GRB081203A: Ashra-1 observation of
early optical and VHE-neutrino emission

GCN Circular #8632

Y.Aita, Y.Asaoka, T.Chonan, Y.Higashi, K.Noda, and M.Sasaki
(ICRR,Univ.Tokyo), Y.Morimoto, S.Ogawa (Toho Univ.), J.Learned
(Univ.Hawaii Manoa), R.Fox (Univ.Hawaii Hilo) report on behalf of the
Ashra-1 collaboration:

We have searched for optical and VHE-neutrino emission in the field of
GRB081203A (A. M. Parsons et al., GCN Circ. 8595) around the BAT-triggered
GRB time (T0) with one of the light collector units in the Ashra-1
detector ( on Mauna Loa on Hawaii
Island (latitude = 19.5412 deg. N, longitude =155.5676 deg. W, altitude =3330m).

The Ashra-1 light collector unit has the achieved resolution of a few
arcmin, viewing 42 degree circle region of which center is located at
Alt = 11.7 deg, Azi = 22.1 deg. The sensitive region of wavelength is
similar with the B-band.

We quickly analyzed 83 images covering the field of GRB081203A every
7.2s with 6s exposure time respectively during the observation between
T0-300s and T0+300s. We detected no new optical object within the PSF
resolution around the GRB081203A determined by Swift-UVOT (M. De
Pasquale et al.,GCN Circ. 8603).

As a result of our preliminary analysis, the following 3-sigma
limiting magnitudes are derived:

Starting&Ending Exp.Time,  3-sigma Limit. Mag.
-297.7   -291.7                    11.9
-290.7   -284.7                    11.9
    .        .                       .
    .        .                       .
  -1.7      4.3                    11.9
   5.3     11.3                    11.8
    .        .                       .
    .        .                       .
 287.3    293.3                    11.9
 294.3    300.3                    11.9
The limiting magnitudes were estimated in comparison with stars in
Tycho-2 Catalog to be distributed between 11.7 and 12.0 as above
partly listed.

Adding that we have searched for VHE-neutrino emission with the
primary neutrino energy above 10^16eV by detecting Cherenkov lights
from tau-decay induced air-showers. VHE neutrinos are expected to be
produced at the GRB behind Mauna Kea and converted into tau leptons in
the mountain rock. The GRB position was passed through our triggering
fiducial region viewing onto the face of Mauna Kea from 11:07UT to
11:49UT (2.83 hours to 2.13 hours before the GRB). We have no signal
come out of the mountain. The limiting flux is now under analysis.

Figures of limiting optical magnitudes vs time comparing with the
other measurements and more details on the VHE neutrino observation
can be found at:

This message may be cited.

The collaboration list

Y.Aita, T.Aoki, Y.Asaoka, J.Asou, T.Browder, T.Chonan, S.Dye, R.Fox, J.Hamilton,
Y.Higashi, N.Ishikawa, T.Kimura, H.Kuze, J.Learned, M.Masuda, S.Matsuno,
Y.Morimoto, K.Noda, S.Ogawa, S.Olsen, M.Sasaki, H.Shibuya, K.Shinomiya,
N.Sugiyama, M.Yabuki, M.Yasuda, G.Varner, Y.Watanabe, and Y.Watanabe

We have observed the field of GRB081203A (Parsons, et al., GCN Circ. 8595)
by the Ashra-1 detector on the Mauna Loa Site on Hawaii Island
(latitude = 19d 32' 24" N, longitude = 155d 34' 12" W, altitude = 3330m).


The time of the trigger is 2008/12/03 13:57:11UT
RA 15h 32m 07.58, Dec +63d 31m 14.9s (90%-radius 0.74asec)

field of view : circle, 42 [deg] in diameter
altitude : 11.7 [deg], azimuth : 22.1 [deg]
angular relolution: a few [arcminutes] over the FOV

Figure 1: The Ashra-1 detector for low elevation angle
         Figure 2: trajectory of thr GRB in our field of view.

Significance maps

Around the trigger time, we have obtained data in every 7.2 seconds with 6sec exposure.
The examples of obtained preliminary S/N (signal to noise ratio) maps including the trigger time are shown below.

Figure 3: Examples of obtained preliminary S/N maps around the trigger time.
From left to right, 100sec before, just at, and 100sec after the trigger time, respectively.

Limiting magnitude

The resultant preliminary limiting magnitudes is summarized as below, and seen here.

Start time End time  3-sigma Limit. Mag.
-297.70    -291.70    11.9
-290.70    -284.70    11.9
   .          .        .
   .          .        .
  -1.70       4.30    11.9
   5.30      11.30    11.8
   .          .        .
   .          .        .
 287.30     293.30    11.9
 294.30     300.30    11.9
The detector has its maximum sensitivity in u and b band region.
Here is the dependence of its components on wavelength,
and here is the total dependence.

Summarized lightcurves

Figure 4: Summarized lightcurve around the trigger time.
The horizontal axis is in linear scale. The vertical axis in the right is
only for the data by Swift XRT (gray data), where the scale is arbitrary.
Figure 5: Summarized lightcurve after the trigger time.
The horizontal axis is in log scale.

The references are listed below.
XRT XRT product web page
(URL included in GCN#8609)
Stara Lesna Rc GCN#8604
Z-600 R GCN#8615
Skynet/GORT I GCN#8617
Skynet/GORT R GCN#8617
Xinglong TNT R GCN#8618
MITSuME g' GCN#8619

VHE neutrino observation

We have also searched VHE neutrino emission from the GRB
via the earth-skimming neutrino detection technique.
The introduction for this technique is here.
The observation time for the GRB position (RA 15:32:07.58, Dec +63:31:14.9),
defined as the time when the position is behind the mountain and our trigger region,
was from 11:07UT to 11:49UT, corresponding to from 2.83 hours to 2.13 hours before the GRB.
The threshold of the detection is estimated roughly as about 10^16eV.
No candidate was found so far. and detailed analysis is going on.

This message can be cited.