Gamma-Ray Burst (GRB) phenomenon has been studied in -rays
since 1967 (first publication in 1971) by space born wide field
-ray detectors. More than 4000 events have been registered.
Technology development, onboard GRB source position calculation, rapid
information downlink to tracking station and the Internet, allow looking
for GRB events worldwide with large optic telescopes. Since discovery
of the first optical counterpart of GRB970228 (Guarnieri et al. 1997),
more than 30 GRBs optical afterglows were found, and the cosmological
nature of the phenomenon is proved. The typical brightness of a detected
Optical Transient (OT) is about 18 mag and large telescope response
time is about one day. However, response time is decreasing as more
telescopes join the GRB Coordinates Network (Barthelmy et al. 1994).
Search for prompt optic emission has lead to creation of a new generation
of optical telescopes--fast response wide field cameras, which can
be automatically pointed to a GRB error box having been alerted by
-ray telescopes via the network. However, in only one case
was prompt optical emission successfully registered by robotic telescope:
GRB990123. ROTSE was alerted via network, started observation 22s after
GRB trigger, and recorded optical emission with a maximum brightness
about 8.5 mag (Akerloff et al. 1999).
Even if an alert is generated, transmitted, accepted and
responded to in near real time, the alerted system could not
effectively record prompt emission if time delay after burst
trigger is greater than duration of the burst. And the typical
duration of bursts is 10s. Indeed the necessary time to
generate alert depends on brightness of the burst and its time
profile in -ray domain: e.g., according to current trigger
algorithms a slow rise burst will generate alert later than a fast
rise burst; some bursts were found only in post flight analysis.
Accuracy of GRB source position calculation also depends on
brightness and duration of the burst; sometimes ground analysis
is necessary to reduce an error-box, which also delays the start
of ground based observation.
The class of short bursts and Soft Gamma-Ray Repeaters with specific
duration of about 0.1s cannot be observed by alerted system in real-time.
No afterglow from short burst has been detected till now (Hurley et
al. 2002). Also the optical emission preceding the burst onset (which
is discussed in GRB models (Mezaros et al. 2001, Beloborodov 2001)
cannot be detected by alert-based system at all. Now the detection of
a prompt emission and possible optical precursor as well as variability
investigation of the optical emission is a crucial point in understanding
of physics of -ray bursts.
Synchronicity. To effectively observe prompt optical emission one
needs to monitor simultaneously the same area of the sky by space-born
-ray observatories and an optical observatory. While GRB monitors
do so regularly (at present the HETE-2 and IPN collaboration, consisting
of Konus-Wind, Ulysses, HEND/Mars Odyssey, and GRS/Mars Odyssey, is
operating in space), as far as the authors know, no one optical system
is continuously monitoring the sky synchronously with space labs.
Independence. Monitoring should be independent, to avoid the need for any alerted transmission in real-time. The full data reduction and correlation with space labs could be done after observation, which additionally decreases the uptime requirements and cost of telecommunication channels.
Time scale. As opposed to the case of afterglow
observations, the time resolution for prompt emission search
should be near equal or better than the duration of the event.
Though time scale of the optical emission may significantly
differ from that of the -ray emission, one may estimate the
specific duration of prompt emission from the only one detected,
GRB990123, as 20s. However, prompt emission duration
may be different for different events. On the other hand,
variability measurement requires better time resolution. Because
the search strategy depends on unknown duration of the OT, and
requirements for variability measurement contradict to those for
search strategy, one needs to have a continuous set of frames
with time exposure better than duration of the event which could
be used for subsequent co-adding frames with the aim of
increasing sensitivity.
Wide FOV versus sensitivity. The larger the FOV of a
telescope, the larger is the probability to have in this FOV an
error box of a GRB. On the other hand a more sensitive telescope
has better chance to detect OT if the source of a burst is in
the FOV of the telescope. In the case of searching, the probability
of catch the OT is a function of the two above probabilities.
Let us consider a detector with CCD matrix having fixed pixel
size and number of pixels. The sensitivity S of
the telescope equipped with that matrix is roughly proportional to ,
, where
is a filled size in degrees.
