GRB 070724B

GRB 070724b: First Gamma Ray Burst Localization by SuperAGILE onboard 
AGILE

M. Feroci, E. Costa, E. Del Monte, I. Donnarumma, Y. Evangelista, 
I. Lapshov, F. Lazzarotto, L. Pacciani, M. Rapisarda, P. Soffitta, 
A. Rubini, E. Morelli, M. Mastropietro, G. Di Persio, M. Frutti, 
and S. Mereghetti, A. Pellizzoni, G. Di Cocco, C. Labanti, M. Trifoglio, 
G. Barbiellini, P. Picozza, A. Argan and M. Tavani, 
on behalf of the AGILE Team, P. Giommi, L.A. Antonelli, 
C. Pittori (ASDC), and L. Salotti (ASI) report:

The SuperAGILE experiment onboard the AGILE mission, launched on April 
23rd on an equatorial orbit, is currently performing the in-flight 
check-out and performance verification. The experiment is not yet in its 
optimal configuration and not all the functions are active. In 
particular, the onboard triggering and imaging is not active yet
and the absolute position reconstruction is being calibrated by
using celestial X-ray sources.
During this testing phase SuperAGILE detected and imaged 
GRB 070724b. The event starting time is approximately at 
23:25:09 UT on July 24 and it was observed at 21 degrees off-axis.
The observed duration in the 20-60 keV energy range is about 55 seconds, 
with a multi-peak time structure. 

The burst position was reconstructed as (RA, Dec) (17.629 deg, 
57.673 deg), which is:

RA(J2000) = 01h 10m 31.0s
Dec(J2000) = +57d 40' 23"

with an uncertainty of 20' radius. The given uncertainty is almost 
entirely systematic and it is due to the very preliminary absolute 
position calibration, carried out only on 3 X-ray sources. At this 
preliminary stage the absolute positioning was obtained by the ground 
calibrations, preliminarily checked with these sources. The error box
may be improved over the next hours/days.

An analysis of the AGILE Gamma Ray Imager (GRID) data is in progress.
A quick look analysis does not show any bright gamma-ray transient
in coincidence with the X-ray emission of GRB 070724b.

P. Romano(Uni-Bicocca and OAB), J.A. Kennea(PSU), C. Guidorzi(Uni-Bicocca and OAB),
D. Burrows(PSU) report on behalf of the Swift-XRT team:

We have analysed the first two orbits of Swift-XRT data obtained for
GRB 070724B detected by SuperAGILE (Feroci et al., GCN Circ. 6668
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 ),
which include 3971s of Photon Counting (PC) mode data, beginning
on 2007-07-25 18:32:39 UT, 19.1 hours after the burst trigger time.
Using the PC data we found two sources within the SuperAGILE error circle,
at the following positions:

1) RA, Dec=17.48574, 57.67613, which is equivalent to
RA(J2000)  =  01 09 56.58
Dec(J2000) = +57 40 34.1
with an estimated error radius of 4.5 arcsec (90% confidence).
This position lies 4.6 arcmin  from the SuperAGILE position
(GCN Circ. 6668
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 ). The current observed 0.3-10 keV count rate is
(3.9+/-0.4)E-02 counts/s, which translates into a 0.3-10 keV flux of
~2E-12 erg/cm2/s (assuming a power-law with photon index=2 absorbed
by a column of NH=E21 cm-2).

2) RA, Dec= 17.51024, 57.81898, which is
RA(J2000)  =  01 10 02.46
Dec(J2000) = +57 49 08.3
with an estimated error radius of 6.1 arcsec (90% confidence).
This position lies 9.5 arcmin  from the SuperAGILE position.
The current observed 0.3-10 keV count rate is (0.6+/-0.2)E-02
counts/s, which translates into a 0.3-10 keV flux of ~3E-13 erg/cm2/s.

More data are being collected and a new circular will be issued
when the fading nature of these objects is established.

