GRB 150724B

E. Bissaldi (INFN Bari), J. L. Racusin (NASA/GSFC), and E. Moretti (MPP Munich) 
report on behalf of the Fermi-LAT team: 

At 18:45:36.71 on 2015-07-24, Fermi-LAT detected high-energy emission from GRB 150724B, 
which was also detected by Fermi-GBM (trigger 459456340/150724782). 

The best LAT on-ground location is found to be

RA, Dec = 351.92, 3.67 (J2000) 

with an error radius of 0.3 deg (90% containment, systematic error only). This was 58 deg 
from the LAT boresight at the time of the trigger.

The data from the Fermi-LAT show a ~5 sigma increase in the event rate within 7 degrees 
of the GBM location after the GBM trigger that is spatially and temporally correlated with the 
GBM emission. More than 11 photons above 100 MeV and 1 photons above 1 GeV were 
observed within 300 seconds. The highest-energy photon is a 2 GeV event which is 
observed ~50 seconds after the GBM trigger.

A Swift ToO has been requested for this burst.

The Fermi-LAT point of contact for this burst is Elisabetta Bissaldi (elisabetta.bissaldi@ba.infn.it). 

The Fermi-LAT is a pair conversion telescope designed to cover the energy band from 20 MeV 
to greater than 300 GeV. It is the product of an international collaboration between NASA and 
DOE in the U.S. and many scientific institutions across France, Italy, Japan and Sweden.

P. Veres (UAH), V. Connaughton (USRA) and C. Meegan (UAH)
report on behalf of the Fermi GBM Team:

"At 18:45:37.714 UT on 24 July 2015, the Fermi Gamma-Ray Burst Monitor
triggered and located GRB 150724B (trigger 459456341 / 150724782)
which was also detected by the Fermi/LAT (Bissaldi et al. 2008, GCN
18065
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 ). The GBM on-ground location is consistent with the Fermi/LAT
position.


The on-ground calculated location, using the GBM trigger
data, is RA = 350.92, DEC = +10.160 (J2000 degrees,
equivalent to 23h 24m, 10d 10'), with an uncertainty
of 1.05 degrees (radius, 1-sigma containment,
statistical only).


The GBM light curve consists of two emission episodes
with a total duration (T90) of about 38 s (50-300 keV).
The time-averaged spectrum from T0-6.1 s to T0+46 s is
adequately fit by a power law function with an exponential
high-energy cutoff.  The power law index is -0.75 +/- 0.02 and
the cutoff energy, parameterized as Epeak, is 729 +/- 35 keV.


The event fluence (10-1000 keV) in this time interval is
(3.55 +/- 0.04)E-5 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+40 s in the 8-1000 keV band is 8.3 +/- 0.2 ph/s/cm^2.


The spectral analysis results presented above are preliminary;
final results will be published in the GBM GRB Catalog."

P. A. Evans (U. Leicester) reports on behalf of the Swift team:

Swift has initiated a ToO observation of the Fermi/LAT GRB 150724B. 
Automated analysis of the XRT data will be presented online at
http://www.swift.ac.uk/ToO_GRBs/00020543

Any uncatalogued X-ray sources detected in this analysis will be
reported on this website and via GCN COUNTERPART notices. These are
not necessarily related to the Fermi/LAT event. Any X-ray source
considered to be a probable afterglow candidate will be reported via a 
GCN Circular after manual consideration.

Details of the XRT automated analysis methods are detailed in Evans et
al. (2007, A&A, 469, 379; 2009, MNRAS, 397, 1177 and 2014, ApJS, 210, 8).

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

J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), C. Pagani (U.
Leicester), V. D'Elia (ASDC), A. D'ai (INAF-IASFPA), A. Maselli 
(INAF-IASFPA), L.M. McCauley (PSU), J.A. Kennea (PSU), B. Sbarufatti
(INAF-OAB/PSU) and P.A. Evans (U. Leicester) report on behalf of the
Swift-XRT team:

Swift-XRT has performed follow-up observations of the
Fermi/LAT-detected burst GRB 150724B (Bissaldi et al. GCN Circ. 18065
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 ),
collecting 1.8 ks of Photon Counting (PC) mode data between T0+94.4 ks
and T0+96.2 ks. 

One uncatalogued X-ray source has been detected, it is below the RASS
limit and shows no definitive signs of fading. Therefore, at the
present time we cannot confirm this as the afterglow. Details of this
source are given below:

Source 1:
  RA (J2000.0):  351.8904  =  23:27:33.70
  Dec (J2000.0): +3.8186  =  +03:49:06.8
  Error: 4.3 arcsec (radius, 90% conf.)
  Count-rate: 0.0143 +/- 0.0033 ct s^-1
  Flux: (4.6 +/- 1.1)e-13 erg cm^-2 s^-1 (observed, 0.3-10 keV)

The results of the XRT-team automatic analysis of the XRT observations,
including a position-specific upper limit calculator, are available at
http://www.swift.ac.uk/ToO_GRBs/00020543.

