSS spectroscopy has not yet been performed here.

Data from the SDSS is being placed at 
    http://cmb.as.arizona.edu/~eisenste/grb050813
These include a JPG and 2 files of photometry and astrometry.  We plan
to supply FITS image cutouts from the SDSS next week, but technical
problems have stalled us this weekend.


Inspecting the images and catalogs, the conclusion most immediately
relevant to the topic of short-hard bursts is that there is no bright
galaxy anywhere close to the GRB position.

To the limit of the SDSS imaging, there are only 2 objects within 15" of
the XRT position.  One is a bright star discussed by Berger (GCN 3792
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 ):
RA = 241.98915, Dec = 11.24521, u = 17.286, g = 16.112, r = 15.694,
i = 15.552, z = 15.481.  The photon errors on this photometry are 1%
or less in all bands.  The colors of this object are that of an F star
and the morphology is point-like; it is extremely unlikely that this
object is extragalactic.

The other object is faint: RA = 241.98688, Dec = 11.25030, g = 22.36+-0.1,
r = 21.34+-0.06, i = 20.87+-0.06, and essentially undetected in u and z.
The above are model magnitudes; the Petrosian magnitudes are brighter by
several tenths of a magnitude but have much larger errors.  This object
is also point-like in the SDSS imaging, and the g-r/r-i colors are that
of a K star.  Hence, this object is probably also a star, but it is close
enough to the SDSS detection limit that a deeper image would be needed to
be certain of this.

Going out to 19" from the XRT position, there is a third point-like
object: RA = 241.98325, Dec = 11.24473, g=22.44+-0.1, r=20.95+-0.05,
i=19.74+-03, z=19.18+-0.07.  This is very likely an M star.

There are 2 extended sources at r=21 within 1 arcminute of the XRT position,
a blue galaxy at RA=241.97903, Dec=11.24401 (33" from the XRT position) 
and a red one at RA=241.97762, Dec=11.24578 (36" from the XRT position).
There are no galaxies brighter than r=19.5 within 3 arcminutes of the XRT
position.


The file grb050813.jpg contains a simple JPG made from the SDSS gri
images.  North is *not* up in this image; rather, north is about 2:30
on the clock and east is at 5:30.  The bright star mentioned above is
the one close to the center.  The second object is the nearest faint
object to the upper right.  The XRT position is halfway in between these.
The image is about 4' on a side.


In the file grb050813.sdss_objects.dat, we report photometry of 201
objects detected by SDSS within 3' radius of the Swift BAT position.
We have removed saturated objects and objects fainter than 23.0 in the
r-band model magnitude.

All quantities are standard SDSS photometry, meaning that they are
very close to AB zeropoints and are quoted in asinh magnitudes.
Photometric zeropoints are known to about 2% rms; photon noise can be
much worse, of course.  See documentation for details.  None of this
photometry is corrected for dust extinction.  The Schlegel, Finkbeiner,
and Davis (1998) predictions for this region are A_u = 0.288 mag, A_g =
0.212 mag, A_r = 0.154 mag, A_i = 0.117 mag, and A_z = 0.083 mag.

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
GCN 3792
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 ; we have not checked the offsets in this region.

See the SDSS DR4 documentation for more details: http://www.sdss.org/dr4.

These data have been reduced with the DR4 data pipelines; however, they
are not part of that data release.  We cannot guarantee that the values
here will exactly match those of the data release in which these data
are included.


To aid with calibration of other data sets, we also report the
astrometry and photometry of 147 brighter stars (r<20.5) in a wider
region (roughly 6') around the burst.  These data are in the file
grb050813.sdss_calibstars.dat.  Beware that some of these stars are not
well-detected in the u-band; use the errors to monitor this.


This note can be cited, but please also cite the SDSS data release paper,
Abazajian et al. (AJ, 129, 1755, 2005), when using the data or referring
to the technical documentation.

J. Gorosabel, S. Guziy, A. Sota, A.J. Castro-Tirado,
A. de Ugarte Postigo, M. Jelinek, (IAA-CSIC), report:

"We have taken 7x600s I-band images of the XRT error box
  of GRB 050813 (GCN Circ. 3790
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 ) with the 1.5m OSN
  telescope. Several objects are seen close/consistent
  with the XRT error circle. A finding chart  can be found at:

   http://www.dsri.dk/~jgu/grb050813/FCs/GRB050813.OSN.gif

  Objects B and C show an extended shape. Further observations
  will required to see variability of those (and other) objects."

