GRB 160530A

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

Swift has initiated a series of observations, tiled on the sky, of the
COSI GRB 160530A. Automated analysis of the XRT data will
be presented online at http://www.swift.ac.uk/xrt_products/TILED_GRB00055

Any uncatalogued X-ray sources detected in this analysis will be
reported on this website and via GCN COUNTERPART notices. The probability of finding
serendipitous sources, unrelated to the COSI event is high: 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.

John A. Tomsick (UC Berkeley/Space Sciences Laboratory) reports on behalf 
of the Compton Spectrometer and Imager (COSI) team:

COSI is a large field of view (1/4-sky) gamma-ray (0.1-10 MeV) instrument on 
a Superpressure Balloon flight that began on May 16.  

A likely Gamma-ray Burst (GRB 160530A) was detected by COSI in both the 
cesium iodide shields and the germanium strip detectors (GeDs).  The GRB 
started at 2016 May 30, 07:03:46 UT and lasted until 07:04:23 UT (based on 
the shield rates).  During the burst, the light curve shows several large amplitude 
fluctuations.

Based on the GeD rates and assuming a Band function with alpha=1.0, 
beta=2.5, and E_break = 150 keV, we calculated a preliminary burst fluence of 
1.3E-5 erg/cm2.  This value may be revised in the future using the spectral 
capabilities of the GeDs.

The position of the GRB is R.A. = 120.2 deg, Decl. = -26.2 deg (J2000, 90% 
confidence error radius = 1.5 degrees), which corresponds to Galactic coordinates 
of l = 243.9 deg, b = 2.1 deg.  Due to the proximity to the Galactic plane, a Galactic 
origin cannot be ruled out.

We searched the SIMBAD database for possible counterparts, and three known 
gamma-ray sources are present in the COSI error region:  GRB 080516, 
2FGL J0758.0-2615c, and 3FGL J0755.2-2633 are, respectively, 0.4, 0.6, and 
1.3 degrees from the center of the COSI error circle. 

Follow-up observations with Swift/XRT are underway (see P.A. Evans et al., GCN 
Circ. 19472
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 ) to search for an afterglow that may allow us to improve the position 
and provide a definitive classification of the COSI burst.

D. Svinkin, S. Golenetskii, R. Aptekar, D. Frederiks, A. Kozlova,
and T. Cline on behalf of the Konus-Wind team,

K. Hurley, on behalf of the IPN,

A. von Kienlin, X. Zhang, A. Rau, V. Savchenko, E. Bozzo,
and C. Ferrigno, on behalf of the INTEGRAL SPI-ACS GRB team, report:

The long-duration, very bright GRB 160530A (COSI detection Tomsick GCN 
Circ. 19473
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 ) has been detected by Konus-Wind and INTEGRAL (SPI-ACS), so 
far, at about 25426 s UT (07:03:46).

We have triangulated it to a Konus-SPI-ACS annulus centered at 
RA(2000)=50.332 deg (03h 21m 20s)  Dec(2000)=+12.855 deg (+12d 51' 19"), 
whose radius is 77.066 +/- 0.305 deg (3 sigma).

The COSI position reported by Tomsick (GCN Circ. 19473
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 ) is
consistent with the annulus. The annulus combined with the COSI
(90% confidence) error circle produces the following preliminary error
box:
----------------------------------
RA(2000), deg   Dec(2000), deg
----------------------------------
Center:
118.942        -25.764
Corners:
118.644        -25.659
118.758        -26.966
119.239        -25.868
119.707        -24.768
----------------------------------
The error box area is 3404.3 sq. arcmin, and its maximum
dimension is 2.36 deg (the minimum one is 18 arcmin).
The Sun distance was about 67 deg.

This box may be improved.

A triangulation map is posted at
http://www.ioffe.ru/LEA/GRBs/GRB160530_T25426/IPN/

The Konus-Wind time history and spectrum will be given in a forthcoming
GCN Circular.

