GRB 150428B

K. L. Page (U Leicester), M. M. Chester (PSU),
F. E. Marshall (NASA/GSFC), T. Sakamoto (AGU) and M. H. Siegel (PSU)
report on behalf of the Swift Team:

At 03:12:03 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 150428B (trigger=639278).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 292.654, +4.116 which is 
   RA(J2000) = 19h 30m 37s
   Dec(J2000) = +04d 06' 57"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a double-peaked 
structure with a duration of about 30 sec.  The peak count rate
was ~1800 counts/sec (15-350 keV), at ~3 sec after the trigger. 

The XRT began observing the field at 03:13:50.9 UT, 107.5 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source located at RA, Dec 292.63764, 4.12529
which is equivalent to:
   RA(J2000)  = 19h 30m 33.03s
   Dec(J2000) = +04d 07' 31.0"
with an uncertainty of 3.7 arcseconds (radius, 90% containment). This
location is 67 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. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of 3.26
x 10^21 cm^-2 (Willingale et al. 2013). 

The initial flux in the 2.5 s image was 8.91e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 117 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. Because of the density of catalogued stars, further
analysis is required to report an upper limit for any afterglow in the
sub-image. The 8'x8' region for the list of sources generated on-board covers
100% of the XRT error circle. Because of the density of catalogued stars,
further analysis is required to report an upper limit for any afterglow in the
region. No correction has been made for the expected extinction corresponding
to E(B-V) of 0.40. 

Burst Advocate for this burst is K. L. Page (klp5 AT leicester.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.)

A.P. Beardmore, P.A. Evans, M.R. Goad and J.P. Osborne (U. Leicester) 
report on behalf of the Swift-XRT team.

Using 1321 s of XRT Photon Counting mode data and 1 UVOT
images for GRB 150428B, we find an astrometrically corrected X-ray
position (using the XRT-UVOT alignment and matching UVOT field sources
to the USNO-B1 catalogue): RA, Dec = 292.63941, +4.12505 which is equivalent
to:

RA (J2000): 19h 30m 33.46s
Dec (J2000): +04d 07' 30.2"

with an uncertainty of 1.6 arcsec (radius, 90% confidence).

This position may be improved as more data are received. The latest
position can be viewed at http://www.swift.ac.uk/xrt_positions. Position
enhancement is described by Goad et al. (2007, A&A, 476, 1401) and Evans
et al. (2009, MNRAS, 397, 1177).

This circular was automatically generated, and is an official product of the
Swift-XRT team.

J. R. Cummings (GSFC/UMBC), S. D. Barthelmy (GSFC), W. H. Baumgartner (GSFC/UMBC), 
N. Gehrels (GSFC), H. A. Krimm (GSFC/USRA), A. Y. Lien (GSFC/UMBC), 
C. B. Markwardt (GSFC), K. L. Page (U Leicester), D. M. Palmer (LANL), 
T. Sakamoto (AGU), M. Stamatikos (OSU), T. N. Ukwatta (LANL) (i.e. the Swift-BAT team): 

Using the data set from T-239 to T+963 sec from the recent telemetry downlink, 
we report further analysis of BAT GRB 150428B (trigger #639278) 
(Page, et al., GCN Circ. 17766
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 ).   The BAT ground-calculated position is 
RA, Dec = 292.661, 4.114 deg which is 
 RA(J2000)  =  19h 30m 38.8s 
 Dec(J2000) = +04d 06' 52.2��� 
with an uncertainty of 1.3 arcmin, (radius, sys+stat, 90% containment).  
The partial coding was 40%. 

The mask-weighted light curve shows three peaks.  The first peak starts at T-40 sec, 
peaks at T-25 sec and ends at T-20 sec.  The second peak starts at T-15 sec, peaks 
at T+5 sec and ends at T+50 sec.  And the third peak starts at T+55 sec, peaks at T+65 sec 
and ends at T+120 sec.  T90 (15-350 keV) is 130.9 +- 15.6 sec 
(estimated error including systematics).  

The time-averaged spectrum from T-40.1 to T+121.7 sec is best fit by a power law 
with an exponential cutoff.  This fit gives a photon index 1.0 +- 0.5, and Epeak 
of 55 +- 10 keV (chi squared 37.44 for 56 d.o.f.).  For this model the total 
fluence in the 15-150 keV band is 3.7 +- 0.3 x 10^-6 erg/cm2 and the 1-sec peak flux 
measured from T+1.36 sec in the 15-150 keV band is 1.5 +- 0.3 ph/cm2/sec.  A fit 
to a simple power law gives a photon index of 1.87 +- 0.10 
(chi squared 49.44 for 57 d.o.f.).  All the quoted errors are at the 90% confidence level. 

