2002-2006 SPASE-2 TeV Gamma-Ray Source Search
Unblinding
Progress:
We have spent the last week looking for ways to improve the
background fluctuations.
To do that we have tested different cuts on the data set (2005
only) and weightings:
These include
- a cut on the X2 in the shower reconstruction.
The normalized X2 peaks at 1 but has a long tail
extending to 10. Showers with X2 exceeding
10 are declared non analyzable. We tested X2
of 2 and 5.
- a cut on T2 which is a measure of the direction
reconstruction quality and is not normalized in
the reconstruction code. A cut on T2 < 50
improves the background fluctuations.
- weighting of the data set sidereal time as
recommended by Alan Watson. Sidereal time variations
are quite large in 2005 with a couple of bins (out
of 60) exceeding 3 sigma.
- weighting of events by azimuth. Since RA is a
combination of sidereal time and azimuth we
decided to correct on the azimuthal anisotropy of
the detector that may be a function of the
magnetic field direction at South Pole, that
spreads the shower front and increases the
shower threshold energy.
- a weighting on the rate of showers was also attempted
but did not improve anything.
All these cuts and weights were applied separately to
see what their effects are. Although the effects of cuts
and weighting are not significant, we decided to
apply the two quality cuts and weighting on sidereal
time and azimuth.
Binning for the background and the signal were changed
during the last week. We decided to adopt Tom Gaisser's
idea of using equal solid angle circular bins that
correspond better to the calculated angular resolution
that is binned in space angle. The shape of the
ellipses used in the determination of the background
are shown for the three zenith angle bands of the
three sources in the graphics below. Since the angular
resolution of SPASE 2 does not change much in this
energy range we will use equal zenith angle bins.
This changes the shape of the bins - at higher zenith
angle we can fit more circles in the zenith angle band.
Small bins are for S30 > 3, large ones for S30 < 3.
We will use Li & Ma algorithm for calculation of the
significance of the source fluxes after unblinding.
Ellipse sizes which preserve solid angle
Source
|
zenith angle
|
S30>3
1.58x2.1°=3.3° |
S30<3
1.58x3.3°=5.2°
|
ZEN
minor axis
|
RA
major axis
|
no. of
backgr bins
|
ZEN
minor axis |
RA
major axis |
no. of
backgr bins |
| RX
J0852-4622 |
43.8
|
3.3
|
3.7
|
47
|
5.2
|
5.9
|
28
|
| MSH15-58 |
32.0
|
3.3
|
4.2
|
41
|
5.2
|
6.6
|
25
|
| HESS
J1303-63.1 |
26.9
|
3.3
|
5.7
|
30
|
5.2
|
9.0
|
18
|
