GRB 151021A

A. Melandri (INAF-OAB), S. D. Barthelmy (GSFC),
P. D'Avanzo (INAF-OAB), H. A. Krimm (CRESST/GSFC/USRA),
N. P. M. Kuin (UCL-MSSL), D. M. Palmer (LANL) and
B. Sbarufatti (INAF-OAB/PSU) report on behalf of the Swift Team:

At 01:29:12 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 151021A (trigger=660671).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 337.611, -33.199 which is 
   RA(J2000) = 22h 30m 27s
   Dec(J2000) = -33d 11' 55"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a complex multi-peaked
structure with a duration of at least 100 sec.  The peak count rate
was 2500 counts/sec (15-350 keV), at 10 sec after the trigger,
although the lightcurve around the time of trigger is not
immediately available. 

The XRT began observing the field at 01:30:43.2 UT, 90.7 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
uncatalogued X-ray source located at RA, Dec 337.64619, -33.19907 which
is equivalent to:
   RA(J2000)  = 22h 30m 35.09s
   Dec(J2000) = -33d 11' 56.7"
with an uncertainty of 3.6 arcseconds (radius, 90% containment). This
location is 106 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 destiny in excess of the Galactic value (1.13 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 3.9
(+2.55/-2.23) x 10^21 cm^-2 (90% confidence). 

The initial flux in the 0.1 s image was 2.76e-08 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 99 seconds after the BAT trigger. No credible afterglow candidate has
been found in the initial data products. The 8'x8' region for the list of 
sources generated on-board covers 100% of the XRT error circle. 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 0.01. 

Burst Advocate for this burst is A. Melandri (andrea.melandri AT brera.inaf.it). 
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. de Ugarte Postigo, (IAA-CSIC, DARK/NBI), D. Malesani (DARK/NBI),
D. Xu (NAOC/CAS) report on behalf of a larger collaboration:

We have observed the field of GRB 151021A (Melandri et al., GCN 18425
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 ) 
with X-shooter at the VLT (Paranal Observatory, Chile). Observation 
started at 02:13 UT (44 min after the burst) and cover the range between
3000 and 24800 AA.

In the close proximity of the XRT error circle, we detect a bright, uncatalogued 
object in the r'-band acquisition image, at coordinates (J2000, 0.5" error):

RA = 22:30:34.369
Dec = -33:11:49.45

We suggest that this is the optical afterglow of GRB 151021A and we measure 
for it R = 18.2 (Vega), derived from comparison with R-band magnitudes for the 
USNO-B1.0 catalogue. An object at consistent coordinates is also visible in the 
UVOT white-band image.

On a preliminary reduction, the spectrum shows a bright continuum with multiple 
absorption features, including SII, OI, SiII, SiIV, CIV, AlII, ZnII, CrII, FeII, NiII, MnII, 
MgII, MgI, as well as fine-structure lines from FeII* and NiII* and Ly-alpha 
absorption at a common redshift of z = 2.330, which we identify as the redshift of 
the GRB. We also identify an intervening system at z = 1.490 with features of 
FeII, MgII, and MgI.

We acknowledge the excellent support given by the Paranal staff, and in 
particular Andrea Mehner, Julien Mili and Jose Velasquez.

V. Lipunov, E. Gorbovskoy,   N.Tyurina, V.Kornilov, P.Balanutsa,
A.Kuznetsov, D.Kuvshinov, D. Vlasenko
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.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-Net: http://observ.pereplet.ru) 
located in SAAO was pointed to the  GRB151021A 25 sec after notice time 
and 50 sec after trigger time at 2015-10-21 01:30:06 UT in two 
polarizations. On our first (10s exposure)  set we  found 1 optical 
transient within SWIFT error-box (Melandri et. al GCN 18425
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 ) brighter then 
16.


date       time      Tmid-T0      Coord2000                       Mag
2015-10-21 01:30:06   55.0   (22h 30m 34.42s , -33d 11m 50.3s)    14.86

The 5-sigma upper limit has been about 16.0 mag
The message may be cited.

[GCN OPS NOTE(21oct15) Per author's request, the GBB name in the Subject-line
was changed from 0921A to 1021A.]

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

Using 2547 s of XRT Photon Counting mode data and 3 UVOT
images for GRB 151021A, 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 = 337.64347, -33.19731 which is equivalent
to:

RA (J2000): 22h 30m 34.43s
Dec (J2000): -33d 11' 50.3"

with an uncertainty of 1.7 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.

