GRB 150523A

H.-F. Yu (MPE) reports on behalf of the Fermi GBM Team:

"At 09:29:48.08 UT on 23 May 2015, the Fermi Gamma-Ray Burst Monitor
triggered and located GRB 150523A (trigger 454066191 / 150523396).

The on-ground calculated location, using the GBM trigger
data, is RA = 121.2, Dec = -37.3 (J2000 degrees, equivalent 
to 08h 05m, -37d 20'), with an uncertainty of 1.7 degrees 
(radius, 1-sigma containment, statistical only; there is 
additionally a systematic error which is currently estimated 
to be 2 to 3 degrees).

The trigger resulted in an Autonomous Repoint Request (ARR) 
that was accepted and the spacecraft slewed to the GBM in-flight 
location. The initial angle from the Fermi LAT boresight
is 22 degrees.

The GBM light curve consists of two pulses with a duration (T90) 
of about 74.2 s (50-300 keV). The time-averaged spectrum 
from T0-4.352 s to T0+40.705 s is best fit by a power law function 
with an exponential high-energy cutoff. The power law index 
is -0.64 +/- 0.03 and the cutoff energy, parameterized as Epeak, 
is 574 +/- 24 keV.

The event fluence (10-1000 keV) in this time interval is
(3.57 +/- 0.05)E-05 erg/cm^2. The 1.024-sec peak photon flux 
measured starting from T0+32.769 s in the 10-1000 keV band
is 8.34 +/- 0.25 ph/s/cm^2.

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

E. Bissaldi (INFN Bari), G. Vianello (Stanford University)
and M. Yassine (LUPM) report on behalf of the Fermi-LAT team:



At 09:29:48.08 UT on May 23, 2015, Fermi-LAT detected
high-energy emission from GRB 150523A, which was also
detected by Fermi-GBM (trigger 454066191 / 150523396)
(Yu et al., GCN #17863
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 ).
The best LAT on-ground location is found to be

RA, Dec = 115.36, -45.40 (J2000)

with an error radius of 0.10 deg (90 per cent containment,
statistical error only).
This was 26 deg from the LAT boresight at the time of
the trigger and triggered an autonomous
repoint of the spacecraft.

The data from the Fermi-LAT show a significant increase
in the event rate that is spatially and temporally
correlated with the trigger with high significance.
The highest-energy photon is a 7 GeV event, which is
observed 118 s 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.

A.P. Beardmore (U. Leicester), A. Amaral-Rogers (U. Leicester), P.
D'Avanzo (INAF-OAB), V. D'Elia (ASDC), A. D'a��� (INAF-IASFPA), D.N.
Burrows (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 150523A collecting 1.4 ks of Photon
Counting (PC) mode data between T0+33.0 ks and T0+34.3 ks. 

An uncatalogued X-ray source is detected and is above the RASS limit,
and is therefore likely the GRB afterglow.  The position of this source
is RA, Dec=115.2859, -45.4209 which is equivalent to:

RA (J2000): 07:41:8.62
Dec(J2000): -45:25:15.1

with an uncertainty of 3.7 arcsec (radius, 90% confidence).  This
position is 3.4 arcmin from the Fermi/LAT position.  The light curve is
consistent with a constant source of mean count rate 1.2e-01 ct/sec. A
power-law fit gives an index of -1.41 (+4.59, -0.05).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.71 (+0.29, -0.21). The
best-fitting absorption column is  consistent with the Galactic value
of 3.5 x 10^21 cm^-2 (Willingale et al. 2013). The counts to observed
(unabsorbed) 0.3-10 keV flux conversion factor deduced from this
spectrum  is 4.5 x 10^-11 (5.9 x 10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     3.5 (+/-1.3) x 10^21 cm^-2
Galactic foreground: 3.5 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.71 (+0.29, -0.21)

The results of the XRT-team automatic analysis of the likely afterglow
are at http://www.swift.ac.uk/ToO_GRBs/00020520/index_1.php.
The results of the full analysis of the XRT observations are available
at http://www.swift.ac.uk/ToO_GRBs/00020520.

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

S. Dichiara, C. Guidorzi (U. Ferrara), C.G. Mundell (Bath U.),
A. Gomboc (U. Ljubljana) on behalf of a larger collaboration report:

The 1-m LCOGT unit in Cerro Tololo began observing Fermi GBM
and LAT GRB 150523A (Yu et al GCN 17863
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 ; Bissaldi et al. GCN 17864
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 )
on May 23 at 23:17 UT (~13.8 hours after the burst trigger)
with SDSS r and i filters.
Inside the Swift-XRT error circle (Beardmore et al GCN 17866
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 )
we detect an uncatalogued object at the following position:

RA(J2000) =   07:41:08.68
Dec(J2000)=  -45:25:12.3

with an uncertainty of ~1" and a magnitude of R=20.2 +-0.1 calibrated
against nearby USNOB-1 stars. Presently we cannot establish whether
the source is fading.

