GRB 150301A

A. Y. Lien (GSFC/UMBC), S. D. Barthelmy (GSFC), D. N. Burrows (PSU),
J. R. Cummings (NASA/UMBC), M. De Pasquale (INAF-IASFPA),
L. M. Z. Hagen (PSU), J. A. Kennea (PSU), C. B. Markwardt (NASA/GSFC),
A. Maselli (INAF-IASFPA), D. M. Palmer (LANL) and M. H. Siegel (PSU)
report on behalf of the Swift Team:

At 01:04:28 UT, the Swift Burst Alert Telescope (BAT) triggered and
located a source which could be a short GRB or a new SGR (trigger=632995). 
Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 244.286, -48.697, which is 
   RA(J2000)  =  16h 17m 09s
   Dec(J2000) = -48d 41' 48"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows a single peak
structure with a duration of about 0.1 sec.  The peak count rate
was ~8500 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 01:05:26.6 UT, 58.3 seconds after
the BAT trigger. XRT found a bright, uncatalogued X-ray source located
at RA, Dec 244.3047, -48.7131 which is equivalent to:
   RA(J2000)  = +16h 17m 13.13s
   Dec(J2000) = -48d 42' 47.2"
with an uncertainty of 5.0 arcseconds (radius, 90% containment). This
location is 73 arcseconds from the BAT onboard position, within the BAT
error circle. No event data are yet available to determine the column
density using X-ray spectroscopy. 

UVOT took a finding chart exposure of 113 seconds with the White filter
starting 65 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 none of
the XRT error circle. The coverage of the XRT error circle by the 8'x8' region
for the list of sources generated on-board is uncertain because the large
number of sources filled the available telemetry. The list of sources is
typically complete to about 18 mag. No correction has been made for the large,
but uncertain extinction expected. 

We note that this source is near the Galactic Plane and Bulge 
(lat = 1.36, lon = 334) and so this might be a previously-unknown
Soft Gamma Repeater. The burst has significant emission above 100 keV,
which is unusual, but not unprecedented, for an SGR.  Further analysis
of the full data set and continued observation is required to
determine the nature of this source. 

Burst Advocate for this burst is A. Y. Lien (amy.y.lien 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.)

Valerie Connaughton and Peter Jenke (UAH) and Adam Goldstein (NASA MSFC)
report on behalf of the Fermi GBM Team:

"At 01:04:28.65 UT on 01 March 2015, the Fermi Gamma-Ray Burst Monitor
triggered and located GRB 150301A (trigger 446864671 / 150301045)
which was also detected by the Swift/BAT (Lien et al. 2015, GCN 17510
Loading...
 
 ).
The GBM on-ground location is consistent with the Swift position.

The angle from the Fermi LAT boresight is 105 degrees.

The GBM light curve consists of a single peak about 50 ms in duration.
The counts registered in the NaI detectors are strongly peaked in the
50 - 300 keV energy range typical for GRBs.  From the quicklook
data, the softness ratio for the event (20 - 100 keV / 100 - 500 keV)
is 0.4 +/- 0.3, typical for fairly hard GRBs.  By contrast,
the corresponding softness ratios for recent triggers from
SGR 1935+2154 and AXP 4U 0142+61 were
4.4 +/- 1.8, 3.4 +/- 1.0, and 2.2 +/- 0.4.

The time-averaged spectrum from T0-0.016 s to T0+0.032 s is
best fit by a power law function with an exponential
high-energy cutoff.  The power law index is -0.53 +/- 0.35 and
the cutoff energy, parameterized as Epeak, is 185 +/- 49 keV.

The event fluence (10-1000 keV) in this time interval is
(1.2 +/- 0.2)E-07 erg/cm^2. The 16-msec peak photon flux measured
starting from T0-0.016 s in the 10-1000 keV band
is 29 +/- 4 ph/s/cm^2.

The spectral characteristics of this burst are more like a GRB than
an SGR.  We think it is therefore unlikely that this event is a burst
from a new SGR and conclude that it is, instead, a short GRB.


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

M. Tanga, C. Delvaux, J. Greiner (all MPE Garching) and D. A. Kann (TLS
Tautenburg) report on behalf of the GROND team:

We observed the field of GRB 150301A (Swift trigger 632995; Lien et al.,
GCN #17510
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 06:02 UT on 01-03-2015, 4.97 hrs after the GRB
trigger. They were performed at an average seeing of 1.4" and at an
average airmass of 1.5.

We found two point sources within the 5.0" Swift-XRT error circle reported
by Lien et al. (GCN #17510
Loading...
 
 ). The first source is at

RA (J2000.0) = 16 h 17 m 13.08 s
DEC (J2000.0) = -48d 42' 45.7"

The second source is at

RA (J2000.0) = 16 h 17 m 13.34 s
DEC (J2000.0) = -48d 42' 49.7"�

with an uncertainty of 0.3" in each coordinate for both sources.

Based on the first 50 min of total exposures in g'r'i'z' and 40 min in
JHK, we estimate the following preliminary magnitudes (all in AB system):

For the first source:

g' > 24.4 mag,
r' > 24.5 mag,
i' = 22.0 +/- 0.1 mag,
z' = 21.0 +/- 0.3 mag,
J = 18.8 +/- 0.2 mag,
H = 18.3 +/- 0.2 mag, and
K > 18.3 mag.