D. Svinkin, S. Golenetskii, R. Aptekar, D. Frederiks, P. Oleynik,
M. Ulanov, A. Tsvetkova, A. Lysenko, A. Kozlova, and T. Cline
on behalf of the Konus-Wind team, report:

The long-duration, very intense GRB 160530A
(COSI detection: Tomsick, GCN Circ. 19473
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 ;
IPN triangulation: Svinkin et al., GCN circ. 19476
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 )
triggered Konus-Wind at T0=25426.549 s UT (07:03:46.549).

The burst light curve shows a multipeaked structure
started at ~T0-0.1 s with a total duration of ~37 s.
The emission is seen up to ~5 MeV.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB160530_T25426/

As observed by Konus-Wind, the burst
had a fluence of 1.30(-0.04,+0.04)x10^-4 erg/cm2,
and a 64-ms peak flux, measured from T0+2.416 s,
of 3.70(-0.38,+0.38)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+38.912 s)
is best fit in the 20 keV - 5 MeV range
by a power law with exponential cutoff (CPL) model:
dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep)
with alpha = -0.93(-0.03,+0.03)
and Ep = 638(-33,+36) keV (chi2 = 73/73 dof).
Fitting by a GRB (Band) model yields the same alpha and Ep,
and an upper limit on the high energy photon index: beta < -3.5
(chi2 = 73/72 dof)

The spectrum near the maximum count rate
(measured from T0+1.792 to T0+2.560 s)
is best fit in the 20 keV - 7 MeV range
by the CPL model
with alpha = -0.23(-0.09,+0.10)
and Ep = 714(-50,+55) keV (chi2 = 53/62 dof).
Fitting by a GRB (Band) model yields the same alpha and Ep,
and an upper limit on the high energy photon index: beta < -2.9
(chi2 = 50/61 dof)

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

V. D���Elia (ASDC), L. Izzo (IAA-CSIC), P. A. Evans (U. Leicester), A.  
A. Breeveld (UCL-MSSL), J.A. Kennea (PSU), D. Malesani (DARK/NBI), C.  
B. Markwardt (NASA/GSFC) report on behalf of the Swift team:

Swift has performed follow-up observations of the COSI-detected burst  
GRB 160530A (Tomsick, GCN Circ. 19473
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 ) in a series of observations  
tiled on the sky. The total exposure time is 24 ks, distributed over  
46 tiles; the maximum exposure at a single sky location was 1.4 ks.  
The data were collected between T0+19.0 ks and T0+158 ks, and are  
entirely in Photon Counting (PC) mode.

One uncatalogued X-ray source has been detected by XRT in the  
intersection of the COSI and the IPN error regions (Svinkin et al.,  
GCN Circ. 19476
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 ), however, this source is below the RASS limit at its  
location, and we cannot currently determine whether it is fading.  
Therefore, at the present time we cannot state if it is the afterglow.  
The source was imaged and detected in the time interval T0+145 ks to  
T0+158 ks and comprises 920 s of exposure time. More XRT data are  
expected in the forthcoming days. Further details of this source are  
given below:

RA (J2000.0):  118.9741  =  07:55:53.77
Dec (J2000.0): -25.4638  =  -25:27:49.7
Error: 6.6 arcsec (radius, 90% conf.)
Count-rate: 0.0222 +/- 0.0070 ct s^-1
Flux: (6.3 +/- 2.0)e-13 erg cm^-2 s^-1 (observed, 0.3-10 keV)

No optical afterglow consistent with the XRT source position is  
detected in the initial UVOT exposures. The preliminary 3-sigma upper  
limit for the summed U-band exposures (926 s) is 20.8 mag.

V. D���Elia (ASDC), L. Izzo (IAA-CSIC), A. A. Breeveld (UCL-MSSL), C. B.  
Markwardt (NASA/GSFC) report on behalf of the Swift team:

The source reported in D���Elia et al. (GCN Circ. 19481
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 ) as an X-ray  
afterglow candidate for GRB 160530A (Tomsick, GCN Circ. 19473
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 ; Svinkin  
et al., GCN Circ. 19476
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 ) does not show any sign of fading during the  
Swift-XRT observations. Several X-ray sources have been detected  
during the XRT follow-up campaign, but again, none of them show  
evidence of fading. We thus conclude that the X-ray afterglow  
associated with GRB 160530A was not identified by our observations.

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