GRB 150915A

V. D'Elia (ASDC), N. Gehrels (NASA/GSFC), L. Izzo (URoma/ICRA),
F. E. Marshall (NASA/GSFC), D. M. Palmer (LANL) and M. H. Siegel (PSU)
report on behalf of the Swift Team:

At 21:18:24 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 150915A (trigger=655721).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 319.731, -34.854 which is 
   RA(J2000) = 21h 18m 55s
   Dec(J2000) = -34d 51' 12"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  As is usual for an image trigger, there
is no obvious variation in the immediately available BAT lightcurve. 

The XRT began observing the field at 21:20:33.5 UT, 128.7 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source located at RA, Dec 319.65874,
-34.91306 which is equivalent to:
   RA(J2000)  = 21h 18m 38.10s
   Dec(J2000) = -34d 54' 47.0"
with an uncertainty of 3.6 arcseconds (radius, 90% containment). This
location is 301 arcseconds from the BAT onboard position. This position
may be improved as more data are received; the latest position is
available at http://www.swift.ac.uk/sper. 

Although this position is outside the nominal 90% BAT error circle, 
it is probably the GRB. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density in excess of the Galactic value (8.65 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 2.7
(+2.23/-1.97) x 10^21 cm^-2 (90% confidence). 

The initial flux in the 2.5 s image was 5.57e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 133 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 8'x8' region for the list of sources generated on-board covers 19% 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
0.10. 

Burst Advocate for this burst is V. D'Elia (delia AT asdc.asi.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.)

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

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

K.Ivanov, O.Gres, N.M.Budnev, S.Yazev,
Irkutsk State University

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

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

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

V.Krushinski, I.Zalozhnih
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 IAC (Tenerife, Spain) was pointed to the  GRB150915A 22 sec 
after notice time and 95 sec after trigger time at 2015-09-15 21:20:03 UT 
in two polarizations. On our first single  (20s exposure) and coadd  (760s 
exposure)  sets we haven`t found optical transient  within SWIFT error-box 
(Elia et al. GCN 18315
Loading...
 
 ).
The 5-sigma upper limit has been about 17.3 on single and 19.8 on coadd 
image.

MASTER II  robotic telescope located in SAAO (South Africa) was pointed to 
the  GRB150915A 25 sec after notice time and 100 sec after trigger time at 
2015-09-15 21:20:07 UT in two polarizations. On our first (20s exposure) 
set we haven`t found optical transient  within SWIFT error-box .
The 5-sigma upper limit has been about 17.2 mag

The message may be cited.

R. Yates, J. Bolmer and J. Greiner (all MPE Garching) report on behalf of
the GROND team:

We observed the field of GRB 150915A (D'Elia et al., GCN 18315
Loading...
 
 ) 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 23:28 UT on 2015-09-15, 2.2 hours after the GRB trigger.

Based on a 42 minutes exposure at a mid-time of 3.7 hours after the BAT 
trigger, at an average seeing of 1.6" and at an average airmass of 1.0, 
we detect an optical counterpart at the edge of the Swift-XRT error-circle at
  RA = 21 18 37.84
  Decl. = -34 54 48.3
(+/- 0.3")

at the following with magnitudes (AB system)

g' = 23.0 +/- 0.1 mag,
r' = 23.1 +/- 0.1 mag,
i' = 22.9 +/- 0.2 mag,
z' = 22.7 +/- 0.2 mag,
J  > 19.9 mag,
H  > 19.3 mag,
K  > 17.9 mag.

The given magnitudes are derived based on calibrating the images against
GROND zeropoints in the optical and 2MASS in the NIR, and are not corrected 
for the expected Galactic foreground extinction corresponding to a reddening 
of E_(B-V) = 0.10 mag in the direction of the burst (Schlegel et al. 1998).

V. D'Elia (ASI/ASDC and INAF-OAR), T. Kruehler (MPE), D. Xu (NAOC/CAS), Pall Jakobsson (Univ. Iceland), N. R. Tanvir (U. Leicester), A. de Ugarte Postigo (IAA/CSIC), D. Malesani (DARK/NBI) and J. P. U. Fynbo (DARK/NBI), report on behalf of a larger collaboration:

We observed the optical counterpart of GRB 150915A (D'Elia et al., GCN 18315
Loading...
 
 ) with the ESO VLT equipped with the X-shooter spectrograph. Four spectra with individual exposure time of 1200 s were secured covering the wavelength range 3,000-21,000 AA. Spectroscopy began 3.3 hr after the BAT trigger. The afterglow is well detected in our acquisition image at the position of the optical counterpart (Yates et al., GCN 18317
Loading...
 
 ).

We detect a number of absorption features at a common redshift of z = 1.968, including Ly-a, C IV, Al II, Si II, Fe II, Mg II. We also detect fine-structure lines at the same redshift (e.g. FeII* 2396 / 2389, and Si II* 1533). Finally, we report strong emission lines of [O II], H-alpha and H-beta at a similar redshift, thus confirming z=1.968 as the GRB/host galaxy redshift.

We thank the kind assistence of the observing staff at Paranal, in particular Steve Ertel.

A. A. Breeveld (UCL-MSSL) and V. D'Elia (ASDC) report on behalf of the
Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 150915A
295s after the BAT trigger (D'Elia et al., GCN Circ. 18315
Loading...
 
