New Multispectral Approaches
to Study of Gamma Ray
Bursts
René Hudec
Astronomical Institute, CZ-251 65 Ondrejov, Czech Republic
Email: rhudec@asu.cas.cz
The identification of GRB afterglows: X-rays, optical, radio
Recently, the GRBs are discovered ONLY in ramma rays. However,
 Nearly 90 % of all GRBs detected by BeppoSAX have an X-ray afterglow
 Nearly 45% of detected GRBs by BeppoSAX have an optical afterglow
 Nearly 40% of detected GRBs by BeppoSAX have a radio afterglow
These results justify the searches for GRBs INDEPENDENTLY at other
wavelengths, especially in X-rays and in optical. The GRBs can be detected not
only in gamma rays but at other wavelengths as well.
Since the GRB emission is believed to be beamed, with opening angle increasing
with wavelength and with time, the fraction of GRBs observable at longer
wavelengths if compared with gamma-rays is believed to be larger
Advantages of observing GRBs at other wavelengths:
- larger sample and hence better statistics: the total number of detected triggers
is expected to be larger
- this means also better statistics of related host galaxies, redshift distributions,
etc.
- precise astrometry and localization accuracy (if compared with gamma rays)
- constraints on beaming and other parameters
- cost effective approach
- multispectral analyses in real time, better physical understanding
- The UV flashes predicted by some theories such as Protheroe and Bednarek
1999 could be detected and studied. The corresponding delays regarding GRBs
could serve to study the nature of the sources. This can be addressed only by
surveys, not by follow-up devices since the flashes may preceed the GRBs
( Protheroe and Bednarek, 1999).
Feasibility of Independent X-ray Searches
The independent X-ray searches for GRBs require sensitive X-ray monitoring of
the sky. The X-ray telescopes in recent use however mostly have FOV limited to 1
deg or even less.
Recently designed and developed wide field Lobster-Eye X-ray telescopes offer
one very promising possibility. FOV of 1000 deg2 or even more is easily feasible
in these devices.
The recently developed prototypes of wide field X-ray telescopes of Lobster Eye
type confirm the technical feasibility of such analyses.
The first prototypes of Lobster Eye Telescopes have been designed, developed
and tested in the Czech Republic. The tests both in optical as well as in X-rays
confirm the performance identical with the calculations. One module has FOV of
~ 40 deg2 ; more can be obtained by multiplying of analogous modules.
LE Telescopes and GRB X-ray afterglows
 the expected rate of GRBs is 1 per day, however the theoretical prediction
assumes larger beaming angle in X-rays if compared with gamma rays, hence
the actual rate of X-ray afteglows may be larger
 the sensitivity of LE telescopes is sufficient enough to detect the recently
discovered X-ray GRB afterglows
 the localization accuracy of the LE telescopes is of order of 1 arcmin,
substantially exceeding the recent localization accuracy of most gamma ray
instruments (2 deg and more)
The proposed strategy for locating GRBs upon their X-ray fading counterparts:
 a pointed LE telescope with FOV of 12 x 12 deg + a crude positioning
instrument like BATSE/CGRO
 the 2D telescope will be pointed immediately at the GRB crude position and
stay on it for ~ 24 hrs
 then the sensitivity of ~ 10-14 ergs/cm-2 s-1 or better may be achieved (0.5 - 3
keV energy range)
 this is sufficient to position the fading X-ray GRB counterpart to 1 arcmin or
better, as well as obtain the light curve
More science with LE telescopes
 the additional science of LE X-ray telescopes includes supernova explosions,
high energy binary sources, AGNs, blazars, X-ray novae, X-ray flares on stars,
X-ray transients, X-ray flashes etc
 the use of LE telescopes will allow these objects to be detected and studied by
sky patrol monitoring
Discussion LE X-ray Telescopes
Results of analyses and simulations of lobster-eye X-ray telescopes have
indicated that they will be able to monitor the X-ray sky at an unprecedented level
of sensitivity, an order of magnitude better than any previous X-ray all-sky
monitor.
Limits as faint as 10-12 erg cm-2 s-1 for daily observation in soft X-ray range are
expected to be achieved, allowing monitoring of all classes of X-ray sources, not
only X-ray binaries, but also fainter classes such as AGNs, coronal sources,
cataclysmic variables, as well as fast X-ray transients including gamma-ray
bursts and the nearby type II supernovae.