This circular is an official product of the XRT team.

[GCN OPS NOTE(26jul07): Per author's request, DB's affiliation was added,
"17.1 hours" was changed to "19.1 hours", and the exponent of the count rate
in the second source changed from "03" to "02".]

A.Chen, S.Vercellone, A.Giuliani, A.Pellizzoni, F.Fornari, S.Mereghetti, 
F.Perotti, M.Fiorini, P.Caraveo (IASF-Milano), C.Labanti, M.Marisaldi, 
F.Fuschino, M.Galli, A.Bulgarelli, F.Gianotti, M.Trifoglio, G.Di Cocco 
(IASF-Bologna), M.Tavani, G.Pucella, F.D'Ammando, E.Costa, M.Feroci, 
A.Trois, A.Argan (IASF-Roma), G.Picozza (INFN-Roma), F.Longo, 
G.Barbiellini (INFN-Trieste) on behalf of the AGILE Team

and  

C. Pittori, P.Giommi, L.A.Antonelli (ASDC), and L. Salotti (ASI) report:


Further analysis of the AGILE GRID (Gamma-Ray Imaging Detector) data of 
GRB 070724B (Feroci et al. GCN 6668
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 ) confirms the absence of significant 
emission at energy above 50 MeV. Only one photon consistent with the 
direction of GRB 070724B has been detected by the GRID in a ~100 second 
interval around the time of the burst. This is consistent with the 
measured background rate.

We note that a gamma-ray flux similar to that observed with EGRET from 
GRB930131 (Sommer et al.1994, ApJ 422, L66), would have given about 20 
counts in the GRID in the same time interval.

We also analyzed the data of the AGILE Mini-Calorimeter, which provides a 
nearly whole-sky monitor in the energy range ~0.35-3 MeV (no directional 
information), without finding any statistically significant count rate 
increase in correspondence of GRB 070724B.

[GCN OPS NOTE(26jul06): Per author's reuqest, the Subject line was changed
from "070734B" to "070724B".]

S. Golenetskii, R.Aptekar, E. Mazets, V. Pal'shin, D. Frederiks, and
T. Cline on behalf of the Konus-Wind team report:

The long soft GRB 070724B localized by SuperAGILE (Feroci et al., GCN 
6668
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 ), triggered Konus-Wind at T0=84307.346 s UT (23:25:07.346).

The Konus-Wind light curve shows the main multipeaked pulse
with a total duration of ~50 s followed by a weak pulse
at T-T0 ~102 sec with a duration of ~10 s.
As observed by Konus-Wind the burst
had a fluence of 1.80(-0.25, +0.04)x10^-5 erg/cm2,
and a 64-ms peak flux measured from T0+29.744 s
2.17(-0.45, +0.34)x10^-6 erg/cm2/s
(both in the 20 - 500 keV energy range).

The time-integrated spectrum of the burst
(from T0 to T0+49.408 s) can be fitted (in the 20 - 500 keV range)
by a power law with exponential cutoff model:
dN/dE ~ E^(-alpha)*exp(-E*(2-alpha)/Ep)
with alpha = 1.15 +/- 0.13
and Ep = 82 +/- 5 keV (chi2 = 40/42 dof).

All the quoted errors are at the 90% confidence level.

The Konus-Wind light curve of this GRB is available
at http://www.ioffe.rssi.ru/LEA/GRBs/GRB070724_T84307/

A. Endo, M. Tashiro, Y. Urata, K. Onda, M. Suzuki, N. Kodaka,
K. Morigami (Saitama U.), Y. Terada, T. Tamagawa (RIKEN),
M. Ohno, T. Uehara, T. Takahashi, Y. Fukazawa, C. Kira (Hiroshima U.),
K. Yamaoka, Y. E. Nakagawa, S. Sugita (Aoyama Gakuin U.),
T. Enoto, R. Miyawaki, K. Nakazawa, K. Makishima (Univ. of Tokyo),
E. Sonoda, M. Yamauchi, S. Maeno, H. Tanaka, R. Hara (Univ. of Miyazaki),
M. Kokubun, M. Suzuki, T. Takahashi (ISAS/JAXA), S. Hong (Nihon U.),
on behalf of the Suzaku WAM team, report:

