GRB 170531A

P.A. Evans (U Leicester), S. D. Barthelmy (GSFC),
H. A. Krimm (NSF/USRA), N. P. M. Kuin (UCL-MSSL),
A. Y. Lien (GSFC/UMBC), F. E. Marshall (NASA/GSFC),
K. L. Page (U Leicester), D. M. Palmer (LANL), M. H. Siegel (PSU) and
T. N. Ukwatta (LANL) report on behalf of the Swift Team:

At 20:07:32 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 170531A (trigger=755343).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 121.261, -14.033 which is 
   RA(J2000)  =  08h 05m 03s
   Dec(J2000) = -14d 01' 57"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  As is typical for image triggers, there is nothing
significant in the real-time light curve.  We note that this trigger occurred
while coming out of the SAA. 

The XRT began observing the field at 20:09:35.5 UT, 122.6 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
uncatalogued X-ray source located at RA, Dec 121.25380, -14.02292 which
is equivalent to:
   RA(J2000)  = 08h 05m 00.91s
   Dec(J2000) = -14d 01' 22.5"
with an uncertainty of 4.5 arcseconds (radius, 90% containment). This
location is 44 arcseconds from the BAT onboard position, within the BAT
error circle. This position may be improved as more data are received;
the latest position is available at http://www.swift.ac.uk/sper.  We
cannot determine whether the source is fading at the present time. No
spectrum from the promptly downlinked event data is yet available to
determine the column density. 

The initial flux in the 2.5 s image was 2.92e-10 erg cm^-2 s^-1 (0.2-10
keV). 

No UVOT data are available at this time. 

Burst Advocate for this burst is P.A. Evans (pae9 AT star.le.ac.uk). 
Please contact the BA by email if you require additional information
regarding Swift followup of this burst. In extremely urgent cases, after
trying the Burst Advocate, you can contact the Swift PI by phone (see
Swift TOO web site for information: http://www.swift.psu.edu/too.html.)

M. H. Siegel (PSU) and P. A. Evans (U Leicester) report on behalf of the Swift/UVOT team:

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 3522 seconds after the BAT trigger. No credible afterglow candidate
has been found in the initial data products. The 2.7'x2.7' sub-image covers
100% of the XRT error circle. The typical 3-sigma upper limit has been about
19.6 mag. The coverage of the XRT error circle by the 8'x8' region for the list
of sources generated on-board is uncertain because the large number of sources
filled the available telemetry. The list of sources is typically complete to
about 18 mag. No correction has been made for the expected extinction
corresponding to E(B-V) of 0.09.

G. Pignata (Univ. Andres Bello), L. Wang (CASSACA), P. D'Avanzo (INAF-OAB), D. Malesani (Dark/NBI), L. Izzo (IAA-CSIC), S. Campana (INAF OAB), J. Anderson (ESO), R. Cartier (Southampton), L. Galbany (Univ. de Chile), C. Inserra (QUB), E. Kankare (QUB), K. Maguire (ESO), S. Smartt (QUB), K. W. Smith (QUB), M. Sullivan (Southampton), S. Valenti (UC Davis), O. Yaron (Weizmann), D. Young (QUB), I. Manulis (Weizmann)

We observed GRB 170531A (Evans et al., GCN 21170
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 ) under the extended Public ESO Spectroscopic Survey for Transient Objects (ePESSTO; see Smartt et al. 2015, A&A, 579, 40 http://www.pessto.org ). The observations were performed on the ESO New Technology Telescope at La Silla with the EFOSC2 instrument in imaging mode starting from 2017-05-31 at 23:12:07 UT (i.e. 3.1 hours from the burst).

We obtained two consecutive images each one lasting 150s with the r filter. No optical afterglow is detected in the coadded image within the XRT error circle (http://www.swift.ac.uk/xrt_positions/) down to a limiting magnitude of r > 23 (AB; 3sigma c.l.). The photometric calibration was carried out assuming r=17.07 (AB) for the star at RA(J2000), Dec(J2000) = 08:05:00.30, -14:01:21.3, as reported in the PANSTARRS catalogue.

A. Melandri (INAF-OAB), P. D'Avanzo (INAF-OAB), S. J. LaPorte (PSU),
J.A. Kennea (PSU), B. Sbarufatti (INAF-OAB/PSU), J.P. Osborne (U.
Leicester), K.L. Page (U. Leicester), B. Mingo (U. Leicester), V.
D'Elia (ASDC) and P.A. Evans report on behalf of the Swift-XRT team:

We have analysed 6.2 ks of XRT data for GRB 170531A (Evans et al. GCN
Circ. 21170
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 ), from 254 s to 44.6 ks after the  BAT trigger. The data
are entirely in Photon Counting (PC) mode. The best available XRT
position  (using the promptly downlinked event data, the XRT-UVOT
alignment and matching UVOT field sources to the USNO-B1 catalogue) is
RA, Dec = 121.2524, -14.0227 which is equivalent to:

RA (J2000): 08 05 00.57
Dec(J2000): -14 01 21.8

with an uncertainty of 3.5 arcsec (radius, 90% confidence).