S. Dichiara, C. Guidorzi (U. Ferrara), S.Kobayashi (LJMU),
C.G. Mundell (U. Bath), A. Gomboc (U. Ljubljana) on behalf
of a larger collaboration report:

On May 24 we re-observed GRB 150523A (Yu et al GCN 17863
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 ;
Bissaldi et al. GCN 17864
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 ) using the 2-m Faulkes Telescope South
(FTS) with SDSS r and i filters, starting from 08:53 UT (~23.4 hours
post burst). Compared with our previous report (Dichiara et al.
GCN 17868
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 ) the optical candidate appears to have significantly
faded, as follows:

Mid time(hrs)    Exp(s)       Filter   Magnitude
-----------------------------------------------
13.87            10x120          r'    20.45 +- 0.15
23.69            10x120          r'    21.20 +- 0.22
-----------------------------------------------

The corresponding average power-law decay index is 1.3 +- 0.5.
Magnitudes are calibrated against USNOB-1 nearby stars.
We therefore confirm the afterglow nature of the optical
candidate previously reported.

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 GRB 150523A
(Fermi-GBM detection: Yu, GCN Circ. 17863
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 ;
Fermi-LAT detection: Bissaldi, et al., GCN Circ. 17864
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 )
triggered Konus-Wind at T0=34189.641 s UT (09:29:49.641).

The burst light curve consists of two pulses
with a total duration of ~36 s.
The emission is seen up to ~10 MeV.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB150523_T34189/

As observed by Konus-Wind, the burst
had a fluence of 4.44(-0.69,+0.71)x10^-5 erg/cm2,
and a 64-ms peak flux, measured from T0+31.792 s,
of 5.52(-1.52,+1.53)x10^-6 erg/cm2/s
(both in the 20 keV - 10 MeV energy range).

The time-averaged spectrum of the burst
(measured from T0 to T0+41.216 s)
is best fit in the 20 keV - 12 MeV range
by the GRB (Band) model with the following parameters:
the low-energy photon index alpha = -0.31(-0.14,+0.16),
the high energy photon index beta = -2.66(-0.99,+0.32),
the peak energy 399(-42,+52) keV
(chi2 = 94/97 dof)

The spectrum near the maximum count rate
(measured from T0+24.832 to T0+33.024 s)
is best fit in the 20 keV - 12 MeV range
by the GRB (Band) model with the following parameters:
the low-energy photon index alpha = -0.35(-0.15,+0.17),
the high energy photon index beta = -2.66(-1.35,+0.34),
the peak energy 389(-48,+58) keV
(chi2 = 104/97 dof)

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

F. E. Marshall (NASA/GSFC) and A. Vargas (PSU)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 150523A
32968 s after the LAT trigger (Bissaldi et al. GCN Circ. 17864
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 ).
A source consistent with the XRT position (Beardmore et al. GCN Circ. 17866
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 )
and the optical afterglow position (Dichiara et al. GCN Circ. 17868
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  and 17869
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 )
is detected in the initial UVOT exposures.

The preliminary UVOT position is:
    RA  (J2000) =  07:41:08.67 = 115.28613 (deg.)
    Dec (J2000) = -45:25:12.4  = -45.42011 (deg.)
with an estimated uncertainty of 0.78 arc sec. (radius, 90% confidence).

Preliminary detections using the UVOT photometric system
(Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the early exposures are: 

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

white            33590        39627         1150         21.83 +/- 0.24
v                34212        40619         1433        >20.5
u                32968        44748         2077        >20.5

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.30 in the direction of the burst
(Schlegel et al. 1998).

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

Swift has initiated a ToO observation of the Fermi/LAT GRB 150523A. 
Automated analysis of the XRT data will be presented online at
http://www.swift.ac.uk/ToO_GRBs/00020521

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.

T. Fujinuma,  M. Tashiro, Y. Terada, T. Yasuda, S. Takeda,
T. Nagayoshi, J. Enomoto, S. Nakaya, S. Matsuoka, S. Yabe (Saitama U.),
M. Yamauchi, N. Ohmori, R. Kinoshita (Univ. of Miyazaki),
W. Iwakiri (RIKEN), K. Yamaoka (Nagoya U.), S. Sugita (Ehime U.),
M. Ohno, T. Kawano, Y. Fukazawa (Hiroshima U.),
Y. E. Nakagawa, M. Kokubun, T. Takahashi (ISAS/JAXA),
Y. Urata (NCU), K. Nakazawa, K. Makishima (Univ. of Tokyo)
on behalf of the Suzaku WAM team, report:

The long GRB 150523A(Fermi GBM trigger: Yu et al.,GCN 17863;
Fermi-LAT detection:Bissaldi et al.,GCN 17864;
Konus-Wind observation: Golenetskii et al.,GCN 17870) triggered the 
Suzaku Wide-band All-sky Monitor (WAM) which covers an energy range
of 50 keV - 5 MeV at 09:29:50 UT (=T0).

The observed light curve shows double-peaked structure starting at
T0-4 s, ending at T0+33 s, with a duration (T90) of about 32 seconds.
The fluence in 100 - 1000 keV was 1.81(+0.23/-0.25) x 10^-5 erg/cm^2.
The 1-s peak flux measured from T0+31 s was
3.52(+0.19/-0.34) photons/cm^2/s in the same energy range.

Preliminary result shows that the time-averaged spectrum from T0-4s
to T0+33s is well fitted by a power-law with exponential cutoff model:
dN/dE ~  E^{-alpha} * exp(-(2-alpha)*E/Epeak) with
alpha    0.70(+0.48/-0.56), and
Epeak    417(+56/-41) keV (chi^2/d.o.f. = 61.26/52).

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

The light curves with 1-sec time resolution for this burst will be 
appeared at:
http://www.astro.isas.jaxa.jp/suzaku/HXD-WAM/WAM-GRB/grb/untrig/grb_table.html