The long GRB 070724B (Feroci et al., GCN 6668
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 ) triggered the Suzaku
Wide-band All-sky Monitor (WAM) which covers an energy range of 50 keV
- 5 MeV at 23:25:09 UT (=T0).  The observed light curve with 1/64 sec
time resolution shows a multi-peaked structure with a duration (T90) of
about 41 sec.  The fluence in 100-1000 keV was 1.04 (+0.08,-0.07) X10^-5
erg/cm^2. The 1-s peak flux measured from T0+39 sec to T0+40 sec was
3.33 (+/- 0.40) photons/cm^2/s in the same energy range.

Preliminary result shows that the time-averaged spectrum from T0-2 sec to
T0+43 sec is well fitted by a single power-law with a steep photon index
of 2.8 +/- 0.2 (chi^2/d.o.f. = 17.3/21).

All the quoted errors are at statistical 90% confidence level,
in which the systematic uncertainties are not included.

The light curves for this burst are available at:
http://www.astro.isas.jaxa.jp/suzaku/HXD-WAM/WAM-GRB/grb/trig/grb_table.html

P. Romano, C. Guidorzi(Uni-Bicocca and OAB), A. Moretti(OAB),
M. Chester(PSU) report on behalf of the Swift-XRT team:

We have analysed all downlinked Swift-XRT data obtained for
GRB 070724B detected by SuperAGILE (Feroci et al., GCN Circ.
6668
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 ), for a total of 7973s of Photon Counting (PC) mode data,
beginning on 2007-07-25 18:32:39 UT.
Of the two sources proposed in Romano et al. (GCN Circ. 6669
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 ),
we can exclude number 2 (RA, Dec= 17.51024, 57.81898) as an
afterglow candidate since no fading is found in X-ray data
and there is a catalogued V=11.6 star within the XRT error
circle (2.9 arcsec away).

We propose source number 1 as the most likely afterglow candidate.
The Photon Counting mode image provides a refined XRT
position at RA,DEC(J2000)=17.48497, 57.67616, which is

RA(J2000)  =  01 09 56.39
Dec(J2000) = +57 40 34.2

with an uncertainty of 4.0 arcsec (radius, 90% containment).
This is 1.5 arcsec from the initial X-ray position (Romano et
al., GNC Circ. 6669) and 4.6  arcmin from the initial SuperAGILE
position (Feroci et al., GCN Circ. 6668
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 ).
The light curve exhibits a fading behaviour, with a power-law
slope of 1.1+/-0.3.

The spectrum (150 photons) can be fit using Cash statistics
with an absorbed power law with a photon index of 2.6+/-0.5
and a column density of (9+/-3)E21 cm^-2 in excess
of the Galactic value (3.1E21 cm^-2; Kalberla et al. 2005).
The absorbed (unabsorbed) 0.3-10.0keV flux for the spectrum is
1.2E-12 (4.4E-12) ergs cm^-2 s^-1, at mean time of
T+1.54d.

Since the second set of XRT data were taken with an offset with
respect to the first set (GCN Circ. 6669
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 ), more of the
SuperAGILE error circle was covered. A new, weak source was
found (Source number 3) at RA,DEC(J2000)=17.24912, 57.79786, or
RA(J2000)  =  01 08 59.79,
Dec(J2000) = +57 47 52.3
with an uncertainty of 6.0 arcsec (radius, 90% containment).
This position lies 14.2 arcmin from the SuperAGILE position
(GCN Circ. 6668
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 ). The current observed 0.3-10 keV count rate is
(5.3+/-1.6)E-03 counts/s, which translates into a 0.3-10 keV flux
of ~2.6E-13 erg/cm2/s (assuming a power-law with photon index=2
absorbed by a column of NH=E21 cm-2).