The results of the batgrbproduct analysis are available at 
http://gcn.gsfc.nasa.gov/notices_s/639278/BA/

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

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

K.Ivanov, S.Yazev, N.M.Budnev, O.Gres, O.Chuvalaev, V.A.Poleshchuk
Irkutsk State University

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

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

V.Krushinski, I.Zalozhnih,  A. Popov
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-SAAO: http://observ.pereplet.ru) 
located in SAAO was pointed to the  GRB150428B 24 sec after notice time 
and 59 sec after trigger time at 2015-04-28 03:13:03 UT in two 
polarizations. On our first (10s exposure)  set we haven`t found optical 
transient  within SWIFT BAT (Page et. al GCN 17766
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 ) and XRT (Beardmore et 
al. GCN 17773
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 ) error-boxes .
We have 3x2 images during Gamma Ray activity (see Page et al., GCN 17777
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 ) 
in two polarizations.
So our observations is prompt observations.
We obtain a followin upper limits on single and coadd images:

Date start  T_start -T_trig  T_mid-T_trig  Expt.  Limit Coadd

03:13:03.3        59             64         10    16.5   no
03:13:03.3        59             97         50    17.4    3
03:13:03.3        59            529        810    19.3   12


Data analysis was hampered by a high density of stars in the error box. 
XRT position covered by  bright (~15m) star USNO B1 0941-0468603.



The message may be cited.

P. D'Avanzo (INAF-OAB), V. D'Elia (ASDC), J.A. Kennea (PSU), B.
Sbarufatti (INAF-OAB/PSU), D.N. Burrows (PSU), P.A. Evans (U.
Leicester), A. Amaral-Rogers (U. Leicester), J.P. Osborne (U.
Leicester), M. de Pasquale (INAF-IASFPA) and K.L. Page report on behalf
of the Swift-XRT team:

We have analysed 8.3 ks of XRT data for GRB 150428B (Page et al. GCN
Circ. 17766
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 ), from 100 s to 34.4 ks after the  BAT trigger. The data
comprise 187 s in Windowed Timing (WT) mode (the first 6 s were taken
while Swift was slewing) with the remainder in Photon Counting (PC)
mode. The enhanced XRT position for this burst was given by Beardmore
et al. (GCN Circ. 17773
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 ).

The light curve can be modelled with a series of power-law decays. The
initial decay index is alpha=2.31 (+0.16, -0.20). At T+170 s  the decay
steepens to an alpha of 4.92 (+0.15, -0.16) before breaking again at
T+618 s to a final decay with index alpha=0.29 (+/-0.07).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.93 (+0.06, -0.05). The
best-fitting absorption column is  3.51 (+0.28, -0.25) x 10^21 cm^-2,
consistent with the Galactic value of 3.3 x 10^21 cm^-2 (Willingale et
al. 2013). The PC mode spectrum has a photon index of 1.90 (+0.16,
-0.08) and a best-fitting absorption column consistent with the
Galactic value. The counts to observed (unabsorbed) 0.3-10 keV flux
conversion factor deduced from this spectrum  is 3.9 x 10^-11 (5.6 x
10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     3.3 (+0.3, -0.0) x 10^21 cm^-2
Galactic foreground: 3.3 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.90 (+0.16, -0.08)

If the light curve continues to decay with a power-law decay index of
0.29, the count rate at T+24 hours will be 0.027 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.1 x
10^-12 (1.5 x 10^-12) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00639278.

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

M. M. Chester (PSU) and K. L. Page (U Leicester)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 150428B 117
s after the BAT trigger (Page et al., GCN Circ. 17766
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 ). No optical
afterglow consistent with the enhanced XRT position (Beardmore et al., GCN
Circ. 17773
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 ) is detected in the initial UVOT exposures.
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) exposures and subsequent exposures are:

Filter      T_start(s)   T_stop(s)   Exp(s)    UL (3-sigma)

white_FC       117          267        146       >20.9
u_FC           329          579        246       >20.1
white          117         5414        588       >21.6
v              659         5825        313       >19.6
b              584         6598        445       >20.8
u              329         6440        717       >20.8
w1             708         6235        293       >20.0
m2             683         6030        294       >19.9
w2             634         5620        313       >20.2

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.40 in the direction of the burst
(Schlegel et al. 1998).

K. L. Murata (Nagoya U.), S. Nishiyama, A. Iwamatsu, I. Kawamata (Miyagi
University of Education), and T. Nagayama (Kagoshima U.)

We observed the field of GRB 150428B ( Page et al., GCN Circular #17766
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 ) with
the near-infrared (J, H, Ks) simultaneous imaging camera SIRIUS attached to 1.4
m telescope IRSF ( InfraRed Survey Facility) in Sutherland observatory, South
Africa.

The observations started on 2015-04-28 03:59:59 UT (~ 48 min. after the burst).
We could not detect the afterglow within the XRT error circle reported in the
GCN circular in the three bands.
We have obtained the following preliminary upper limits (Vega magnitude system):
 J  > 17.4
 H  > 16.8
 Ks > 15.3.

Given magnitudes were calibrated against 2MASS point sources in this field.
The upper limits were determined as the magnitudes of the faintest star within 1
arcmin from the XRT position.

This observation was carried out by IRSF and OISTER collaboration.