GRB 060427

V. Mangano (INAF-IASFPA), S. D. Barthelmy (GSFC),
S. Campana (INAF-OAB), J. R. Cummings (NASA/ORAU),
J. A. Kennea (PSU), H. A. Krimm (GSFC/USRA),
F. E. Marshall (NASA/GSFC), K. L. Page (U Leicester),
D. M. Palmer (LANL), P. Romano (INAF-OAB) and D. E. Vanden Berk (PSU)
report on behalf of the Swift Team:

At 11:43:10 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 060427 (trigger=207281).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is RA,Dec 124.303, +62.617
{08h 17m 13s, +62d 37' 02"} (J2000) with an uncertainty of 3 arcmin
(radius, 90% containment, including systematic uncertainty).  This was an
image trigger so the TDRSS lightcurve does not show any significant activity. 

The XRT began observing the field at 11:45:20 UT, 130 seconds after
the BAT trigger. XRT found a fading, uncatalogued X-ray source
located at RA(J2000) = 08h 17m 04.5s, Dec(J2000) = +62d 40' 17.4",
with an estimated uncertainty of 5.4 arcseconds (90% confidence
radius). This location is 204 arcseconds from the BAT on-board
position, outside the BAT error circle. The initial flux in the 2.5s
image was 9.6e-10 erg/cm2/s (0.2-10 keV). 

UVOT took a finding chart exposure of 400 seconds with the V filter
starting 133 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 typical 3-sigma upper limit
has been about 18.5 mag. The 8'x8' region for the list of sources
generated on-board covers 100% of the XRT error circle. The list of
sources has an upper limit of 19.2 mag. No correction has
been made for the expected extinction of about 0.2 magnitudes.

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

"We have observed the field covering the error circle of
GRB060427 (GCN 5006
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 ) with the unfiltered CCD camera on
the 30-cm telescope at University of Miyazaki.
The observation was started 11:44:48 UT on Apr.27(98sec after the Swift trigger).
After co-adding a set of 13 images (11:44:48 - 12:00:21 UT)
of 30 sec exposures, we have compared with the USNO A2.0 catalog.
Preliminary analysis shows there is no new source brighter
than 15.1 mag."

V. Mangano (INAF-IASFPA), G. Tagliaferri (INAF-OAB),
D. N. Burrows (PSU) report on behalf of the Swift XRT Team:


We have analyzed the first orbit data of GRB060417
(Mangano et al 2006, GCN 5006
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 ).
The XRT data set consists of 88 s exposure in Windowed
Timing (WT) mode followed by 843 s exposure in Photon
Counting (PC) mode.

The refined position of the source is

    RA  (2000) =  08h 17m  04.4s  
    Dec (2000) = +62d 40'  18.3"

with an uncertainty of 3.4 arcsec (90% confinement).
This position is only 1 arcsec from the onboard
detected position (Mangano et al 2006, GCN 5006
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 ).

The 0.2-10 keV X-ray light curve shows an initial very
fast decay with slope -4.3 +/- 0.2, a break at 247 +/- 12 s
from the trigger and a final decay slope of -1.7 +/- 0.13.

The WT and PC spectra were fitted separately with an
absorbed power law model and showed photon indices of
2.9 +/- 0.1 and 2.37 +/- 0.14, respectively (90% confidence level).
The WT spectrum, covering the time interval 138-227 s, accounts
for the pre-break phase of the light curve.
Both spectra show evidence of an absorption column at the
level of (1.7 +/- 0.2) x 10^21 cm^-2 in excess with respect
to the Galactic value of 4.3 x 10^20 cm^-2.
The 0.2-10 keV unabsorbed fluxes of the two spectra are
1.3 x 10^-9 and 6.6 x 10^-11 erg cm^-2 s^-1, respectively.

If decaying at the present rate the source will reach the
flux level of 10^-14 erg cm^-2 s^-1 (corresponding to
a count rate level of 1.5 x 10^-4 counts s^-1)
after one day.

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

H. Krimm (GSFC/USRA), L. Barbier (GSFC), S. Barthelmy (GSFC),
J. Cummings (GSFC/NRC), E. Fenimore (LANL), N. Gehrels (GSFC),
D. Hullinger (BYU-Idaho), V. Mangano  (INAF-IASFPA), C. Markwardt (GSFC/UMD), 
D. Palmer (LANL), A. Parsons (GSFC), T. Sakamoto (GSFC/NRC),
G. Sato (ISAS), J. Tueller (GSFC)
on behalf of 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 060427 (trigger #207281)
(Mangano, et al., GCN 5006
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 ).  The BAT ground-calculated position is RA,Dec =
124.175,+62.654 deg {8h 16m 42.0s,+62d 39' 13.4"} (J2000) +- 2.1 arcmin,
(radius, sys+stat, 90% containment).  The partial coding was 71%.

