GRB 050416

T. Sakamoto (GSFC/NRC), L. Barbier (GSFC), S. Barthelmy (GSFC), 
D. Burrows (PSU), M. Chester (PSU), J. Cummings (GSFC/NRC), 
E. Fenimore (LANL), N. Gehrels (GSFC), D. Hullinger (UMD), 
J. Kennea (PSU) , H. Krimm (GSFC/USRA), C. Markwardt (GSFC/UMD), 
D. Palmer (LANL), A. Parsons (GSFC), G. Sato (ISAS), M. Suzuki (Saitama),
M. Tripicco (GSFC-SSAI), J. Tueller (GSFC), D. Burrows (PSU),
on behalf of the Swift team:

At 11:04:44.5 UT, the Swift Burst Alert Telescope (BAT) triggered and 
located on-board GRB050416 (trigger #114753).  The BAT on-board 
calculated location is RA, dec 188.478, 21.043 (J2000) with an 
uncertainty of 3.00 arcmin (radius, 3-sigma, including estimated 
systematic uncertainty). The BAT light curve shows double 
structures with a duration of about three seconds. The peak 
count rate measured by BAT was about 3000 counts/sec in the 15 -
350 keV band.  The burst signal is mostly seen in 15-50 keV.  

The XRT observed the target field at 11:06:12 UT and detected 17 counts in  
a single CCD frame, just under the number needed for a reliable on-board  
centroid.  Initial indications based on the raw spectra and lightcurve  
transmitted via TDRSS suggest that there is a real X-ray source in the  
field.  We will issue an update following the next ground station pass at  
12:18 UT.

S. Bradley Cenko and Derek B Fox report on behalf of the Caltech-NRAO-
Carnegie GRB Collaboration:

We have imaged the error circle of the Swift GRB050416 with the
automated Palomar 60-inch Telescope.  We find the presence of a bright,
stationary source not present in the DSS at the location:

RA: 12:33:54.6
Dec: +21:03:26.7
J2000.0

We tentatively identify this object as the afterglow of GRB050416

Gemma Anderson, Marilena Salvo, Josh Rich and  Brian P Schmidt 
(Australian National University) report on behalf of a larger collaboration

"We observed a 6' region covering the BAT position of  the SWIFT burst 
GRB 050416
using the ANU 2.3m telescope on April 12.51 UT in R band.  The object 
identified by Cenko and Fox is mvisible and clearly has faded in 
comparison with their image.  This indicate that this object is likely 
the afterglow of GRB 050416

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

"We observed the field centered on the BAT position of the Swift burst
GRB 050416 (GCN#3264
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 ) using the VLA at a frequency of 8.46 GHz and
starting at 11:42 UT, about 37 minutes after the burst. There are no
bright radio sources (> 260 microJy) within 1 arcminute of the BAT
position. At the position of the afterglow candidate of Cenko and Fox
(GCN#3265
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 ), the peak radio flux is 20 +/- 51 microJy.

Further observations are planned."

J. A. Kennea, J. L. Racusin, D. N. Burrows (PSU), V. Mangano 
(IASF/Palermo), T. Sakamoto (GSFC/NRC), N. Gehrels (NASA/GSFC) report on 
behalf of the Swift XRT team:

The Swift BAT instrument detected a GRB at 11:04:45 UT on 16th April 2005 
(GCN Circ 3264
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 ). The observatory executed an automated slew to the BAT 
position and the XRT began taking data at 11:06:03 UT.  The XRT was in 
Auto state but was not able to centroid on the afterglow due to low source 
brightness. From downlinked data we find a uncataloged fading X-ray source 
located at:

RA(J2000) = 12:33:54.6,
Dec(J2000) = +21:03:24.

We estimate an uncertainty of about 5 arcseconds radius (90% containment). This 
position is 49 arcseconds from the BAT position reported in GCN 3264
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 , and 
2
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 .7 arcseconds from the P60 optical afterglow candidate reported in GCN 
3265
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 .