The light curve can be modelled with a power-law decay with a decay
index of alpha=0.62 (+0.20, -0.16).

If the light curve continues to decay with a power-law decay index of
0.62, the count rate at T+24 hours will be 1.3 x 10^-3 count s^-1

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00755343.

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

V. Lipunov, V.Kornilov, N.Tyurina, E.Gorbovskoy, D.Kuvshinov, A.V.Krylov,
I.Gorbunov, P.Balanutsa, A.Kuznetsov, V.V.Chazov, D. Vlasenko
Lomonosov Moscow State University, Sternberg Astronomical Institut of MSU

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

D.Buckley
South African Astronomical Observatory


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

N.M.Budnev, O.Gres, O. Ershova
Irkutsk State University

A.Gabovich, V.Yurkov, Yu.Sergienko
Blagoveschensk Educational State University, Blagoveschensk

R.Podesta, C.Lopez, F. Podesta Observatorio Astronomico Felix Aguilar (OAFA),
National University of San Juan, Argentina

H. Levato, C. Saffe
Instituto de Ciencias Astronomicas,de la Tierra y del Espacio (ICATE),
San Juan, Argentina

MASTER-SAAO  robotic telescope (MASTER-Net: http://observ.pereplet.ru)
located in SAAO was pointed to the  GRB170531.84 25 sec after notice time
and 99 sec after trigger time at 2017-05-31 20:09:15 UT. On our first (20s
exposure)  set we haven`t found optical transient  within SWIFT error-box
(ra=121.258 dec=-14.0328 r=0.05).
The 5-sigma upper limit has been about 15.5mag


The observations made on zenit distance = 84 degrees, galaxy latitude b =
10 degree.
The moon (43 % bright part) is 19 degrees above the horizon. The distance
between  moon and  object is 41
The sun  altitude  is -56.5 degree.


MASTER-IAC robotic telescope (2x4 square degrees, MASTER-Net: 
http://observ.pereplet.ru)  was pointed to the  GRB170531A (Evans et al.,
GCN #21170
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 ; Siegel et al. GCN #21172
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 ) 895 sec after notice time and 2115 
sec after trigger time on the 2017-05-31 20:42:48 UT.

The observations were made on zenit distance = 74 degrees during 
the twilight. The sun  altitude  was  -9.6 degree.
The moon (43 % bright part) is 58 degrees above the horizon. The distance 
between  moon and  object is 41deg.

On our 5-th (180s exposure)  set we haven`t found optical 
transient  within SWIFT error-box (ra=121.258 dec=-14.0328 r=0.1).
The 5-sigma upper limit m~13.9mag (unfiltered) on the first image and


MASTER-OAFA  robotic telescope (MASTER-Net: http://observ.pereplet.ru) 
located in OAFA was pointed to the  GRB170531.84 15674 sec after notice 
time and 15751 sec after trigger time at 2017-06-01 00:30:03 UT. On our 
first (180s exposure)  set we haven`t found optical transient  within 
Master error-box (ra=121.25 dec=-14.0225 r=0.1).
The 5-sigma upper limit has been about 18mag
The message may be cited.


The observations made on zenit distance = 63 degrees, galaxy latitude b = 
10 degree.
The moon (45 % bright part) is 38 degrees above the horizon. The distance 
between  moon and  object is 43
The sun  altitude  is -35.4 degree.
The object can be observed till 2017-06-01 02:41:37.

M. H. Siegel (PSU) and P. A. Evans (U. Leicester)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 170531A
3523 s after the BAT trigger (Evans et al., GCN Circ. 21170
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 ).
No optical afterglow consistent with the XRT position
(Melandri et al., GCN Circ. 21180
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 ) is detected in the initial UVOT exposures.
We note that the XRT position is 3.5� away from a known USNO-B1 source, which
we detect. There is no indication of a second source within the error circle.
Preliminary 3-sigma upper limits using the UVOT photometric system
(Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the first
finding chart (FC) exposure and subsequent exposures are:

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

white_FC          3523         3673          147         >18.1
white             3523         4292          344         >18.2
v                 4502         4702          197         >17.5
b                 3887         4087          197         >18.2
u                 3682         3881          197         >18.0
w2                4298         4497          197         >19.4

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.09 in the direction of the burst
(Schlegel et al. 1998).