For pointed observations, limits better than 10-14 erg sec-1cm-2 (0.5 to 3 keV)
could be obtained, sufficient enough to detect X-ray afterglows to GRBs.
The first prototypes of both Schmidt as well as Angel arrangements have been
produced succesfully for the first time, demonstrating the possibility to construct
these lenses by innovative but feasible technologies.
This makes the proposals for space projects with very wide field lobster eye
optics possible.
Feasibility of Independent Optical Searches
 The recent detection of optical afterglows and optical transients of gamma ray
bursts allows considering optical ground-based independent detection of these
phenomena.
 The optical surveys achieving lim mag better than 19...23 for stars and/or 10 for
1 min exposures may detect OAs and OTs of GRBs
 This opens the possibility of independent optical searches
 These searches must be of large field of view i.e. CCD surveys and/or deep
patrol plates are suitable
The rate of OAs - estimated beaming
- it is expected that sources emit jets from which the gamma ray emission is
more beamed than the subsequent optical afterglow radiation due to the
deceleration of the jet by the ambient gas and the corresponding decline in its
relativistic beaming with time (Rhoads 1997)
- because the shift to lower frequencies accompanies the shift to lower bulk
Lorentz factor, the minimum solid angle into which the transient can radiate
increases with time
- a jet geometry hence implies a higher rate of OAs detections
- if bursts are highly collimated, the gamma rays will radiate into a small solid
angle, the optical light into a larger one, and radio into a still larger one
- if bursts emit isotropically, we do not expect OAs unaccompanied by GRBs
- the ratio of transients detected hence allows the ratio of the mean solid angle
into which transients radiate to be estimated
- the GRBs rate known already
- the OA rate with characteristics typical for the observed OAs can be estimated
by present and future sky survey programs
The estimated beaming for the OAs observed so far
Recently, the evidence for collimation in GRBs has grown:
 /4 > 0.11 for 970508 Sari et al. 1999
< 0.003 for 980519 Halpern et al. 1999
< 0.01 for 990123 Anderson et al. 1999, Kulkarni et al. 1999
< 0.003 for 990510 Harrison et al. 1999
 The strongest evidence for collimation is in the brightest sources
 This evidence is based on the observed profiles of the light curves - steeper
slopes and even breaks in the slope as a collimated flow slows and spreads
laterally
 the number of optically selected OAs could be greater than the number of
gamma-ray selected GRBs by a factor of (0/a)2 where the go is the initial
Lorentz factor, the initial gamma-ray emission is beamed to an angle of about
1/0 and the afterglow emission is produced when the fireball has been
decelerated to a modest Lorentz factor a
 since typical bursts have 0 < than 102-3 (e.g. Fenimore et al. 1993, Woods and
Loeb 1995) while the optical afterglow emission occurs at a nearly 101-2, hence
this could boost the expected OAs rates by up to four orders of magnitude (we
will call this boosting factor)
The estimated rates of OAs and OTs in real experiments
The recent digitization of deep sky patrol plates as well as CCD surveys provide
valuable input data for these searches.
Optical Transients (prompt emission)
-
observed rate of GRBs (by BATSE) is about 1 GRB/day
beaming factor 1 … 10 (no more since the time after GRB is small)
boosting factor of 1 … 100
actual GRB fraction with OTs 0.01 - 1 (brighter than mag 12)
estimated OT rate 0.01 - 100 OTs/day for the whole sky sphere
this means 2.5 x 10-7 …. 2.5 x 10-3 deg-2
examples: astrograph plate (100 deg2) 2.5 x 10-5 … 2.5 x 10-1 OTs per plate
Optical Afterglows (delayed emission)
-
observed rate of GRBs (by BATSE) is about 1 GRB/day
beaming factor 1 … 100
boosting factor of 1 … 10 000
actual GRB fraction with OAs 0.5 (brighter than mag 23) and/or 0.05 (brighter
than mag 18)
estimated OA rate 0.5 … 5 000 OAs/day for the whole sky sphere lim mag 23
or 0.05 … 500 OAs/day for the whole sphere lim mag 18
this means 1 x 10-5 …. 1 x 10-1 deg-2 lim mag 23 and 1 x 10-6 … 1 x 10-2 lim mag
18
not in contrast with results obtained by plate searches
Example: UKSTU plate area of 41 deg2
23 mag limit ... 4 x 10-4 ... 4 objects per plate
18 mag limit ... 4 x 10-5 ... 4 x 10-1 objects per plate
- Constraints from observations (Schmidt 1999, Hudec
events/square degree lim mag 23 i.e. boosting factor < 10 000
1999)
<
0.15
- Note however that the background by SNe, variable AGNs, variable stars, flare
stars etc is higher! Depending on galactic latitude, their integrated rate may
achieve ~ 1 000 to 5 000 variable objects per UKSTU plate, lim mag 23.