S. Bradley Cenko, Derek B. Fox, and Avishay Gal-Yam (Caltech) report on
behalf of the Caltech-NRAO-Carnegie GRB Collaboration:

We have imaged the field of GRB050416 with the Wide-Field Infrared
Camera (WIRC) mounted on the Palomar 200-inch Hale Telescope.  Images
were taken in Ks-band and began approximately 6 minutes after the burst.
The afterglow candidate reported in GCN 3265
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  is clearly detected.  The
object fades by 0.8 mag from approximately 11:15 UT to 12:05 UT (10 - 50
minutes after the burst), confirming the results of Anderson et al. (GCN
3266
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 ).

W. Li, R. Chornock, S. Jha, and A. V. Filippenko (University of 
California, Berkeley) report:

"The Katzman Automatic Imaging Telescope (KAIT) at Lick Observatory
robotically observed Swift GRB 050416 (Trigger #114753; Sakamoto et
al., GCN 3264
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 ). The first 3x15s R-band grid image started at 11:05:25 
UT (40.5s after the burst), which did not detect the afterglow as
observed by Cenko and Fox (GCN 3265
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 ) to a limiting magnitude of about
16.5 when compared to the USNO A2.0 catalog. Our subsequent images
have varying exposure times from 15s to 360s, and were observed with
R, I, V, and z filters. Due to the depth of the images, only a 60s
I-band image started 11:12:09 UT (444.5s after the burst) has a
marginal detection of the afterglow. Further analysis is in progress."

E.Sonoda,S.Maeno,Y.Tokunaga,M.Yamauchi
(University of Miyazaki)


"We have observed the field covering the error circle of
GRB 050416 (GCN3264
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  ; Swift-BAT Trigger time is 11:04:57 UT)
with the unfiltered CCD camera on the 30-cm telescope
at University of Miyazaki.
The observation was started 11:09:18 UT on Apr.16.
After co-adding a set of 5 images (11:09:18 - 11:14:52 UT)
of 30 sec exposures, we have compared with the USNO A2.0 catalog.
Preliminary analysis shows there is no new source brighter than
16.7 mag. at the reported position by S.Bradley Cenko et al.
(GCN3265
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 ) and J.A.Kennea et al.(GCN3268
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 )."

K. Torii (Osaka U.) reports

 "The error region of GRB 050416 (Sakamoto, et al. GCN 3264
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 ) was
observed with the ART 14 inch telescope. VRcIc imaging started at
11:05:36 UT (52s after trigger) and 60s exposure was repeated.

 As a result of the preliminary analyses, the optical afterglow (Cenko 
and Fox GCN 3265
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 ; Anderson, et al. GCN 3266
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 ) is not detected in our
frames and the following upper limit is derived relative to USNO-A2.0
(R) magnitude.

Start(UT) Magnitude Exposure 
11:07:53 >16.6Rc 60s
"

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

At 11:04:44.5  UT Swift-BAT detected GRB 050416 (trigger=114753)
(GCN Circ 3264
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 , Sakamoto et al.).  The refined BAT ground position
is (RA,Dec) = 188.490,  21.053, [deg; J2000] +- 3 arcmin, (95%
containment).  This is 0.7 arc minutes from the position determined
by the XRT (GCN Circ 3268
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 , Kennea, et al.).

The BAT mask weighted light curve shows a single peak of about one 
second duration followed by a small bump.  Most of the emission is 
in the 15-50 keV band.  T90 (15-350 keV) is (2.4 +- 0.2) seconds 
(estimated error including systematics).

The photon index of the spectrum is 2.9 +- 0.2.  The fluences in 
the 15-50 keV band and in the 15-350 keV band are 
(2.8 +- 0.2) x 10^-7 erg/cm2 and (3.8 +- 0.4) x 10^-7 erg/cm2 respectively.  
The peak photon fluxes in the 15-50 keV band and in the 15-350 band are 
(4.4 + 0.8) ph/cm2/s and (4.8 +- 0.6) ph/cm2/s respectively.  All the 
quoted errors are in 90% confidence level. 

The BAT team cautions that while there appears to be a deficit of 
counts below 20 keV, this is likely due to systematic errors in the 
response matrix being accentuated by the steep spectrum.

