GRB 160501A

J. R. Cummings (NASA/UMBC), A. P. Beardmore (U Leicester),
J. A. Kennea (PSU), F. E. Marshall (NASA/GSFC),
K. L. Page (U Leicester), D. M. Palmer (LANL), T. Sakamoto (AGU),
M. H. Siegel (PSU) and T. N. Ukwatta (LANL) report on behalf of the
Swift Team:

At 00:40:31 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 160501A (trigger=684679).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 286.377, -17.240 which is 
   RA(J2000) = 19h 05m 30s
   Dec(J2000) = -17d 14' 22"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  As is usual for an image trigger, there
is no clear variation visible in the immediately-available lightcurve
data. 

The XRT began observing the field at 00:42:39.8 UT, 128.1 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source located at RA, Dec 286.38344,
-17.24088 which is equivalent to:
   RA(J2000)  = 19h 05m 32.03s
   Dec(J2000) = -17d 14' 27.2"
with an uncertainty of 3.7 arcseconds (radius, 90% containment). This
location is 22 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 in excess of the Galactic value (1.62 x
10^21 cm^-2, Willingale et al. 2013), with an excess column of 2.7
(+3.06/-2.65) x 10^21 cm^-2 (90% confidence). 

The initial flux in the 2.5 s image was 1.08e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 136 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 upper limit is 19.6 magnitude. 
The 8'x8' region for the list of sources generated on-board covers
100% of the XRT error circle. The typical upper limit is 18.0. 
No correction has been made for the expected extinction corresponding
to E(B-V) of 0.18. 

Burst Advocate for this burst is J. R. Cummings (jayc AT milkyway.gsfc.nasa.gov). 
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.)

Z. Cano (Univ. Iceland), N. Tanvir (Univ. Leicester), D. Malesani
(DARK/NBI)  & J. Saario (NOT) report on behalf of a larger collaboration:

We observed the field of GRB 160501A (Cummings et al.; GCN Circ. 19365
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 )
with the 2.5-m Nordic Optical Telescope equipped with the StanCam and the
NOTCam, starting at 01:57 UT on 01 May 2016.  We obtained 15 minutes in the
I-band (StanCam) and 18 minutes in the H-band (NOTCam).  In our co-added
optical and NIR images we do not detect any new source within the XRT error
circle to the following limits:

I > 22.3 mag
H > 19.0 mag

These upper limits (Vega) are for a single point source in the GRB field,
they are calibrated against the USNO-B1 (optical) and 2MASS (NIR)
catalogues, and they have not been corrected for foreground extinction.

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

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

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.Senik
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)

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

MASTER II  robotic telescope (MASTER-Net: http://observ.pereplet.ru) 
located in SAAO was pointed to the  GRB160501A 24 sec after notice time 
and 106 sec after trigger time at 2016-05-01 00:42:21 UT in two 
polarizations. On our first (20s exposure)  set we haven`t found optical 
transient  within SWIFT error-box (Cummings et al GCN 19365
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 ).
The 5-sigma upper limit has been about 17.5 mag  on single (20 s exposure) 
and   19.1 mag  at coadd of first 10 images (with total exposure 780 s).
Not very depth upper limits are explained by the proximity to the Moon.
The message may be cited.

Klotz A., Turpin D., Atteia J.L. (CNRS-OMP-IRAP),
Boer, M., Laugier, R. (CNRS-ARTEMIS),
Gendre B. (UVI - Etelman Obs.) report:

We imaged the field of GRB 160501A detected by SWIFT
(trigger 684679) with the TAROT robotic telescope (D=25cm)
located at the Calern observatory, France.

The observations started 50 min after the GRB trigger.
The elevation of the field increased from
20 degrees above horizon and weather conditions
were good.

We co added images to increase detectivity:

t1(s) t2(s)  Rmag_lim  Site
3594  5526   18.5      TAROT Calern
5526  7863   18.5      TAROT Calern

Magnitudes were estimated with the nearby USNO-B1 stars
and are not corrected for galactic dust extinction.

N.B. Galactic coordinates are lon= 18.9676 lat=-10.7948
and the galactic extinction in R band is about 1.0 magnitudes
estimated from D. Schlegel et al. 1998ApJ...500..525S.

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

Using 2151 s of XRT Photon Counting mode data and 1 UVOT
images for GRB 160501A, 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 = 286.38402, -17.24138 which is equivalent
to:

RA (J2000): 19h 05m 32.16s
Dec (J2000): -17d 14' 29.0"

with an uncertainty of 1.9 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.

