GRB 151001A

M. Stamatikos (OSU/NASA/GSFC), S. D. Barthelmy (GSFC),
P. A. Evans (U Leicester), N. Gehrels (NASA/GSFC),
S. L. Gibson (U Leicester), J. A. Kennea (PSU),
H. A. Krimm (CRESST/GSFC/USRA), N. P. M. Kuin (UCL-MSSL),
A. Y. Lien (GSFC/UMBC), F. E. Marshall (NASA/GSFC),
C. Pagani (U Leicester), B. Sbarufatti (INAF-OAB/PSU) and
M. H. Siegel (PSU) report on behalf of the Swift Team:

At 15:04:22 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 151001A (trigger=657286).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 233.731, +10.971 which is 
   RA(J2000) = 15h 34m 55s
   Dec(J2000) = +10d 58' 16"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows a couple overlapping peaks
with a duration of about 100 sec.  The peak count rate
was ~1000 counts/sec (15-350 keV), at ~10 sec after the trigger. 

The XRT began observing the field at 15:05:26.9 UT, 64.0 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source with an enhanced position: RA, Dec 233.7289, 10.9669 which
is equivalent to:
   RA(J2000)  = 15h 34m 54.94s
   Dec(J2000) = +10d 58' 00.8"
with an uncertainty of 2.0 arcseconds (radius, 90% containment). This
location is 16 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 (3.53 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 1.3
(+1.39/-1.27) x 10^21 cm^-2 (90% confidence). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 67 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	15:34:54.90 = 233.72874
  DEC(J2000) = +10:58:02.1  =  10.96724
with a 90%-confidence error radius of about 0.62 arc sec. This position is 3.8
arc sec. from the center of the XRT error circle. The estimated magnitude is
17.94 with a 1-sigma error of about  0.14. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.04. 

Burst Advocate for this burst is M. Stamatikos (Michael.Stamatikos-1 AT 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.)

B. Mailyan (UAH), P. Veres (UAH) and C. Meegan (UAH)
report on behalf of the Fermi GBM Team:

"At 15:04:22.41  UT on 01 October 2015, the Fermi Gamma-Ray Burst Monitor
triggered and located GRB 151001A (trigger 465404666 / 151001628)
which was also detected by the Swift/BAT and XRT
(M. Stamatikos et al. 2015, GCN 18373
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 )
The GBM on-ground location is consistent with the Swift position.


The angle from the Fermi LAT boresight at the GBM trigger time is 22 degrees.


The GBM light curve has a duration (T90) of about 15.1 s (50-300 keV).
The time-averaged spectrum from T0-6.1 s to T0+4.1 s is
adequately fit by a power law function with an exponential
high-energy cutoff.  The power law index is -0.54 +/- 0.21 and
the cutoff energy, parameterized as Epeak, is 213.80 +/- 36.90 keV.


The event fluence (10-1000 keV) in this time interval is
(1.798 +/- 0.195)E-06  erg/cm^2. The 1-sec peak photon flux measured
starting from T0+2.75 s in the 10-1000 keV band
is 2.2 +/- 0.2 ph/s/cm^2.

The spectral analysis results presented above are preliminary;
final results will be published in the GBM GRB Catalog."

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

Using 2627 s of XRT Photon Counting mode data and 5 UVOT
images for GRB 151001A, 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 = 233.72875, +10.96720 which is equivalent
to:

RA (J2000): 15h 34m 54.90s
Dec (J2000): +10d 58' 01.9"

with an uncertainty of 1.5 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.

A. Melandri (INAF-OAB), D.N. Burrows (PSU), T.G.R. Roegiers (PSU), L.M.
McCauley (PSU), K.L. Page (U. Leicester), C. Pagani (U. Leicester),
A.P. Beardmore (U. Leicester), P. D'Avanzo (INAF-OAB), V. D'Elia (ASDC)
and M. Stamatikos report on behalf of the Swift-XRT team:

We have analysed 8.2 ks of XRT data for GRB 151001A (Stamatikos et al.
GCN Circ. 18373
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 ), from 49 s to 36.0 ks after the  BAT trigger. The data
comprise 9 s in Windowed Timing (WT) mode (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.
18376
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 ).