B. Kahharov, M. Ibrahimov,  D. Sharapov (MAO), A.Pozanenko (IKI)
V.Rumyantsev (CrAO),  G.Beskin (SAO) on behalf of larger collaboration
report:

We observed the  Swift GRB050416  (Sakamoto et al. GCN3264
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 ) with 1.5m
telescope of  the Maidanak Astronomical Observatory (MAO), Uzbekistan  The
observation started at (UT) 15:34 , April 16 in R and B bands.  We found a
source at the position afterglow (Cenko & Fox GCN3265
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 ).   A preliminary
R-photometry of first frames against of USNO-B1.0 Catalog is the following:

Mean time    Exposure    Mag.
(UT)            (s)

15:44         3x300     20.85 +/- 0.12

Observation is continuing.

G. Cusumano, V. La Parola, T. Mineo, V. Mangano (IASF/Palermo), G. 
Chincarini, S. Campana, G. Tagliaferri, P. Romano, P. Giommi, M. Perri 
(INAF-OAB), M. Capalbi (ASDC), D. N. Burrows, J. A. Kennea (PSU), N. 
Gehrels (NASA/GSFC)

We have analyzed the Swift-XRT data from the first orbit observation of 
GRB050416 (Sakamoto et al. 2005, GCN3264
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 ). The new refined coordinates 
are:

RA(J2000) = 12:33:54.8
Dec(J2000) = +21:03:25.1

This position is 17.7 arcseconds from the BAT position given in GCN 3273
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(Sakamoto et al. 2005) and 1.3 arcsec from the preliminary XRT position 
(GCN3268
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 , Kennea et al. 2005). We estimate an uncertainty of 5 arcseconds 
radius (90% containment).

The [0.2-10] keV light curve in Photon Counting (PC) mode starts ~90 
seconds from the BAT trigger (T0). We clearly detect a rapidly fading 
source. The light curve can be fitted with a single power law with alpha= 
-0.6 +/- 0.3.

A preliminary spectral fit to the PC data gives a spectral power law 
photon index of 2.02+/-0.21 in the [0.2-10] keV band, with a column 
density of (0.25+/-0.05)E22 cm^-2 (the Galactic line-of-sight absorption 
is 2.06E20 cm^-2). The average estimated unabsorbed 0.2-10 keV flux is 
1.7E-11 erg cm^-2 s^-1.

P. Schady (MSSL), T. Sakamoto (GSFC/NRC), K. McGowan (MSSL), P. Boyd 
(GSFC/UMBC), P. Roming (PSU), J. Nousek (PSU), N. Gehrels (GSFC), on behalf of 
the Swift UVOT team.

The Swift Ultra-Violet/Optical Telescope (UVOT) began settled observations of 
the field of GRB 050416 (Sakamoto et al; GCN 3264
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 ) at 11:05:49 UT, 65s 
after the BAT trigger. A comparison of the 100s finding chart in the V 
filter against the Digitized Sky Survey reverals a new source inside the 
XRT error circle at

RA = 12 33 54.56 Dec = +21 03 27.73

and consistent with the P60 positition reported by Bradley Cenko & Fox (GCN 
3265
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 ). The V band magnitude is V = 19.38. The magnitude is based on preliminary 
zero-points, measured in orbit and will require refinement with further 
calibration.

Further detailed analysis is ongoing.

P. Schady (MSSL), T. Sakamoto (GSFC/NRC), K. McGowan (MSSL), P. Boyd
(GSFC/UMBC), P. Roming (PSU), J. Nousek (PSU), N. Gehrels (GSFC), on behalf of
the Swift UVOT team.

Starting with the data taken ~207s after the BAT trigger (Sakamoto et al., GCN 
3264
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 ) we have co-added the early time data of GRB 050416 in each of the 
Ultra-Violet/Optical Telescope (UVOT) U, B, and V bands. The co-added images 
are made up of 9 x 10s exposures. A source is detected at the position of the 
afterglow (Cenko & Fox, GCN 3265
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 ) in the U and B band with magnitudes U = 19.34 
+/- 0.2 and B = 19.85 +/- 0.2. No source is detected in the V co-added 
image down to a 5-sigma limiting magnitude V > 19.57. There is ~173s 
difference between the start of the finding chart exposure, in which a 
source was detected (Schady et al., 3276) and the co-added image.