(The sensitivity is defined as a detection limit with the
same confidence level for the same exposure time.)
The larger
the FOV, the worse is the sensitivity. The cumulative
distribution of OT versus their brightness may be written as
, where
is a number of sources with
brightness more than
. (In case of 3-dimensional
Euclidean space and uniform distribution of sources it is
.) Finally, the number of
observed potential sources of OT is proportional to
because of isotropic distribution of GRB. Combining these
formulas one can obtain that the number of detected OT is
Strategy of prompt emission search depends on the parameter
. One can estimate the slope of
distribution by extrapolating already observed afterglow light
curves backward to some standard moment of time after burst
onset. The estimation gives
. Of course this estimation
is not fully correct due to possible bias and possible change in
a power law index of OT light curve backward to the beginning
of the burst. The parameter
could be estimated from
theoretical predictions including cosmological nature of GRB,
possible evolution of source intensity in cosmological reference frame,
beaming optical emission, etc. In this case the
is
varying from 0.5 to 1.5 depending on many uncertain
suggestions. Because of
a telescope with wider FOV is
more preferable.
All sky optical survey, which is widely discussed (e.g., Nemiroff
& Rafert 1999; Paczynski 2001) may resolve the problem of
GRB optical prompt emission and fast OT search while
technological problems and limitations, including financial one,
are evident. The most important factor is a large scanning time
in comparison with short duration of the OT. To avoid some of the
limitations and to increase the probability of synchronous
coverage of the same part of the sky we propose to monitor
by ground based optical system only
``small'' part of the sky, in particular the field of view of
spaceborn X- and -ray telescopes. The data obtained
by space lab and ground based
observatory will be correlated later for joint search of
transients. This approach has been already discussed (Beskin et
al. 1999).
Taking into account the above requirements we have developed a
low cost wide field camera (
) with relatively high time
resolution (0.13-10s) which will be able to monitor FOV of
current and future X-ray telescopes, in particular Wide field
X-ray Camera (WXC) of HETE-2. The main components of the optical
camera are: (1) Main objective: focal length 180 mm, aperture diameter 150
mm;
(2) Image intensifier: photocathode S25, quantum efficiency
0.1; input fiber optic window D=80 mm, output glass window,
scaling factor 4.5:1; amplification coefficient 120;
(3) Adapting objective: constructed from two commercial
objectives AVENIR SE2509;
(4) CCD camera: commercial TV-CCD camera equipped with SONY 2/3
IXL285 matrix,
pixels,
variable exposure 1/7.5-10s, readout noise 20
/pixel.
To obtain wide FOV and to use the low cost commercial CCD matrix we use image intensifier for both image scaling and compensating light loss in adapting objective. The FOV of the camera is 20$^$, and the spatial resolution of the system is about 50 arcsec/pixel. The spatial resolution in fast observation is less important because the precise localization can be done later by observations of the afterglow with large telescopes. Because of large readout noise of TV-CCD the sensitivity of the camera is restricted by the noise at minimum exposure time and by sky background at maximum exposures. The modeled detection level of the system is about 12.5 mag at 0.13s exposure and about 14 mag at 1s for a dark night. The prototype of the camera (TT600 telescope of Kosmoten observatory) is now being used for current alerted observation of GRB afterglow.
The software includes frame comparator in video processor for
ion and particle events elimination, storage system
management, and a buffer
for frame accumulation.
Frame will be accumulated in different time windows and
compared in real time. According to predefined criterion
the system can generate alert in case of bright transient detection.
This part of the system is similar to a usual trigger scheme of
-ray burst detectors.
We estimate the rate of successful simultaneous observation of
GRB error-box with WXC of HETE-2 as 1.6 per year. More events
will be investigated after correlation of weak bursts which
occur in FOV of WXC but not sufficiently intense for source coordinates
calculation. The same time-spatial correlation of events
registered in our camera with HETE and time correlation with
space born GRB detectors may help to resolve the problem of
absence/presence of OT from short bursts and problem of
possible GRB-orphans (GRBs which are not observed in
-rays).
This investigation is partially supported by U.S. Civilian Research and Development Foundation (CRDF).
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