Further observations with Swift are planned.

This circular is an official product of the Swift XRT team.

M.M. Chester (PSU) and P. Romano (Uni-Bicocca and OAB) report on behalf
of the Swift UVOT team:

The Swift/UVOT observed the field of GRB 070724B (detected by
SuperAGILE; Feroci et al., GCN Circ. 6668
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 ) beginning 2007-07-27 00:02:11
UT, 48.6 hours after the burst trigger time.

We do not find any new source in any of the UVOT observations inside the
refined Swift/XRT error circle for the likely candidate, source #1
(RA,DEC(J2000)= 17.48497, 57.67616, Romano et al., GCN Circ. 6675
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 ).  We
note a cataloged star, V=18.2, 7.6 arcsec from the XRT position.

The 3-sigma upper limits for detecting a source inside the XRT error
circle in the co-added frames are:

Filter   Tstart   Tstop    Exp     Magnitude
            (s)     (s)     (s)    (3-sigma UL)
White    179270  197091   1776       21.3
v        175022  196602   2023       19.8

The values quoted above are not corrected for the expected Galactic
extinction corresponding to a reddening of E_{B-V} = 0.51 mag in the
direction of the burst (Schlegel et al. 1998).

S. B. Cenko and A. Rau (Caltech) report on behalf of a larger
collaboration:

We have imaged the field of GRB070724B (Feroci et al., GCN 6668
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 ), the
first GRB detected by SuperAGILE, with the automated Palomar 60-inch
telescope.  Observations consisted of 5 x 180 s images in the i' filter at
a mean epoch of approximately UT 06:46:30 on 2007 July 26 (~ 31.3 hours
after the burst).  We do not detect any sources inside the error circle of
the likely XRT afterglow (Romano et al., GCN 6675
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 ) to a limiting
magnitude of i' > 21.0 (calculated with respect to several USNO-B stars
in the field).

A. Pelangeon & J-L. Atteia (LATT-OMP) report:

We have used the spectral parameters obtained for the most intense part
of GRB 070724B -- from T0+24.8 to T0+41.2 s, Konus time
(V. Pal'shin, Ioffe Inst., private communication) --
observed by Konus-Wind (Golenetskii et al., GCNC 6671)
to compute the spectral pseudo-redshift(**) of the first burst
localized by SuperAGILE (Feroci et al., GCNC 6668).

We obtain: pz= 1.10 +/- 0.20

We thank V. Pal'shin for having kindly performed the spectrum analysis
and providing us the spectral parameters.


(**) cf. http://www.ast.obs-mip.fr/grb/pz

P. Romano, C. Guidorzi(Uni-Bicocca and OAB), A. Moretti(OAB),
report on behalf of the Swift-XRT team:

Using all data collected by Swift/XRT obtained for GRB 070724B
detected by SuperAGILE (Feroci et al., GCN Circ. 6668
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 ),
for a total of 16.5 ks (between T+19.1 hours and T+7.5 days),
we can now confirm that the candidate proposed in GCN Circ. 6675
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(Romano et al.) is the afterglow of GRB 070724B.
Its final position is RA,DEC(J2000)=17.48477,57.67620, which is

RA(J2000)  =  01 09 56.34
Dec(J2000) = +57 40 34.3

with an uncertainty of 3.9 arcsec (radius, 90% containment).
This is 0.4 arcsec from the X-ray position given in Romano et
al., GCN Circ. 6675
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 ) and 4.6 arcmin from the initial SuperAGILE
position (Feroci et al., GCN Circ. 6668
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 ).
The light curve can be fit with a power-law power-law with
slope of 1.19+0.23-0.19 (90% cl).
No further observations with Swift are planned.

This circular is an official product of the Swift XRT team.