GRB 170306B

P. A. Evans (U. Leicester) reports on behalf of the Swift team:

Swift has initiated a series of observations, tiled on the sky, of the
Fermi/LAT GRB 170306B. Automated analysis of the XRT data will
be presented online at http://www.swift.ac.uk/xrt_products/TILED_GRB00065

Any uncatalogued X-ray sources detected in this analysis will be
reported on this website and via GCN COUNTERPART notices. The probability of finding
serendipitous sources, unrelated to the Fermi/LAT event is high: any X-ray source
considered to be a probable afterglow candidate will be reported via a GCN Circular
after manual consideration.

Details of the XRT automated analysis methods are detailed in Evans et
al. (2007, A&A, 469, 379; 2009, MNRAS, 397, 1177 and 2014, ApJS, 210, 8).

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

N. Omodei (Stanford U.), G, Vianello (Stanford U.), D. Tak(U. of 
Maryland) and F. Longo (University and INFN Trieste) report on behalf of 
the Fermi-LAT team:
At 14:07:22.27 on March 06, 2017, Fermi-LAT detected high-energy 
emission from GRB 170306B, which was also detected by Fermi-GBM (trigger 
510502047).
The best LAT on-ground location is found to be RA, Dec 154.62, 51.59 
(degrees, J2000) with an error radius of 0.61 deg (90% containment, 
statistical error only).
This was 55 deg from the LAT boresight at the time of the trigger and 
triggered an autonomous repoint of the spacecraft.
The data from the Fermi-LAT show a significant increase in the event 
rate within 4.3 degree of the GBM location after the GBM trigger that is 
temporally correlated with the GBM emission with high significance.
More than 10 photons above 100 MeV are observed within 300 seconds.
The highest-energy photon is a 500 MeV event which is observed 46 
seconds after the GBM trigger.
A Swift ToO has been requested for this burst.
The Fermi-LAT point of contact for this burst is Nicola Omodei 
(nicola.omodei@stanford.edu).

The Fermi-LAT is a pair conversion telescope designed to cover the 
energy band from 20 MeV to greater than 300 GeV.
It is the product of an international collaboration between NASA and DOE 
in the U.S. and many scientific institutions across France, Italy, Japan 
and Sweden.

P. Veres (UAH), C. Meegan (UAH) and A. von Kienlin (MPE)
report on behalf of the Fermi GBM Team:

"At 14:07:22.27 UT on 6 March 2017, the Fermi Gamma-Ray Burst Monitor
triggered and located GRB 170306B (trigger 510502047 / 170306588),
which was also detected by Fermi/LAT (Omodei et al., GCN 20826
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 ). The
GBM on-ground location is consistent with the LAT position.


The trigger resulted in an Autonomous Repoint Request (ARR)
by the GBM Flight Software owing to the high peak flux
of the GRB. This ARR was accepted and the spacecraft slewed
to the GBM in-flight location.

The GBM light curve shows multiple overlapping pulses
with a duration (T90) of about 18.7 s (50-300 keV).
The time-averaged spectrum from T0-3.8 s to T0+26.9 s is
best fit by a power law function with an exponential
high-energy cutoff.  The power law index is -0.73 +/- 0.03 and
the cutoff energy, parameterized as Epeak, is 289 +/- 10 keV.


The event fluence (10-1000 keV) in this time interval is
(3.07 +/- 0.06)E-5 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+8.7 s in the 10-1000 keV band
is 12.4 +/- 0.3 ph/s/cm^2.

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

V. D'Elia (ASDC), A. D'Ai (INAF-IASFPA), B. Sbarufatti (INAF-OAB/PSU),
D.N. Burrows (PSU), A. Cholden-Brown (PSU), J.P. Osborne (U.
Leicester), K.L. Page (U. Leicester), B. Mingo (U. Leicester), A.
Melandri (INAF-OAB) and P.A. Evans (U. Leicester) report on behalf of
the Swift-XRT team:

Swift-XRT has performed follow-up observations of the
Fermi/LAT-detected burst GRB 170306B in a series of observations tiled
on the sky. The total exposure time is 4.5 ks, distributed over 8
tiles; the maximum exposure at a single sky location was 1.4 ks. The
data were collected between T0+17.4 ks and T0+29.9 ks, and are entirely
in Photon Counting (PC) mode. 