Alan M. Watson (UNAM), Nat Butler (ASU), Alexander Kutyrev (GSFC),
William H. Lee (UNAM), Michael G. Richer (UNAM), Ori Fox (STScI), J.
Xavier Prochaska (UCSC), Josh Bloom (UCB), Antonino Cucchiara
(GSFC/STScI), Eleonora Troja (GSFC), Owen Littlejohns (ASU), Enrico
Ramirez-Ruiz (UCSC), Jos�� A. de Diego (UNAM), Leonid Georgiev (UNAM),
Jes��s Gonz��lez (UNAM), Carlos Rom��n-Z����iga (UNAM), Neil Gehrels (GSFC),
Harvey Moseley (GSFC), John Capone (UMD), V. Zach Golkhou (ASU), and
Vicki Toy (UMD) report:

We observed the field of GRB 160501A (Cummings et al., GCN Circ. 19365
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 )
with the Reionization and Transients Infrared Camera (RATIR;
www.ratir.org) on the 1.5m Harold Johnson Telescope at the Observatorio
Astron��mico Nacional on Sierra San Pedro M��rtir from 2016/05 1.33 to
2016/05 1.47 UTC (7.23 to 10.70 hours after the BAT trigger), obtaining
a total of 1.73 hours exposure in the r and i bands and 0.66 hours
exposure in the Z, Y, J, and H bands.

For a source within the enhanced Swift-XRT error circle (Evans et al.,
GCN Circ. 19369
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 ), in comparison with the USNO-B1 and 2MASS catalogs, we
obtain the following 3-sigma upper limits:

 r	> 22.94
 i	> 22.76
 Z	> 21.69
 Y	> 21.22
 J	> 20.81
 H	> 19.90

These magnitudes are in the AB system and are not corrected for Galactic
extinction in the direction of the GRB.

We thank the staff of the Observatorio Astron��mico Nacional in San Pedro
M��rtir.

T.G.R. Roegiers (PSU), L.M. McCauley (PSU), J.A. Kennea (PSU), C.
Pagani (U. Leicester), A.P. Beardmore (U. Leicester), P.A. Evans (U.
Leicester), P. D'Avanzo (INAF-OAB), V. D'Elia (ASDC), A. D'Ai
(INAF-IASFPA) and J.R. Cummings report on behalf of the Swift-XRT team:

We have analysed 6.8 ks of XRT data for GRB 160501A (Cummings et al.
GCN Circ. 19365
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 ), from 118 s to 46.0 ks after the  BAT trigger. The
data comprise 161 s in Windowed Timing (WT) mode (the first 8 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 Evans
et al. (GCN Circ. 19369
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 ).

The light curve can be modelled with a series of power-law decays. The
initial decay index is alpha=2.76 (+/-0.16). At T+261 s  the decay
steepens to an alpha of 4.4 (+0.6, -0.3) before breaking again at T+918
s to a final decay with index alpha=0.26 (+0.36, -0.20).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 2.96 (+0.16, -0.15). The
best-fitting absorption column is  2.9 (+/-0.4) x 10^21 cm^-2, in
excess of the Galactic value of 1.6 x 10^21 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 2.06 (+0.25, -0.18)
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 3.4 x 10^-11 (4.6 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.6 (+0.7, -0.0) x 10^21 cm^-2
Galactic foreground: 1.6 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index:	     2.06 (+0.25, -0.18)

If the light curve continues to decay with a power-law decay index of
0.26, the count rate at T+24 hours will be 7.9 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 2.7 x
10^-13 (3.6 x 10^-13) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00684679.

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

A. A. Breeveld (UCL-MSSL) and J. R. Cummings (NASA/UMBC)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 160501A
136s after the BAT trigger (Cummings et al., GCN Circ. 19365
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 ).
No optical afterglow consistent with the XRT position (Evans et al. GCN 
Circ. 19369
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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) exposure and subsequent exposures are:

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

white_FC           136          286          147         >20.3
u_FC               295          544          246         >19.7
white              575         1543          245         >21.1
v                  625         1594          117         >18.9
b                  551         1518           97         >19.9
w1                 674         1643          117         >19.2
m2                 649         1618          117         >19.7
w2                 600         1569          117         >19.2

The magnitudes in the table are not corrected for the Galactic 
extinction due to the reddening of E(B-V) = 0.18 in the direction of the 
burst (Schlegel et al. 1998).

W. Fong and P. Milne (University of Arizona) report:

"We observed the field of GRB 160501A (Cummings et al., GCN 19365
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 ) with the
Wide Field Camera (WFCAM) mounted on the 3.8-m United Kingdom Infrared
Telescope (UKIRT) on Mauna Kea beginning on 2016 May 1.533 UT (12.1 hr
post-burst). We obtained observations in the J- and K-bands in 0.75"
seeing. Using the quick-look pipeline ORAC-DR, we do not detect any near-IR
source in or around enhanced X-ray position (Evans et al., GCN 19369
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 ).
Calibrated to 2MASS, we therefore place 3-sigma limits of J(AB)>19.7 mag
and K(AB)>19.2 mag on the near-IR afterglow of GRB 160501A at 12.1 hr
post-burst. We note that this analysis is preliminary and a final reduction
of the images are expected to be at least ~1.5 mag deeper.