H. A. Krimm (GSFC/USRA), S. D. Barthelmy (GSFC), J. R. Cummings (GSFC/UMBC),
N. Gehrels (GSFC), A. Y. Lien (GSFC/UMBC), C. B. Markwardt (GSFC),
A. Melandri (INAF-OAB), 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-240 to T+352 sec from the recent telemetry downlinks,
we report further analysis of BAT GRB 151021A (trigger #660671)
(Melandri, et al., GCN Circ. 18425
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 ).  The BAT ground-calculated position is
RA, Dec = 337.647, -33.189 deg, which is 
   RA(J2000)  =  22h 30m 35.3s 
   Dec(J2000) = -33d 11' 21.8" 
with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 10%.
 
The mask-weighted light curve shows many overlapping peaks starting
at ~T-50sec, with a maxiximum peak at ~T+1 sec, and ending at ~T+130 sec.
T90 (15-350 keV) is 110.2 +- 3.7 sec (estimated error including systematics).
 
The time-averaged spectrum from T-21.6 to T+127.4 sec (with a 1.5 sec data gap
in the middle) is best fit by a simple power-law model.  The power law index
of the time-averaged spectrum is 1.57 +- 0.04.  The fluence in the 15-150 keV band
is 2.8 +- 0.1 x 10^-5 erg/cm2.  The 1-sec peak photon flux measured from T-0.13 sec
in the 15-150 keV band is 9.7 +- 0.8 ph/cm2/sec.  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/660671/BA/

J.A. Kennea (PSU), P.A. Evans (U. Leicester), S.L. Gibson (U.
Leicester), J.P. Osborne (U. Leicester), A. D'ai (INAF-IASFPA), A.
Maselli  (INAF-IASFPA), B. Sbarufatti (INAF-OAB/PSU), D.N. Burrows
(PSU) and A. Melandri report on behalf of the Swift-XRT team:

We have analysed 6.7 ks of XRT data for GRB 151021A (Melandri et al.
GCN Circ. 18425
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 ), from 80 s to 29.2 ks after the  BAT trigger. The data
comprise 184 s in Windowed Timing (WT) mode (the first 9 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 Osborne et
al. (GCN Circ. 18428
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 ).

The light curve can be modelled with a series of power-law decays. The
initial decay index is alpha=1.3 (+1.8, -1.6). At T+102 s  the decay
steepens to an alpha of 3.90 (+0.33, -0.29). The light curve breaks
again at T+230 s to a decay with alpha=0.91 (+0.07, -0.06),  before a
final break at T+2618 s s after which the decay index is 1.40
(+/-0.08).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.68 (+/-0.04). The
best-fitting absorption column is  1.47 (+0.20, -0.19) x 10^22 cm^-2,
at a redshift of 2.33, in addition to the Galactic value of 1.1 x 10^20
cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index
of 1.96 (+0.10, -0.09) and a best-fitting absorption column of 2.3
(+0.5, -0.4) x 10^22 cm^-2. The counts to observed (unabsorbed) 0.3-10
keV flux conversion factor deduced from this spectrum  is 3.5 x 10^-11
(4.4 x 10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Galactic foreground: 1.1 x 10^20 cm^-2
Intrinsic column:    2.3 (+0.5, -0.4) x 10^22 cm^-2 at z=2.33
Photon index:	     1.96 (+0.10, -0.09)

If the light curve continues to decay with a power-law decay index of
1.40, the count rate at T+24 hours will be 0.017 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 5.9 x
10^-13 (7.5 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/00660671.

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

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 151021A (Melandri et al., GCNC 18425)
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 2015-10-21 09:35:46 UT (8.1 h after
the burst). We could not detect the previously reported afterglow
(de Ugarte Postigo et al., GCNC 18426; Lipunov et al., GCNC 18427)
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
-----------------------------------------------------
0.37964    10:35:52    6180.0   >19.5  >19.7  >19.1
-----------------------------------------------------
T0+ : Elapsed time after the burst [day]
T-EXP: Total Exposure time [sec]

L. M. McCauley (PSU), and A. Melandri (INAF-OAB)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 151021A
99 s after the BAT trigger (Melandri et al., GCN Circ. 18425
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 ).
A source consistent with the XRT position
(Osborne et al. GCN Circ. 18428
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 )
is detected in the initial UVOT exposures.

The preliminary UVOT position is:
    RA  (J2000) =  22:30:34.43 = 337.64335 (deg.)
    Dec (J2000) = -33:11:50.3  = -33.19715 (deg.)
with an estimated uncertainty of 0.42 arc sec. (radius, 90% confidence).

Preliminary detections and 3-sigma upper limits using the UVOT photometric
system
(Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the early exposures
are:

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

white                  99         249      294   16.89
white                1668        5949      216  >20.86
v                      81          90        9   15.82
v                    1545        6359      235  >19.28
b                    1644        5744      216  >20.19
u                    1619        5538      216  >19.77
uvw1                 5133        5333      196  >19.53
uvm2                 4928        7484      316  >19.64
uvw2                 5955        6155      196  >19.97


The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.01 in the direction of the burst
(Schlegel et al. 1998).

