GRB 160521B

H.-F. Yu (MPE) and P. Veres (UAH) report on behalf of the Fermi GBM Team:

"At 09:13:58.03 UT on 21 May 2016, the Fermi Gamma-Ray Burst Monitor
triggered and located GRB 160521B (trigger 485514842 / 160521385).

The on-ground calculated location, using the GBM trigger
data, is RA = 156, Dec = 76 (J2000 degrees, equivalent 
to 15h 25m, 76d 17'), with an uncertainty of 1 degree 
(radius, 1-sigma containment, statistical only; there is 
additionally a systematic error which we have characterized 
as a core-plus-tail model, with 90% of GRBs having a 3.7 deg 
error and a small tail suffering a larger than 10 deg
systematic error. [Connaughton et al. 2015, ApJS, 216, 32] ).

This burst was also independently detected by INTEGRAL SPI-ACS.

The trigger resulted in an Autonomous Repoint Request (ARR)
by the GBM Flight Software owing to the high peak flux of the GRB. 
This ARR was accepted and the spacecraft slewed to the GBM in-flight
location. The initial angle from the Fermi LAT boresight to
the GBM ground location is 47 degree.

The GBM light curve shows a single pulse with sub-structures
with a duration (T90) of about 2.8 s (50-300 keV).
The time-averaged spectrum from T0+0.002 s to T0+3.520 s is
best fit by a Band function with Epeak = 155 +/- 5 keV,
alpha = -0.45 +/- 0.03, and beta = -2.54 +/- 0.07.

The event fluence (10-1000 keV) in this time interval is
(1.25 +/- 0.02)E-05 erg/cm^2. The 1.024-sec peak photon flux measured
starting from T0+0.960 s in the 10-1000 keV band
is 42.4 +/- 0.4 ph/s/cm^2.

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

M. Axelsson (KTH Stockholm), E. Bissaldi (Politecnico and INFN Bari), J. McEnery
(NASA/GSFC), F. Dirirsa (Univ. of Johannesburg) and J.M. Burgess (KTH Stockholm)
report on behalf of the Fermi-LAT team:

At UT 09:13:58.03 on May 21, 2016, Fermi-LAT detected high-energy emission
from GRB 160521B, which was also detected by Fermi-GBM
(trigger 485514841/160521385).

The best LAT on-ground location is found to be

RA, Dec = 148.17, 79.01 (degrees, J2000)

with an error radius of 0.17 deg (90% containment, statistical error only).
This was 47 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.
More than 10 photons above 100 MeV are observed within 3000 seconds, with
the highest-energy photon being a 12 GeV event observed 420 seconds after
the GBM trigger.

A Swift ToO has been approved for this burst.

The Fermi-LAT point of contact for this burst is Magnus Axelsson (magaxe@kth.se<mailto:magaxe@kth.se>).

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

Swift has initiated a ToO observation of the Fermi/LAT GRB 160521B. 
Automated analysis of the XRT data will be presented online at
http://www.swift.ac.uk/ToO_GRBs/00020615

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.

K.L. Page (U. Leicester), P. D'Avanzo (INAF-OAB), V. D'Elia (ASDC), A.
D'Ai (INAF-IASFPA), T.G.R. Roegiers (PSU), L.M. McCauley (PSU), J.A.
Kennea (PSU), C. Pagani (U. Leicester), A.P. Beardmore (U. Leicester)
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 160521B (Axelsson et al. GCN Circ. 19444
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 ),
collecting 2.0 ks of Photon Counting (PC) mode data between T0+41.9 ks
and T0+47.7 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):  147.6669  =  09:50:40.06
  Dec (J2000.0): +79.0302  =  +79:01:48.9
  Error: 4.6 arcsec (radius, 90% conf.)
  Count-rate: 0.0122 +/- 0.0028 ct s^-1
  Flux: (5.3 +/- 1.2)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/00020615.

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

F. E. Marshall (NASA/GSFC) reports on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of GRB 160521B centered on
the LAT position (Axelsson et al., GCN Circ. 19444
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 ) 41882 seconds after
the trigger. The exposures for the optical filters (white, b, and u)
cover about 20% of the LAT statistical error circle, and
the uv filters (uvw1 and uvm2) cover about 44%.
Only the exposures with the uv filters cover
the possible XRT counterpart (Page et al., GCN Circ. 19448
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 ).
The preliminary 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           42794        43127          324          >20.8
b               41882        49001         1180          >20.7
u               47789        48695          885          >20.2
uvw1            46882        54226         1578          >20.1
uvw2            52616        53065          886          >20.4

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.02 in the direction of the burst
(Schlegel et al. 1998).

R. Ruffini, Y. Aimuratov, L. Becerra, C.L. Bianco, M. Kovacevic, R.
Moradi, M. Muccino, A.V. Penacchioni, G.B. Pisani, D. Primorac, J.
Rueda, Y. Wang report:


From preliminary data of GRB 160521B detected by the Fermi satellite (Yu
& Veres, GCN 19443
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 ) and from the presence of both gamma-ray (Fermi-GBM,
8 keV - 40 MeV) and GeV emission (Fermi-LAT, 0.1-100 GeV, see Axelsson
et al., GCN 19444
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 ) it is likely that this source is a Binary driven
Hypernova (BdHN) at very high redshift.