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

Nat Butler (ASU), Alan M. Watson (UNAM), Alexander Kutyrev (GSFC), William 
H. Lee (UNAM), Michael G. Richer (UNAM), Chris Klein (UCB), Ori Fox (UCB), 
J. Xavier Prochaska (UCSC), Josh Bloom (UCB), Antonino Cucchiara 
(ORAU/GSFC), Eleonora Troja (GSFC), Owen Littlejohns (ASU), Enrico 
Ramirez-Ruiz (UCSC), José A. de Diego (UNAM), Leonid Georgiev (UNAM), 
Jesús González (UNAM), Carlos Román-Zúñiga (UNAM), Neil Gehrels (GSFC), 
and Harvey Moseley (GSFC) report:

We observed the field of the LAT GRB 150724B (Bissaldi et al., GCN 18065
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 ) 
with the Reionization and Transients Infrared Camera (RATIR; 
www.ratir.org) on the 1.5m Harold Johnson Telescope at the Observatorio 
Astronómico Nacional on Sierra San Pedro Mártir from 2015/07 26.28 to 
2015/07 26.48 UTC (36.02 to 40.86 hours after the BAT trigger), obtaining 
a total of 2.39 hours exposure in the r and i bands.

For a source within the Swift XRT error circle (Osborne et al., GCN 
18069
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 ), in comparison with the SDSS DR9 catalog, we obtain the following 
detections:

   r 23.24 +/- 0.34
   i 22.39 +/- 0.15

These magnitudes are in the AB system and are not corrected for Galactic 
extinction in the direction of the GRB.

At this moment we have no information on fading. Further observations are 
planned.

We thank the staff of the Observatorio Astronómico Nacional in San Pedro 
Mártir.

S. Golenetskii, R.Aptekar, D. Frederiks, V. Pal'shin, P. Oleynik,
M. Ulanov, D. Svinkin, A. Tsvetkova, A. Lysenko, and T. Cline
on behalf of the Konus-Wind team, report:

The long-duration, hard-spectrum GRB 150724B
(Fermi-LAT detection: Bissaldi, Racusin & Moretti, GCN 18065
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 ;
Fermi GBM detection: Veres, Connaughton & Meegan, GCN 18066
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 )
triggered Konus-Wind at T0=67574.792 s UT (18:46:14.792).

The burst light curve shows  a double-peaked structure
which starts ~30 s before the trigger and has a total duration of ~41 s.
The emission is seen up to ~5 MeV.

As observed by Konus-Wind, the burst
had a fluence of 3.63(-0.29,+0.69)x10^-5 erg/cm2,
and a 64-ms peak energy flux, measured from T0+2.064 s,
of 6.01(-1.60,+1.62)x10^-6 erg/cm2/s
(both in the 20 keV - 10 MeV energy range).

Only the brightest part of the burst is covered by the
KW triggered mode data and, therefore, available for
the multi-channel spectral analysis. The time-averaged
spectrum measured from T0 to T0+8.448 s
is best fit in the 20 keV - 5 MeV range
by a power law with exponential cutoff model:
dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep)
with  alpha = -0.65(-0.11,+0.12),
and Ep = 663(-69,+82) keV (chi2 = 77/73 dof).
Fitting by the GRB (Band) function yields the same alpha and Ep,
and an upper limit on the high energy photon index:
beta < -3.0 (chi2 = 77/72 dof).

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB150724_T67574/

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

N.P.M. Kuin(MSSL/UCL) reports on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 150724B
88.6ks after the LAT trigger (Bissaldi et al., GCN Circ. 18065
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 ),
which was also observed at high energy by Fermi-GBM (Veres et al.,
GCN Circ 18066
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 ) and Konus-Wind (Golenetskii et al. GCN Circ. 18074
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 ).
Five UVOT exposures were obtained of which the first two show a
weak detection at the XRT position (Osborne et al., GCN Circ. 18069
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 ).

Preliminary magnitudes and 3-sigma upper limits using the UVOT
photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373)
are:

Filter         T_start(s)   T_stop(s)      Exp(s)         Mag

white            88619      88799          178        19.64+-0.12
white            88802      90608         1765        20.99+-0.14
white            90614      90748           60        >19.53
white            94379      94558          176        >20.82
white            94562      96236         1648        >22.01

The brightness is consistent with the RATIR observation at 147ks
in r and i (Butler et al, GCN 18072
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 ).

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.06 in the direction of the burst
(Schlegel et al. 1998).