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

Y. Qiu, C.L. Lu (BAO), Y.Q. Lou (THCA), K.Y. Huang (NCU), Y. Urata (RIKEN) 
on behalf of EAFON report

" We have imaged the GRB 050416 optical afterglow position (Bradley et
al, GCN 3265
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 ) using 0.8-m telescope at XingLong Observatory, China.
The observations started at 18.65 UT (~ 7.57 hours after the
burst). Due to the bad weather condition (seeing ~ 4"), the limiting
magnitude of our R band co-added image is ~ 18.6 compared with
USNOB1.0 stars. No new source was detected under the limiting
magnitude.

This message may be cited."

K. Yanagisawa (OAO/NAO), H. Toda, and N. Kawai (Tokyo Tech) report on
behalf of the Mitsume Collaboration:

"The field of GRB050416 (Sakamoto et al.; GCN 3264
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 ) was observed by
the 3-color Mitsume 50cm Telescope at Okayama, Japan starting at 
13:27 (UT).  With the first 60 min exposure, we did not detect a
source brighter than V=18.0, R=19.6, and I=19.0 at the position of 
the afterglow reported by Cenko and Fox (GCN 3265
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 )."

P.A. Price (IfA), T. Minezaki (U. Tokyo), L. Cowie (IfA) and Y. Yoshii
(U. Tokyo) report on behalf of the UH-MAGNUM GRB collaboration:

We observed the optical afterglow of GRB 050416 (Cenko & Fox, GCN #3265
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 )
with the automated MAGNUM telescope + MIP dual-beam optical-NIR imager,
commencing at April 16.53 UTC.  The observations were performed in poor
seeing with clouds which prevented detection in the I and K bands. 
Nevertheless, we detected the afterglow in the R, J and H bands.  Based
on comparison with a single USNO-B1 star, we estimate that the optical
afterglow was about R ~ 21.3 mag at the epoch of our observations.

This message may be cited.

Derek B. Fox (Caltech) reports on behalf of a larger collaboration:

"We have created a summed image from the first 10.14 ksec of exposure
on GRB050416 (BAT trigger #114753; Sakamoto et al., GCN 3264
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 ) through
the "UVW2" filter of the Swift UVOT.  Earlier reports have discussed
the detection of the optical afterglow of this event (Cenko & Fox, GCN
3265
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 ) in the U, B, and V filters (Schady et al., GCN 3280
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 ).  The UVW2
sequence consists of nine ~10-second exposures taken from 4 to 15
minutes after the burst; two ~100-second exposures taken 18 and 29
minutes after the burst, and 11 ~900-second exposures taken from 92
minutes to 2 days after the burst.  The final summed frame was
astrometrically registered to the USNO B-1.0 catalog.

At the position of the optical afterglow we detect a source at
>3-sigma confidence with a mean count rate of 6.0 +/- 0.7 cts/ksec
according to the default uvotdetect analysis.  Examination of
individual frames suggests that the source is not distinguishable
above background in the individual short (10-s and 100-s) exposures,
being most prominent in the early ~900 second exposures of the
sequence.

The UVW2 filter bandpass is centered at 188 nm with a width of 76 nm.
Since it is unlikely that significant afterglow emission would be
detected blueward of the host galaxy Lyman limit at 91 nm -- with
significant suppression redward to the Lyman-alpha resonance at 122 nm
-- we suggest that this detection implies a redshift constraint for
GRB050416 of z <~ 1.0."

S. B. Cenko, S. R. Kulkarni, A. Gal-Yam (Caltech) and E. Berger
(Carnegie Observatories) report on behalf of the Caltech-NRAO-Carnegie
Collaboration:

We have obtained 2 x 30 minute spectra of the host galaxy of GRB 050416
(a) with the Low Resolution Imaging Spectrometer mounted on the 10-m
Keck I Telescope.  Observations were taken on 6 June, 2005 UT.  We
identify several emission lines including [OII], H-beta, H-gamma, and H-
delta at a redshift of z = 0.6535 +/- 0.0002.

The spectrum indicates the host galaxy is faint and blue with large
amounts of ongoing star formation.  We note the redshift is consistent
with the prediction of Fox (3408) based on the afterglow detection in
the Swift UVOT UVW2 filter.  

Using a fluence of 3.8e-7 erg/cm^2 (10 - 350 keV, GCN 3273
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 ), we find an
isotropic gamma-ray energy release of 7.0e50 ergs.