I. Bikmaev, A. Galeev, N. Sakhibullin (KSU/AST), 
R. Burenin, M. Pavlinsky, R. Sunyaev (IKI),
Z. Aslan, I. Khamitov, U. Kiziloglu (METU), A. Alpar (SabUni),  

report: 
 
We have observed the error box of GRB 050813 (GCN 3788
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  and GCN 3790
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 )
with Russian-Turkish 1.5-m telescope (Bakyrlytepe, Turkey), using TFOSC
(the imaging and spectroscopic camera of the TUBITAK National
Observatory, Turkey). We made series of 60s exposures in R during UT
18:35 - 20:25, August 13, 2005.

We found almost all faint objects shown in GCN 3796
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  (Gorosabel et al.)
in our co-added image with limiting magnitude R ~= 23.
 
A finding chart can be found at:
 
http://hea.iki.rssi.ru/~rodion/050813/r.jpg
 
This message may be cited.

Mike Gladders, Edo Berger (Carnegie Observatories), Nidia Morrell, and
Miguel Roth (Las Campanas Observatory) report:

"We imaged the field of GRB 050813 (GCN 3788
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 ) with the PANIC IR camera on
the Magellan/Baade 6.5-m telescope starting on 2005 Aug. 14.017 UT.
Observations were obtained with the K-s and Y-Carnegie filters, under poor
conditions (variable thick clouds, bright moonlight, and poor seeing of
~1").  Initial processing of these data suggest that this line of sight is
centered on a high-redshift galaxy cluster.  A composite Y-K color image,
as well as a greyscale image of the stacked K frame is shown at:

        http://www.ociw.edu:/~eberger/grb050813_panic.jpg

The apparent centrally concentrated population of faint galaxies with red
and uniform colors is strongly suggestive of a galaxy cluster, and looks
similar to extensive data taken with this instrument on known high-z
galaxy clusters from the Red-Sequence Cluster Survey (Gladders & Yee
2005). The apparent size of the core of this distribution (~1 arcmin) and
the magnitude of the objects suggest this putative cluster is at high
redshift (above z=0.5, and likely at z~1). The objects noted as B and C by
Gorosabel et al. (GCN 3796
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 ) in the XRT error circle are both red and
likely cluster ellipticals, and both are approximately K=19, based on an
initial calibration of the image to the 2MASS magnitude of the bright star
to the immediate SE of these objects.

Given the low probability of a chance coincidence we suggest that GRB
050813 most likely occurred in the cluster environment.  This supports
recent associations of short GRBs with old galaxies (050509b: Bloom et al.
astro-ph/0505480; 050724: Berger et al. astro-ph/0508115).  The likely
higher redshift compared to the latter two events may also explain the
faintness of the X-ray afterglow (GCN 3790
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 )."

Further observations are planned.

E. Berger (Carnegie Observatories) reports:

"We reduced and analyzed spectra of galaxies B and C in the XRT error
circle of GRB 050813 (GCN 3788
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 ) obtained on Aug 14.35 UT with GMOS on the
Gemini north telescope under program GN-2005B-Q-6.  Two 1800-sec exposures
were obtained with a 0.75" slit using the R400 grating.  We find that both
galaxies exhibit clear Ca H&K, H-beta, and H-gamma absorption at a
redshift of z=0.722.  The identical redshifts and the nature of the
galaxies (early types dominated by old stellar populations) support the
conclusion of Gladders et al. (GCN 3798
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 ) from Magellan imaging that the
position of GRB 050813 is centered on a galaxy cluster at z~0.5-1.  At
this redshift, given K~19 mag, galaxies B and C are about 0.5 L* compared
to the 2MASS luminosity function.

Assuming that this is the redshift of GRB 050813, we find an isotropic
equivalent gamma-ray energy of 1.7e50 erg (fluence = 1.24e-7 erg/cm^2;  
GCN 3793
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 ), which is somewhat larger than that of GRB 050724."