We thank Tim Carroll, Watson Varricatt and Tom Kerr for their assistance in
planning and executing these observations."

D. Kuroda, K. Yanagisawa, Y. Shimizu, H. Toda (OAO, NAOJ),
S. Nagayama (NAOJ), M. Yoshida (Hiroshima), K. Ohta (Kyoto)
and N. Kawai(Tokyo Tech)
report on behalf of MITSuME and OISTER collaboration:

We observed the field of GRB 160501A (Cummings et al., GCNC 19365)
with the optical three color (g', Rc and Ic) CCD camera attached
to the MITSuME 50cm telescope of Okayama Astrophysical Observatory.

The observation started on 2016-05-01 17:18:31 UT (~16.6 h after
the burst). We did not find any new point source within the enhanced
XRT circle (Evans et al., GCNC 19369) in all the three bands.

Three sigma upper limits of the OT are listed below.
We used GSC2.3 catalog for flux calibration.

#T0+[day]  MID-UT    T-EXP[sec]   g'    Rc     Ic
-----------------------------------------------------
0.73280    18:15:45    5460.0   >19.4  >19.1  >18.3
-----------------------------------------------------
T0+ : Elapsed time after the burst [day]
T-EXP: Total Exposure time [sec]

J. Bodensteiner, F. Knust, T. Schweyer , J. Greiner (all MPE Garching)
report on behalf of the GROND team:

We observed the field of GRB 160501A (Swift trigger 684679;  Cummings et
al., GCN #19365
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 ) simultaneously in g'r'i'z'JHK with GROND (Greiner et al.
2008, PASP 120, 405) mounted at the 2.2 m MPG telescope at ESO La Silla
Observatory (Chile).

Observations started at 07:15 UT on 2016-05-01, 6h 35min after the GRB
trigger. They were performed at an average seeing of 1.3" and at an
average airmass of 1.09.

We do not detect a source within the Swift-XRT error circle reported by
Evans et al. (GCN #19369
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 ) down to

g' > 24.8 mag,
r' > 24.5 mag,
i' > 23.8 mag,
z' > 23.9 mag,
J > 21.1 mag,
H > 20.7 mag, and
K > 19.1 mag.
(all in AB).

The given limits are derived based on calibration against GROND zeropoints
and 2MASS field stars and are not corrected for the Galactic foreground
extinction corresponding to a reddening of E_(B-V)= 0.18 mag in the
direction of the burst (Schlegel et al. 1998).

C. B. Markwardt (GSFC), S. D. Barthelmy (GSFC), J. R. Cummings (GSFC/UMBC),
N. Gehrels (GSFC), H. A. Krimm (GSFC/USRA), A. Y. Lien (GSFC/UMBC),
C. B. Markwardt (GSFC), 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 160501A (trigger #684679)
(Cummings, et al., GCN Circ. 19365
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 ).  The BAT ground-calculated position is
RA, Dec = 286.389, -17.249 deg which is 
   RA(J2000)  =  19h 05m 33.2s 
   Dec(J2000) = -17d 14' 54.8" 
with an uncertainty of 1.8 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 81%.
 
The mask-weighted light curve a broad peak starting from T-80 sec, peaking 
at T+50 sec and ending at T+90 sec.  T90 (15-350 keV) is 118 +- 19 sec 
(estimated error including systematics).
 
The time-averaged spectrum from T-54.3 to T+72.6 sec is best fit by a power law
with an exponential cutoff.  This fit gives a photon index 0.6 +- 1.3, 
and Epeak of 28 +- 9 keV (chi squared 54.26 for 56 d.o.f.).  For this
model the total fluence in the 15-150 keV band is 9.5 +- 1.3 x 10^-7 erg/cm2
and the 1-sec peak flux measured from T-54.38 sec in the 15-150 keV band is
0.5 +- 0.2 ph/cm2/sec.  A fit to a simple power law gives a photon index
of 2.4 +- 0.2 (chi squared 63.09 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/684679/BA/

B. E. Cobb (GWU) reports:

Using the ANDICAM instrument on the 1.3m telescope at CTIO, we
obtained optical/IR imaging of the error region of GRB 160501A
(GCN 19365
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 , Cummings et al.) with a mid-exposure time of 4.3 hours
post-burst (2016-05-01 05:01 UT). Total summed exposure times
amounted to 36 minutes in I and 30 minutes in J.

No source is detected at the position of the X-ray
afterglow (GCN 19365
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 , Cummings et al; GCN 19369
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 , Evans
et al.) to approximate limiting magnitudes of I > 20.7 and J > 18.5.
Magnitudes are calibrated using USNO-B1.0 stars in I,
and 2MASS stars in J.