Following Ruffini et al. 2016 (arXiv:1602.02732
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 ), it lies in the BdHN
region of the Ep-Eiso plane for selected values of the redshift above
0.5 (see purple filled circles in Fig.[1]). The observed GeV emission
implies that the source cannot be a XRF.

The current paucity of data of the X-ray afterglow (Page et al., GCN
19448
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 ) is surprising and should be further examined up to late times
taking as an example GRB 090423 at z = 8.

Assuming that GRB 160521B is actually a BdHN, we can test the overlap of
its X-ray flux with the prototypical BdHN sources (see Pisani et al.
2013, A&A, 552, L5). In Fig.[2] it is plotted the single available data
point (Page et al., GCN 19448
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 ), not yet observed dimming, of GRB 160521B
X-ray luminosity, assuming 20 different values of the redshift from 0.5
(the lowest blue point) to 10 (the highest one) in steps of 0.5.

From these preliminary data, GRB 160521B is a BdHN with possible
redshift z > 2.5 all the way up to z ~ 10, assuming that the single
observed X-ray data point belongs to the afterglow. A deep search for
the afterglow in the X-ray band is indeed crucial to define the nature
of this source.


[1] http://www.icranet.org/documents/GRB160521b_Fig1.pdf
[2] http://www.icranet.org/documents/GRB160521b_Fig2.pdf

A. Kozlova, S. Golenetskii, R. Aptekar, D. Frederiks, 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 160521B
(Fermi-GBM detection: Yu and Veres, GCN 19443
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 ;
Fermi-LAT detection: Axelsson et al., GCN 19444
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 )
triggered Konus-Wind at T0=33237.337 s UT (09:13:57.337).

The burst light curve shows a broad, multi-peaked pulse
started at ~T0-0.3 s with a total duration of ~6.4 s.
The emission is seen up to ~2 MeV.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB160521_T33237/

As observed by Konus-Wind, the burst
had a fluence of 1.32(-0.15,+0.17)x10^-5 erg/cm2,
and a 64-ms peak flux, measured from T0+1.504 s,
of 1.01(-0.14,+0.14)x10^-5 erg/cm2/s
(both in the 20 keV - 10 MeV energy range).

The time-averaged spectrum of the burst
(measured from T0 to T0+9.984 s)
is best fit in the 20 keV - 2 MeV range
by the GRB (Band) model with the following parameters:
the low-energy photon index alpha = -0.65(-0.14,+0.16),
the high energy photon index beta = -2.72(-0.49,+0.26),
the peak energy Ep = 165(-16,+17) keV
(chi2 = 49/61 dof)

The spectrum near the maximum count rate
(measured from T0+0.256 to T0+1.792 s)
is best fit in the 20 keV - 2 MeV range
by the GRB (Band) model with the following parameters:
the low-energy photon index alpha = -0.43(-0.12,+0.14),
the high energy photon index beta = -3.62(-6.38,+0.62),
the peak energy Ep = 190(-15,+15) keV
(chi2 = 56/50 dof)

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

K.L. Page (U. Leicester), P. D'Avanzo (INAF-OAB), V. D'Elia (ASDC), A.
D'Ai (INAF-IASFPA), T.G.R. Roegiers (PSU), L.M. McCauley (PSU), J.A.
Kennea (PSU), C. Pagani (U. Leicester), A.P. Beardmore (U. Leicester)
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 160521B (Axelsson et al. GCN Circ. 19444
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 ).
The observations now extend from T0+41.9 ks to T0+346.3 ks. The source
previously reported by Page et al. (GCN Circ. 19448
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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=147.6665, +79.0305
which is equivalent to:

RA (J2000): 09:50:39.96
Dec(J2000): +79:01:49.7

with an uncertainty of 4.2 arcsec (radius, 90% confidence).  This
position is 5.9 arcmin from the Fermi/LAT position. 

The light curve can be modelled with a power-law decay with a decay
index of alpha=0.7 (+0.7, -0.4).

The results of the XRT-team automatic analysis of the likely afterglow
are at http://www.swift.ac.uk/ToO_GRBs/00020615/index_1.php.
The results of the full analysis of the XRT observations are available
at http://www.swift.ac.uk/ToO_GRBs/00020615.

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

K. P. Mooley, T. D. Staley, R. P. Fender (Oxford), G. E. Anderson 
(Curtin), T. Cantwell (Manchester), C. Rumsey, D. Titterington, S. 
Carey, J. Hickish, Y. C. Perrott, N. Razavi-Ghods, P. Scott (Cambridge), 
K. Grainge, A. Scaife (Manchester)

We observed the Fermi/LAT GRB 160521B (Veres et al., GCN 19434
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 ) with the 
AMI Large Array at 15 GHz on 2016 May 24.75, May 27.65, and Jun 03.73 
(UT) as part of the 4pisky program. We do not detect any source at the 
XRT location (Page et al., GCN 19448
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 ), with 3sigma upper limits of 108 
uJy, 108 uJy, and 117 uJy respectively.

We thank the AMI staff for scheduling these observations. The AMI-GRB 
database is a log of all GRB follow up observations with the AMI, and is 
available at http://4pisky.org/ami-grb/.