The late-time light curve (from T0+4.8 ks) can be modelled with a
power-law decay with a decay index of alpha=1.18 (+0.23, -0.20).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.78 (+/-0.14). The
best-fitting absorption column is  8.0 (+4.0, -3.6) x 10^20 cm^-2, in
excess of the Galactic value of 3.5 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 (4.2 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     8.0 (+4.0, -3.6) x 10^20 cm^-2
Galactic foreground: 3.5 x 10^20 cm^-2
Excess significance: 2.1 sigma
Photon index:	     1.78 (+/-0.14)

If the light curve continues to decay with a power-law decay index of
1.18, the count rate at T+24 hours will be 2.8 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.0 x
10^-13 (1.2 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/00657286.

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

T. Kruehler (MPE), J. P. U. Fynbo, D. Malesani (DARK/NBI), and P. 
Jakobsson (U. Iceland) report on behalf of a larger collaboration:

We observed the field of GRB 151001A (Swift trigger 657286, Stamatikos 
et al., GCN Circ. 18373
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 ) with the 2.5-m Nordic Optical Telescope (NOT) 
equipped with ALFOSC on the evening of 01-Oct-2015 between 19:55-20:24 
UT (5 hr after the burst). In SDSS filters r and g we clearly detect the 
optical afterglow of GRB 151001A at the position given by UVOT 
(Stamatikos et al., GCN Circ. 18373
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 ). We derive AB magnitudes of

g = 22.18 +/- 0.05
r = 21.51 +/- 0.08

calibrated against SDSS field stars. The g-r color of the afterglow is 
relatively red compared to typical afterglows.

A. A. Breeveld (UCL-MSSL) and M. Stamatikos (OSU/NASA/GSFC) report on 
behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 151001A 46 
s after the BAT trigger (Stamatikos et al., GCN Circ. 18373
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 ).
A fading source consistent with the enhanced XRT position (Beardmore et 
al. GCN Circ. 18376
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 ) is detected in the initial UVOT exposures.

The preliminary UVOT position is:
     RA  (J2000) =  15:34:54.90 = 233.72875 (deg.)
     Dec (J2000) = +10:58:02.2  =  10.96728 (deg.)
with an estimated uncertainty of 0.43 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               67          217          147         17.82 �� 0.06
white              559          578           20         19.5  �� 0.3
v                   46         1403          110         18.78 �� 0.29
b                  534         1502           97         19.60 �� 0.29
u                  279          529          246         19.13 �� 0.15
w1                 657         1453           97        >19.3
m2                 806         1428           39        >18.0
w2                1358         1552           39        >18.4

The magnitudes in the table are not corrected for the Galactic 
extinction due to the reddening of E(B-V) = 0.04 in the direction of the 
burst (Schlegel et al. 1998).

V. Zach Golkhou (ASU), Nat Butler (ASU), Alan M. Watson (UNAM), Alexander Kutyrev (GSFC), William H. Lee (UNAM), Michael G. Richer (UNAM), Chris 
Klein (UCB), Ori Fox (UCB), 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), and Vicki Toy (UMD) report:

We observed the field of GRB 151001A (Stamatikos, et al., GCN 18373
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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 2015/10 2.10 to 2015/10 2.15 UTC (11.31 to 
12.60 hours after the BAT trigger), obtaining a total of 1.07 hours exposure in the r, i, and z bands.

For a source within the Swift-XRT error circle (Beardmore et al., GCN 18376
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 ), in comparison with the SDSS DR9 catalog, we obtain the following 
detections and upper limit (3-sigma):

   r   =  22.69 +/- 0.20
   i   =  22.59 +/- 0.20
   z   >  20.03

These magnitudes are in the AB system and are not corrected for Galactic extinction in the direction of the GRB. The source has faded by about 1 
magnitude relative to the earlier observations by the NOT (Kruehler, et al. GCN 18379
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 ).

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

M. Stamatikos (OSU), 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),
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 151001A (trigger #657286)
(Stamatikos, et al., GCN Circ. 18373
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 ).  The BAT ground-calculated position is
RA, Dec = 233.722, 10.984 deg which is
  RA(J2000)  =  15h 34m 53.2s
  Dec(J2000) = +10d 59' 01.1"
with an uncertainty of 1.1 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 100%.

The mask-weighted light curve shows a single-peaked structure that starts
at ~T-6 s, peaks at ~T0, end ends at~T+5 s. T90 (15-350 keV) is 8.94 +- 1.07 sec
(estimated error including systematics).

The time-averaged spectrum from T-6.28 to T+4.53 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.17 +- 0.11.  The fluence in the 15-150 keV band is 6.7 +- 0.5 x 10^-7 erg/cm2.
The 1-sec peak photon flux measured from T-0.04 sec in the 15-150 keV band
is 1.3 +- 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/657286/BA/