 ).
No optical afterglow consistent with the XRT position (D���Elia et al.,
GGN Circ. 18315) or the GROND position (Yates et al., GCN Circ. 18317
Loading...
 
 )
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

u_FC               295          545          246         >20.0
white              576          769           39         >19.9
v                  625          818           39         >18.2
b                  550          745           39         >19.2
u                  295         4267          434         >20.0

The magnitudes in the table are not corrected for the Galactic
extinction due to the reddening of E(B-V) = 0.10 in the direction of the
burst (Schlegel et al. 1998).

D. Xu (NAOC/CAS), A. de Ugarte Postigo (IAA-CSIC, DARK/NBI), N. R.
Tanvir (Univ. Leicester), D. Malesani (DARK/NBI), T. Kruehler (MPE),
P. Jakobsson (Univ. Iceland), S. Schulze (PUC, MAS), V. D'Elia
(ASI/ASDC and INAF-OAR), K. Karhunen (Turku Univ.), report on behalf
of a larger collaboration:

We observed the field of GRB 150915A (D'Elia et al., GCN 18315
Loading...
 
 ) with
the Nordic Optical Telescope equipped with the ALFOSC camera. Two
observations composed each by 3x300 s exposures in R-band were taken
with the mean times of 0.80 and 3.05 hr after the GRB trigger.

The source detected by GROND and reported by Yates et al. (GCN 18317
Loading...
 
 )
is clearly visible in our images at both epochs, with a magnitude of
R(Vega) = 22.52 +- 0.09 on our first epoch (assuming R = 16.72 for the
star at RA = 21:18:34.76, Dec = -34:53:44.9).

Interestingly, the object is found to brighten between the two epochs
by 0.28 +- 0.11 mag - a behavior not common, though not unprecedented.

K. L. Murata (Nagoya U.), S. Nagatomo (Kyoto U.), and T. Nagayama (Kagoshima U.)

We observed the field of GRB150915A (D'Elia et al., GCN Circular #18315
Loading...
 
 ) 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 2015-09-15 23:45:25 UT (~ 2.5 hours 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  > 16.47
H  > 15.91
Ks > 15.64.

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.

D. M. Palmer (LANL), S. D. Barthelmy (GSFC),  J. R. Cummings (GSFC/UMBC),
V. D'Elia (ASDC), N. Gehrels (GSFC), H. A. Krimm (GSFC/USRA),
A. Y. Lien (GSFC/UMBC), C. B. Markwardt (GSFC), 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 150915A (trigger #655721)
(D'Elia, et al., GCN Circ. 18315
Loading...
 
 ).  The BAT ground-calculated position is
RA, Dec = 319.675, -34.874 deg which is
  RA(J2000)  =  21h 18m 42.0s
  Dec(J2000) = -34d -52' -27.1"
with an uncertainty of 2.3 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 55%.

The mask-weighted light curve shows a weak complex structure that starts
at ~ T0-70 s and ends at ~T0+120 s. There might be additional weak emission
that starts since the beginning of the event data. T90 (15-350 keV)
is 164.7 +- 49.7 sec (estimated error including systematics).

The time-averaged spectrum from T-69.0 to T+116.8 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
2.51 +- 0.42.  The fluence in the 15-150 keV band is 8.0 +- 1.8 x 10^-7 erg/cm2.
The 1-sec peak photon flux measured from T+17.68 sec in the 15-150 keV band
is 0.5 +- 0.2 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/655721/BA/

A. D'ai (INAF-IASFPA), A. Maselli  (INAF-IASFPA), A. Melandri
(INAF-OAB), J.A. Kennea (PSU), B. Sbarufatti (INAF-OAB/PSU), D.N.
Burrows (PSU), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), 
L. Izzo (URoma/ICRA) and V. D'Elia (ASDC) report on behalf of the
Swift-XRT team:

We have analysed 12 ks of XRT data for the Swift/BAT-detected burst GRB
150915A (D'Elia et al. GCN Circ. 18315
Loading...
 
 ), from 118 s to 126.0 ks after
the  Swift/BAT trigger. The data comprise 256 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 refined XRT position is RA,
Dec = 319.6584, -34.9136 which is equivalent to:

RA (J2000): 21 18 38.01
Dec(J2000): -34 54 49.1

with an uncertainty of 3.7 arcsec (radius, 90% confidence). This
position is 5.1 arcmin from the Swift/BAT position. 

The light curve can be modelled with an initial power-law decay with an
index of alpha=0.5 (+/-0.3), followed by a break at T+170 s to an alpha
of 3.94 (+0.15, -0.13).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.79 (+/-0.04). The
best-fitting absorption column is  7.8 (+1.7, -1.6) x 10^21 cm^-2, at a
redshift of 1.968, in addition to the Galactic value of 8.6 x 10^20
cm^-2 (Willingale et al. 2013). The counts to observed (unabsorbed)
0.3-10 keV flux conversion factor deduced from this spectrum  is 3.7 x
10^-11 (4.5 x 10^-11) erg cm^-2 count^-1. 

A summary of the WT-mode spectrum is thus:
Galactic foreground: 8.6 x 10^20 cm^-2
Intrinsic column:    7.8 (+1.7, -1.6) x 10^21 cm^-2 at z=1.968
Photon index:	     1.79 (+/-0.04)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00655721.

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