K.Ivanov, O.Gres, N.M.Budnev, S.Yazev,
Irkutsk State University

V. Lipunov, E. Gorbovskoy,  N.Tyurina, V.Kornilov,
P.Balanutsa,A.Kuznetsov,D.Kuvshinov,
Lomonosov Moscow State University, Sternberg Astronomical Institute

R. Rebolo, M. Serra, N. Lodieu, G. Israelian, L. Suarez-Andres
The Instituto de Astrofisica de Canarias

D.Buckley, S. Potter, A.Kniazev, M.Kotze
South African Astronomical Observatory

A. Tlatov, A.V. Parhomenko, D. Dormidontov, V.Sennik
Kislovodsk Solar Station of the Pulkovo Observatory

V.Yurkov, Yu.Sergienko, D.Varda, E.Sinyakov, A.Gabovich
Blagoveschensk Educational State University, Blagoveschensk

V.Krushinski, I.Zalozhnih
Ural Federal University, Kourovka

Hugo Levato and Carlos Saffe
Instituto de Ciencias Astronomicas, de la Tierra y del Espacio (ICATE)

Claudio Mallamaci, Carlos Lopez and Federico Podest
Observatorio Astronomico Felix Aguilar (OAFA)



MASTER II  robotic telescope (MASTER-Net: http://observ.pereplet.ru) 
located in Tunka was pointed to the  GRB150724B (Bissaldi et al. GCN 
18065
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 ) 9244 sec after notice time and 78080 sec after trigger time
at 2015-07-25 16:26:57 UT.
We do not see any OT brighter than 20.3 inside Fermi LAT error box and at 
Swift XRT transient position (Osborne et al., GCN 18069
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 ).
All magnitudes are unfiltered (0.2B+0.8R) with respect to number USNO B 
stars).

The message may be cited.

Alan M. Watson (UNAM), Nat Butler (ASU), Alexander Kutyrev (GSFC), William 
H. Lee (UNAM), Michael G. Richer (UNAM), Chris Klein (UCB), Ori Fox (UCB), 
J. Xavier Prochaska (UCSC), Josh Bloom (UCB), Antonino Cucchiara 
(ORAU/GSFC), Eleonora Troja (GSFC), Owen Littlejohns (ASU), Enrico 
Ramirez-Ruiz (UCSC), José A. de Diego (UNAM), Leonid Georgiev (UNAM), 
Jesús González (UNAM), Carlos Román-Zúñiga (UNAM), Neil Gehrels (GSFC), 
and Harvey Moseley (GSFC) report:

We reobserved the field of the LAT GRB 150724B (Bissaldi et al., GCN 
18065
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 ) with the Reionization and Transients Infrared Camera (RATIR; 
www.ratir.org) on the 1.5m Harold Johnson Telescope at the Observatorio 
Astronómico Nacional on Sierra San Pedro Mártir from 2015/07 27.24 to 
2015/07 27.47 UTC (8819.11 to 8824.45 hours after the Fermi trigger), 
obtaining a total of 4.01 hours exposure in the r and i bands.

We detect the source we previously reported (Butler et al. GCN 18072
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 ) at 
23:27:33.82 +3:49:06.8 (J2000, �0.5"). In comparison with the SDSS DR9, we 
obtain the following detections:

   r 23.50 ± 0.19
   i 23.99 ± 0.27

These magnitudes are in the AB system and are not corrected for Galactic 
extinction in the direction of the GRB.

A naive comparison to our observations from the previous night would 
suggest that the source has not faded in r but has faded in i. However, 
examining the images in detail, we suspect that our photometry in i from 
the first night might be influenced by a spuriously bright pixel. We 
therefore discount the apparent fading in i.

We thank the staff of the Observatorio Astronómico Nacional in San Pedro 
Mártir.

[GCN OPS NOTE(27jul15): Per author's request, "140724A" was changed to "150724B" 
in the Subject-line.]

Paul Kuin (MSSL/UCL) reports on behalf of the Swift/UVOT
team:

In GCN Circ.18075
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  we reported a detection in the first two
white band images with the Swift UVOT.

Further examination of the UVOT data showed that in the
initial analysis the background region included part off
the detector and gave a false reading. Correcting for that
it is found that for the first two images there is just an
upper limit of  white > 22.00 mag at 88.7ks after the
trigger.

We apologise for the confusion this error may have caused.

J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), C. Pagani (U.
Leicester), V. D'Elia (ASDC), A. D'ai (INAF-IASFPA), A. Maselli 
(INAF-IASFPA), L.M. McCauley (PSU), J.A. Kennea (PSU), B. Sbarufatti
(INAF-OAB/PSU) and P.A. Evans (U. Leicester) report on behalf of the
Swift-XRT team:

Swift-XRT has conducted further observations of the field of the
Fermi/LAT-detected burst GRB 150724B (Bissaldi et al. GCN Circ. 18065
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 ).
The observations now extend from T0+88.6 ks to T0+607.8 ks. The source
previously reported by Osborne et al. (GCN Circ. 18069
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 ), "Source 1", is
fading with 3-sigma significance, and is therefore likely the GRB
afterglow.  The position of this source is RA, Dec=351.8905, +3.8181
which is equivalent to:

RA (J2000): 23:27:33.71
Dec(J2000): +03:49:05.2

with an uncertainty of 4.1 arcsec (radius, 90% confidence).  This
position is 9.1 arcmin from the Fermi/LAT position.  The source is
fading with alpha >0.3.

The results of the XRT-team automatic analysis of the likely afterglow
are at http://www.swift.ac.uk/ToO_GRBs/00020543/index_1.php.
The results of the full analysis of the XRT observations are available
at http://www.swift.ac.uk/ToO_GRBs/00020543.

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