Conclusion astronomical plates/surveys
It seems to be feasible that both flaring (OTs) as well as fading (OAs) optical
emission related to GRBs may be detected by optical sky patrols. Although the
true rate of these triggers remains unknown, in is very probably that their rate is
substantially below the background rate, hence the good knowledge of all
background triggers must be available as well as a reliable technique for their
classification and elimination.
Background:
 false events not related to GRBs but with similar transient behavior
Background sources:
 SN
 AGN flares/brightening
 Stellar flares
 Variable Stars
 OTs of unknown nature and origin
 Nonastrophysical triggers
Supernovae
The supernovae, especially those of Ia type, may represent an important source
of confusion due their occurrence rates, rise and decay timescales, and
magnitudes.
The expected rate of occurrence for faint events (down to R 23 mag) is roughly 2
deg-2 or 0.0015 arcmin-2 (Evans et al. 1989, Pain 1996, Brainerd 1998).
But, at least some, SNe may be probably related to GRBs (SN 1998bw and
GRB980425)
AGN flares
 Typical flare amplitudes are between 0.5 and 1.3 mag, depending on the AGN
type (Smith A. G. Bl. Cont. Var. ASP Conf Ser 110, 1996, 3)
 These are mean values. The spread of particular flare amplitudes is large (0.1
…. 6.7 mag).
 Recently, there is growing evidence for large amplitude ( more than 10 mag)
flares on AGNs.
 AGNs/QSOs surface densities: ~ 20 deg-2 lim mag B/V 20 (Iovino A. et al.
A&ASS 119(1996), 265-269, Hartwick F. D. A. and Scade D. ARA&A 28(1990),
437, Boyle et al. 1988), ~ 100 deg-2 lim mag B 22.5 (Trevese D. et al. AJ
98(1989), 108), most of them variable (Trevese D. and Kron R.G. 1994 in MultiWavelength Continuum Emission of AGNs, 412 (Eds. T. Courvoisier and A.
Blecha)
Stellar flares
 There is growing evidence for large amplitude (5 mag and more) stellar flares.
 archival searches have revealed large-amplitude flares (5-9 mag) from
otherwise typical dMe flare stars
 rate of analogous stellar flares unknown
Variable stars
 Also variable stars may exhibit light behavior similar to OAs and OTs.
 Variable stars are observed more commonly in decline than in increase since
the declines are typically more slowly such as delta Cep stars, U Gem stars,
flare stars, novae etc.
 Example: Y Dra Mira type variable inside GRB910709 detected and positioned
by COMPTEL: light variations between 6 and 15 mag, gradual light decrease
after maximum (period 322 d).
 Surface density: estimated rates for VS brighter than 20 mag:
80 deg-2 for /bII/ < 20 deg
4 deg-2 for /bII/ > 40 deg
(Hudec and Wenzel A&ASS 120 (1996), C707-C710)
but the discovery probability is ~0.1 (blinkmicroscope use)
 No statistics for variable stars below 20 mag available (no systematic surveys)
OTs of unknown nature and origin
There are real OTs of unknown nature but of astrophysical origin detected both
on emulsions and CCDs. Examples (real CCD detections):
 OT 970215: real CCD detection, V 13 mag, nothing down 20 mag on the
position, amplitude more than 7 mag (Vidal-Saiz J. et al., IBVS Budapest 4324,
1996)
 OT 950806: real object: detected on 20 CCD frames, peak magnitude I 7.5,
amplitude more than 10 mag, nothing down mag 21 48 hrs after detection (Toth
I. et al. AA 315, 153, 1996).
 OT triggers found
communication)
by
SNe
searches
(Schmidt
et
al.
1999,
private
 mystery events found at a rate of about 0.15 deg-2/per time scale (between 10
min and 3 days) lim mag R 23.5. In 3 cases, no host galaxy seen down mag R
24, in 1 case, host galaxy clearly visible. 2 events at a low galactic latitude and
hence can be flare stars.
 note that SN searches reject events with timescales less than 3 min (as cosmic
rays) hence can detect OAs but not all OTs
 these searches are very limited so far (6 SN runs done, 2-6 sq. deg. per run,
Schmidt et al, 1999 private comm.)