For the second source:

g' > 24.4 mag,
r' > 24.5 mag,
i' = 23.1 +/- 0.1 mag,
z' = 22.2 +/- 0.1 mag,
J = 19.6 +/- 0.3 mag,
H = 19.1 +/- 0.4 mag, and
K > 18.3 mag.

Both objects appear to be constant over the duration of our observation.
Therefore we suppose they are not related to the burst.

We note the field is crowded. We also notice the presence of a bright star
(~14 mag) west of the Swift-XRT error circle covering around 10% of the
encircled region.

Given magnitudes are calibrated against GROND zeropoints as well as 2MASS
field stars and are not corrected for the expected Galactic foreground
extinction corresponding to a reddening of E_(B-V)=3.31 mag in the
direction of the burst (Schlafly & Finkbeiner 2011).

M. de Pasquale (INAF-IASFPA), A. Maselli  (INAF-IASFPA), 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), A. Melandri (INAF-OAB) and A.Y. Lien report on behalf of
the Swift-XRT team:

We have analysed 147 s of XRT data for GRB 150301A (Lien et al. GCN
Circ. 17510
Loading...
 
 ), from 50 s to 43.7 ks after the  BAT trigger. The data are
entirely in Windowed Timing (WT) mode (the first 6 s were taken while
Swift was slewing). 

The light curve can be modelled with an initial power-law decay with an
index of alpha=0.53 (+0.18, -0.59), followed by a break at T+112 s to
an alpha of 2.6 (+0.6, -0.7).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.71 (+0.21, -0.20). The
best-fitting absorption column is  5.6 (+/-0.7) x 10^22 cm^-2, in
excess of the Galactic value of 1.3 x 10^22 cm^-2 (Willingale et al.
2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 9.1 x 10^-11 (2.0 x 10^-10) erg
cm^-2 count^-1. 

A summary of the WT-mode spectrum is thus:
Total column:	     5.6 (+/-0.7) x 10^22 cm^-2
Galactic foreground: 1.3 x 10^22 cm^-2
Excess significance: 10.5 sigma
Photon index:	     1.71 (+0.21, -0.20)

If the light curve continues to decay with a power-law decay index of
2.6, the count rate at T+24 hours will be 9.7 x 10^-7 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 8.8 x
10^-17 (1.9 x 10^-16) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00632995.

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

T. Sakamoto (AGU), S. D. Barthelmy (GSFC),  W. H. Baumgartner (GSFC/UMBC),
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),
M. Stamatikos (OSU), J. Tueller (GSFC), T. N. Ukwatta (LANL)
(i.e. the Swift-BAT team):

Using the data set from T-210 to T+962 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 150301A (trigger #632995)
(Lien, et al., GCN Circ. 17510
Loading...
 
 ).  The BAT ground-calculated position is
RA, Dec = 244.281, -48.732 deg which is
  RA(J2000)  =  16h 17m 07.5s
  Dec(J2000) = -48d 43' 56.2"
with an uncertainty of 1.9 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 96%.

The mask-weighted light curve shows a single short FRED peak that starts at
~ T0 and ends at ~ T+0.5 s. T90 (15-350 keV) is 0.48 +- 0.14 sec (estimated error
including systematics).

The time-averaged spectrum from T+0.00 to T+0.56 sec fit by a simple power-law
model has a power law index of 1.44 +- 0.23 (chi squared 54.10 for 57 d.o.f.).
The fluence in the 15-150 keV band is 7.4 +- 1.0 x 10^-8 erg/cm2.
The 1-sec peak photon flux measured from T-0.22 sec in the 15-150 keV band
is 1.0 +- 0.2 ph/cm2/sec.

A single blackbody fit to the time-averaged spectrum shows a blackbody
temperature of 15.11 +- 2.92 keV (chi squared 74.66 for 57 d.o.f.).
A thermal bremsstrahlung model fit shows the temperature of 199.36 keV
(chi squared 55.72 for 57 d.o.f.).

Despite being near the Galactic Plane and Bulge (lat = 1.36, lon = 334), both the
fast-decaying XRT light curve and Fermi/GBM observation (Connaughton et al.,
GCN Circ. 17511
Loading...
 
 ) suggests that this source is more likely to be a short GRB than
a SGR. In addition, there are no other BAT detections of this source before or since
this trigger.

The results of the batgrbproduct analysis are available at
http://gcn.gsfc.nasa.gov/notices_s/632995/BA/

L. M. Z. Hagen (PSU) and A. Y. Lien (GSFC/UMBC)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 150301A
66 s after the BAT trigger (Lien et al., GCN Circ. 17510
Loading...
 
 ).
No optical afterglow consistent with the XRT position (Lien et al.
GCN Circ. 17510
Loading...
 
 ) or GROND candidates (Tanga et al. GCN Circ. 17513
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
exposures are:

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

white               66         4931          306        >21.08
v                 5143         5343          196        >19.40
b                 4526        10879          750        >21.16
u                 4321         5926          362        >20.08
uvw1              5553         5752          196        >19.61
uvm2              5347         5547          196        >19.40
uvw2              4937         5137          196        >19.82

The magnitudes in the table are not corrected for the substantial Galactic
extinction due to the reddening of E(B-V) = 3.84 in the direction of the
burst (Schlegel et al. 1998).  We note that the photometry is likely
affected by a nearby bright foreground star.