GRB 050813: Gemini Imaging

J. S. Bloom reports:

"Imaging acquired in i-band with GMOS/Gemini-N on Aug 14.4 UT under
program GN-2005B-Q-6 reveals a compact faint source 2.3 arcseconds to the
South-East of the centroid of galaxy C (Gorosabel et al.; GCN #3796
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 ). We
cannot confirm variability at this time but if it is at the redshift of
this galaxy (z=0.722; Berger GCN #3801
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 ) then it lies 16.5 kpc in
projection from the galaxy center, suggesting it is a viable counterpart
to the short-hard GRB 050813 (Retter et al., GCN 3788
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 )."

An image will be posted shortly at
http://astro.berkeley.edu/~jbloom/grb050813-gemini.jpg

We thank K. Roth and the Gemini Observing Staff for assistance in
acquiring this data.

Edo Berger and Mike Gladders (Carnegie Observatories) report:

"We imaged the field of GRB 050813 (GCN 3788
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 ) with IMACS on the
Magellan/Baade telescope on Aug 14.98 UT.  Observations were obtained in
R-band under mostly clear but non-photometric conditions.  In addition to
galaxies B and C we also detect an object at the position of the source
proposed as a possible afterglow by Bloom (GCN 3802
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 ).  Since the object
has not faded away in over 14 hours (and in addition appears to be
extended) we consider it unlikely that this is the afterglow of GRB
050813."

GRB 050813: Gemini Spectra Revisited

R. J. Foley, J. S. Bloom (University of California, Berkeley), and H.-W.
Chen (MIT) report:

"We have performed an independent reduction and analysis of the spectra of
galaxies B and C in the XRT error circle of GRB 050813 (GCN 3788
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 ), and a
as yet unnamed galaxy to the north-east (referred to as X below) obtained
on Aug 14.35 UT with GMOS on Gemini-North under program GN-2005B-Q-6 and
originally reported in GCN 3801
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  by Berger.  Inspection of the spectrum of
galaxy X shows it to be at redshift z = 0.722 (consistent with the
redshift listed in GCN 3801
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  for galaxies B and C) with obvious Ca H&K and
G-band absorption but no signs of H gamma or H beta absorption.
Inspection of the spectrum of galaxy B shows it to be at redshift z =
0.718 with obvious Ca H&K absorption, but again, no signs of H gamma or H
beta absorption. Inspection of the spectrum of galaxy C shows it to not
definitively be at z = 0.722.  Although there are broad dips in the
spectrum which may correspond to Ca H&K at z = 0.722, there are other
features which conflict with this conclusion.  There are narrow emission
lines at 7353 and 7519 A observed (which would be 4270 and 4367 A at z =
0.722).

The Ca H&K lines of galaxy X have a velocity width of 591 +/- 97 km/s.
The presence of G-band absorption and a lack of emission lines points
towards an old stellar population.  The Ca H&K lines of galaxy B have a
width of 340 +/- 93 km/s.  There is also a strong (EW_observed = -6.0 A)
unidentified absorption line at 7402 A.  If we place this line at the
redshift of the galaxy, we notice that this could be Ti I 4305.  There is
an offset of 116 km/s between the line and the fiducial redshift if this
is the correct identification.  Another possibility is this line is due to
Mg II 4481 at z = 0.65."

This message may be cited.

P. B. Cameron (Caltech) and D. A. Frail (NRAO) report on behalf of the
Caltech-NRAO-Carnegie collaboration:

"We have undertaken VLA observations at a frequency of 8.46 GHz toward
the short-hard burst GRB050813 (GCN 3790
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 , 3793
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 ) centered on 2005
August 14.92 UT. No significant radio emission is seen within the XRT
error radius to a 3-sigma level of 55 microJy. No further observations
are planned.

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

D. Sharapov,  M. Ibrahimov (MAO), A.Pozanenko (IKI), V.Rumyantsev (CrAO) on 
behalf of larger GRB follow up collaboration report:

We observed the error box of GRB050813 (GCNs 3788
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 , GCN 3790
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 )  with 1.5m 
telescope of Maidanak Astronomical Observatory. Set of  I images were taken 
between (UT) 16:47 - 17:44, on August 13, 2005 under poor weather conditions 
(seeing of ~ 1.15").  Limiting magnitude of stacked image (6x300 s) is I ~ 
21.

Detailed photometry of the field objects which is required variable 
background accounting  is underway.

The stacked image can be found in
http://grb.rssi.ru/GRB050813/grb050813_AZT22_I.jpg.

This message may be cited.