The mask-weighted lightcurve shows a broad peak starting at ~T-20 sec
and lasting to ~T+80 sec.  T90 (15-350 keV) is 64 +- 5 sec
(estimated error including systematics).
 
The time-averaged spectrum from T-10.0 to T+60.0 is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.87 +- 0.29.  The fluence in the 15-150 keV band is
5.0 +- 0.9 x 10^-07 erg/cm2.  The 1-sec peak photon flux measured
from T+5.00 sec in the 15-150 keV band is 0.3 +- 0.1 ph/cm2/sec.
All the quoted errors are at the 90% confidence level.

M. De Pasquale (UCL-MSSL), V. Mangano (IASF Palermo) report on
behalf of the Swift/UVOT team:

  The Swift/UVOT began taking data on the field of GRB060427
at 11:45:13 UT on 2006-04-27, 122 s after the BAT trigger
(Mangano et al., GCN 5006
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 ), with the settling exposure. No
afterglow candidate was detected at the refined XRT position
(Falcone et al., GCN  5009
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 ) in summed images from any of
the filters down to the following three-sigma upper
limits.

  Filter  T_range(s)  Exp(s) 3sig_UL

  V       122-6920   1668   20.4
  B       610-6635    416   20.7
  U       585-6430    436   20.3
  UVW1    562-6226    457   20.0
  UVM2    562-6021    456   20.0
  UVW2    626-6840    436   20.5

  These upper limits are uncorrected for the estimated Galactic
reddening of E_{B-V} = 0.051 mag (Schlegel et al. 1998).

A. R. López-Sánchez, J. García-Rojas (IAC La Laguna),
M. Jelínek and A. J. Castro-Tirado (IAA-CSIC Granada),
report on behalf of a larger collaboration:

"Following the detection by SWIFT of GRB 060427 (V. Mangano
et al. GCNC 5006), we have obtained R & I-band images
at the 2.5m Nordic Optical Telescope (+ ALFOSC), starting
on Apr 27.876 UT (i.e. 9.25 hr after the onset of the event).
At the position of the X-ray afterglow detected by SWIFT/XRT
(V. Mangano et al. GCNC 5010), no optical source is detected
in the 600s R-band image, down to a limiting magnitude of
R = 23.5."

This message can be quoted.

M.Zhai, Y.L. Qiu, J.Y. Wei, J.Y. Hu, J.S. Deng and W.K. Zheng, report on
behalf of EAFON:

We have observed the field of GRB060427 (GCN 5006
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 ) with the Xinglong 0.8m
telescope 4.788 hours after the burst, thers is no new source brighter than
18.7(3-sigma) Mag within 4x600s combined R-band image.
                             
This message may be cited.

P.A. Price (IfA, Hawaii), D.B. Fox (Penn State), S.B. Cenko (Caltech),
E. Berger (OCIW), B.P. Schmidt (RSAA, ANU) and L.L. Cowie (IfA, Hawaii)
report on behalf of a larger collaboration:

We have observed the XRT localisation of the afterglow of GRB 060427
(GCN #5010
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 ) with the Gemini North telescope + NIRI under variable
clouds.  Our observations commenced at 2006 Apr 28.23 UTC, and consisted
in 18 x 60 sec exposures in J-band.  Visual inspection of the combined
J-band image does not reveal any afterglow candidate at the XRT position
to a limiting magnitude of J ~ 20.5 mag, based on three 2MASS sources in
the field.  Additional H-band exposures have not yet been examined.

The afterglow is therefore likely faint, rather than at high redshift.


We thank Tom Matheson and the Gemini North observing team for performing
these ToO observations.

V. Lipunov, V.Kornilov, D.Kuvshinov, N.Tyurina, A.Belinski, E.Gorbovskoy,
A.Krylov, G.Borisov, A.Sankovich, N.Shatskiy, V.Vladimirov
S.Korobkin, P.Gritsyk

Sternberg Astronomical Institute, Moscow Union 'Optic'
MASTER  robotic system (http://observ.pereplet.ru) responded to 
GRB060427.4 (GRB_TIME is 2006-04-27 09:04:33.53, V. Mangano, S. D. 
Barthelmy et al.(GCN5006
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 ).
The first image was at 2006-04-27 18:18:07 UT, 09:13:33.47  after the GRB time
The unfiltered image is calibrated relative to USNO A2.0 (0.8 R + 0.2 B).
The robot not find OT-candidate in error box (V. Mangano et al. GCN5010
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 ) 
brighter then  17.5 (s/n=3).
This work is supported by RFFI  04-02-16411 grant.
This message can be cited.
Mailto: lipunov@sai.msu.ru