The SN versus OAs rates
- cosmological GRBs are much rarer than SNe (104 to 106 times, Woods and Loeb
1999)
- the GRB afterglows are however at peak luminosity 103 to 104 brighter than
SNeII (Woods and Loeb 1999)
- in Euclidean space, this implies that GRBs are detected from a volume bigger
by a factor of about (104)3/2 = 106, roughly canceling the factor by which they are
rarer than Sne
- the average detection rate of SNe and OAs may be comparable at lim mag
brighter than mag 18-20 in all colors (U, B, R, K) (Woods and Loeb 1998),
assuming the OAs frequency the same as of gamma ray selected GRBs
- if true, this has a significant consequence: in optical searches down to lim mag
18-20, the roughly identical numbers of SNe and of OAs are expected
- this means that the OAs may be among detected SNe (Hudcová et al. this
conference)
How to distinguish OAs and SNe (and other background
events)
1. Light curve
2. Peak luminosity (only for objects with known redshift)
3. Color information
The color indices of the optical afterglows (OAs) of the GRBs
 Powerful tool to search for the common properties of these events
 Important parameter reflecting the related physical processes
 Specific color indices of OAs give a hope to resolve whether the optical event
is related to GRB even without available gamma-ray detection
 Search for interrelations among the colors, luminosities and the decay rates of
the OAs (if z is known)
Results of study of colors of OAs
 The color variations ((V-R)0, (R-I)0, (B-V)0) during the decline of OAs are small
for 0.8 days < t-T0 < 10 days (Šimon et al. 2001)
 The colors of OAs on the final decline branch concentrate at (V-R)0 =0.44+/0.25, (R-I)0=0.50+/-0.25, (B-V) 0=0.44+/-0.18 (but large scatter of (U-B)0)
very
smooth shape of spectrum of OAs, no bumps or strong lines within the
observed I to B passbands. Slope of the spectrum can be supposed to be
almost constant and may have a powerlaw shape F ~  in this spectral region.
 Concentration of the color indices
the intrinsic reddening (in their host
galaxies) must be quite similar for all these OAs (rather small)
these
GRBs are unlikely to come directly from the inner parts of the star-forming
regions (maybe on our side of a star-forming region)
Conclusions
 the recent results of OAs and OTs searches indicate that GRBs may be
monitored and studied by observing their optical emissions, i.e. independently
on satellite projects
 the rate of OAs may be (significantly) higher than the rate of GRBs due to
different beaming
 it is feasible to use ground based optical devices to monitor OAs and OTs of
GRBs
 this opens a new observing window for GRBs
 these surveys must be of wide field and high sensitivity
 there is however a background of false triggers (not related to GRBs but with
similar transient behavior) with poorly known statistics (for faint magnitudes)
 this background is due to supernovae, AGN/QSOs, stellar flares, variable
stars, optical transients of unknown origin and non-astrophysical triggers
 the expected total number of optically variable sources of astrophysical origin
is large: ~ (0.5-2) in a 5 x 5 arcmin error box, depending on galactic latitude, lim
mag 23 (Hudec, 1999)
 the recent searches (e.g. SN searches) and databases (e.g. UKSTU plate
collection) may be used to detect OAs – there even may be unrecognized OAs
in detected SNe
 the detected OAs and especially OTs (since they will be recorded only once
due to their short duration in most cases) must be further studied in detail to
eliminate them from background triggers
Suitable devices/methods
 OMC (Optical Monitoring Camera) INTEGRAL: 5 x 5 deg2, lim mag 19
 CCD based devices and telescopes (ASPA, ROTSE, OTM, BOOTES,…)
 Digitized plate surveys
 Digitized deep archival plates (e.g. UKSTU plate collection, Siding Springs
Schmidt, 17 000 plates with lim mag 20-23, different filters/colors)
Acknowledgement
The investigations of gamma-ray bursts and optical transients are supported by
the project KONTAKT ES002 provided by the Ministry of Education and Youth of
the Czech Republic and by the grant 205/99/0145 provided by the Grant Agency of
the Czech Republic. The investigation of plate defects has been supported by the
Academic Link between the University of Westminster and Astronomical Institute
Ondrejov provided by the British Council in Prague.
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