No uncatalogued X-ray sources have been detected. The 3-sigma upper
limit in the field (not including the regions where the tiles overlap)
ranges from ~0.007 to ~0.021 ct s^-1, corresponding to a 0.3-10 keV
observed flux of 2.9e-13 to 8.5e-13 erg cm^-2 s^-1 (assuming a typical
GRB spectrum).

Four previously-catalogued X-ray sources have been detected, however
their status as catalogued objects makes them unlikely to be the
afterglow.

The results of the XRT-team automatic analysis of the tiled XRT
observations, including a position-specific upper limit calculator, are
available at http://www.swift.ac.uk/xrt_products/TILED_GRB00065.

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

D. Frederiks, S. Golenetskii, R.Aptekar, 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 GRB 170306B (Fermi-LAT detection: Omodei et al., GCN 20826
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 ;
Fermi-GBM detection: Veres et al., GCN 20827
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 )
triggered Konus-Wind at T0=50850.920 s UT (14:07:30.920)

The light curve shows a multi-peaked structure
with a total duration of ~25 s.
The emission is seen up to ~10 MeV.

As observed by Konus-Wind, the burst had a fluence of
(3.4 �� 0.4)x10^-5 erg/cm2 and a 64-ms peak energy flux,
measured from T0+3.712, of (4.1 �� 0.6)x10^-6 erg/cm2
(both in the 20 keV - 10 MeV energy range).

The time-integrated spectrum (measured from T0 to T0+22.528 s)
is best fit in the 20 keV - 15 MeV range
by the GRB (Band) function with the following model parameters:
the low-energy photon index alpha = -0.58 (-0.13,+0.16),
the high energy photon index beta = -2.62 (-0.61,+0.28),
the peak energy Ep = 237 (-27,+27) keV,
chi2 = 105/97 dof.

The spectrum near the peak count rate (measured from T0 to T0+6.912 s)
is best fit in the 20 keV - 15 MeV range
by the GRB (Band) function with the following model parameters:
the low-energy photon index alpha = -0.40 (-0.11,+0.12),
the high energy photon index beta = -2.70 (-0.84,+0.29),
the peak energy Ep = 214 (-24,+30) keV,
chi2 = 120/97 dof.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB170306_T50850/

All the quoted errors are estimated at the 90% confidence level.
All the presented results are preliminary.

H.L. Xiao (PSI), R. Marcinkowski (PSI)  and W. Hajdas (PSI) report on behalf of the POLAR collaboration:

At 2017-03-06 14:07:20.0 UT(T0), during a routine on-ground search of data, POLAR detected GRB 170306B,
which was also observed by  Fermi-GBM (trigger # 510502047), Fermi-LAT (trigger #170306588) and SPI ACS at 2017-03-06 14:07:25.

The POLAR light curve consists of multiple peaks with duration (T90) of 25.0 +/- 1.0 s measured from T0.
The 1.00 s peak flux at T0 + 11.50 s  is equal to 2380 +/- 100 counts/sec.
POLAR recorded 31640 events from the burst.
Above measurements are in the energy range of about 15 - 300 keV.

LC_URL: http://polar.psi.ch/triggers/GRB_170306B_raw.png
     or http://polar.psi.ch/pub/lc.php?event=GRB+170306B

Using the best location from Fermi-LAT, which is (J2000):
RA : 154.650 [deg]
Dec:  51.630 [deg]
Err:  39.00  [arcmin]
the incident angle in the POLAR coordinate at T0 is:
Theta: 87.0 [deg]
Phi: 98.4 [deg]

The analysis results presented above are preliminary. POLAR is a dedicated Gamma-Ray Burst polarimeter which
was launched on-board the Chinese space laboratory Tiangong-2 (TG-2) on Sep 15, 2016. 

More information about POLAR can be found at http://polar.psi.ch/pub ,
http://polar.ihep.ac.cn/en/ and http://isdc.unige.ch/polar/ .

This message is quotable in publications.