S. Golenetskii, R. Aptekar, D. Frederiks, V. Pal'shin, P. Oleynik,
M. Ulanov, D. Svinkin, A. Tsvetkova, A. Lysenko, A.Kozlova and T. Cline
on behalf of the Konus-Wind team, report:

The long-duration, intense GRB 151021A (Swift-BAT trigger #660671:
Melandri et al., GCN 18425
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 ; Krimm et al., GCN 18429
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 )
triggered Konus-Wind at T0=5336.535 s UT (01:28:56.535).

The burst light curve shows a multi-peaked structure
started at ~T0-6.1 s with a total duration of ~100 s.
The emission is seen up to ~10 MeV.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB151021_T05336/

As observed by Konus-Wind, the burst
had a fluence of 7.66(-0.87,+0.97)x10^-5 erg/cm2,
and a 64-ms peak flux, measured from T0+22.256 s,
of 4.92(-1.44,+1.41)x10^-6 erg/cm2/s
(both in the 20 keV - 10 MeV energy range).

The time-averaged spectrum of the burst
(measured from T0 to T0+131.328 s)
is best fit in the 20 keV - 10 MeV range
by the GRB (Band) model with the following parameters:
the low-energy photon index alpha = -1.14 (-0.14,+0.15),
the high energy photon index beta = -2.46 (-0.32,+0.18),
the peak energy 170 (-18,+23) keV
(chi2 = 94/96 dof)

The spectrum near the maximum count rate
(measured from T0+16.640 to T0+33.024 s)
is best fit in the 20 keV - 10 MeV range
by the GRB (Band) model with the following parameters:
the low-energy photon index alpha = -0.89 (-0.19,+0.19),
the high energy photon index beta = -2.29 (-0.32,+0.14),
the peak energy 200 (-27,+46) keV
(chi2 = 73/96 dof)

Assuming the redshift z=2.330 (de Ugarte Postigo et al., GCN 18426
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 )
and a standard cosmology model with H_0 = 70 km/s/Mpc,
Omega_M = 0.27, and Omega_Lambda = 0.73,
we estimate the following rest-frame parameters:
the isotropic energy release E_iso is ~1.0x10^54 erg,
the peak luminosity L_iso is ~2.2x10^53 erg/s,
and the rest-frame peak energy of the time-integrated spectrum,
Ep,i, is ~666 keV.

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

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

We observed the field of GRB 151021A (Melandri et al., GCN 18425
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 )
in Ks-band with a wide-field near infrared imager at Okayama
Astrophysical Observatory (Japan).  The imager has effective
aperture of 0.91 m.

Observations started from 10:42 UT on 21th October, 9.2h after
the BAT trigger, to 11:11 UT. The total exposure of 11.7 min
was successfully obtained.

In our co-add image, we did not find any new point source within
the enhanced XRT error circle (Osborne et al., GCN 18428
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 ) down
to limiting magnitude of Ks=14.5 (Vega, S/N=10). The photometric
calibration was made against 2MASS field stars.

T0+[sec]    MID-UT     T-EXP[sec]      Ks
---------------------------------------------------
 +34,049    10:56      700.0           >14.5
---------------------------------------------------
T0+ : Elapsed time after the burt [sec]
T-EXP: Total Exposure time [sec]

A. Trotter, D. Reichart, J. Haislip, A. Smith, D. Caton, L. Hawkins, J. Moore, N. Frank, M. Maples, E. Johnson, R. Joyner, J. Martin, C. Salemi, J. A. Crain, K. Ivarsen, A. LaCluyze, and M. Nysewander report:

Skynet observed the Swift BAT/XRT localization of GRB 151021A (Melandri et al., GCN 18425
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 , Swift trigger=660671) with with one 16" telescope (PROMPT 5; I band) of the PROMPT array at CTIO, Chile, and with the 17" telescope at the Appalachian State University Dark Sky Observatory (DSO17; B band) in NC, USA. Starting at 2015-10-21 01:31:14 UT and continuing until 03:00 UT (t=2m-91m post-trigger), Skynet took a total of 38 exposures ranging from 10-160s. 

In the PROMPT I-band images, we clearly detect a fading optical afterglow at the position reported by de Ugarte Postigo et al. (GCN 18426
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 ), with I~14 at t=2m at I~19 at t=75m. A DSO17 B-band image taken at t=2.2m shows no source to a 3-sigma limit of B>15.7. 

A preliminary light curve is at:
http://www.skynet.unc.edu/grb/grb151021a.png

Limiting magnitudes are in the Vega System, calibrated to 4 APASS DR9 stars in the field.  Magnitudes have not been corrected for the small line-of-sight Milky Way dust extinction, with expected E(B-V)=0.009 (Schlafly & Finkbeiner 2011).

No further Skynet observations are scheduled.