GRB 170228A

E. Bissaldi (Politecnico & INFN Bari), F. Longo (University & INFN Trieste),
E. Moretti (MPI Munich), M. Yassine (LUPM) and Z. Acuner (KTH Stockholm)
report on behalf of the Fermi-LAT team:


At 19:03:00.71 UT on February, 28, 2017, Fermi-LAT detected high-energy emission
from GRB 170228A, which was also detected by Fermi-GBM (trigger 170228794/510001385).

The best LAT on-ground location is found to be

RA, Dec = 239.55, -3.59 (J2000)

with an error radius of 0.3 deg (90 % containment, statistical error only).

This was about 70 deg from the LAT boresight at the time of the trigger.
The burst entered the LAT FoV about 80 s post trigger and exited about 800 s post trigger.
The data from the Fermi-LAT show a significant increase in the event rate that
is spatially correlated with the trigger with high significance.
In total, LAT detected about 20 photons with an energy greater than 100 MeV.
The highest-energy photon is a 5 GeV event, which is observed 130
seconds after the GBM trigger.

A Swift ToO has been requested for this burst.

The Fermi-LAT point of contact for this burst is Zeynep Acuner (acuner@kth.se).


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, M. Stanbro and C. Meegan (UAH)
report on behalf of the Fermi GBM Team:

"At 19:03:00.71 UT on 28 February 2017, the Fermi Gamma-Ray Burst
Monitor triggered and located GRB 170228A (trigger 510001385 /
170228794) which was also detected by Fermi/LAT (Bissaldi et al., GCN
20785
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 ). The GBM on-ground location is consistent with the LAT
position.


The GBM light curve shows a single pulse followed by weaker emission
with a duration (T90) of about 62 s (50-300 keV).
The time-averaged spectrum from T0 to T0 + 18.8 s is
best fit by a Band function with Epeak = 755 +/- 76 keV,
alpha = -0.50 +/- 0.07, and beta = -2.35 +/- 0.17.

The event fluence (10-1000 keV) in this time interval is
(2.23 +/- 0.06)E-5 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+1.0 s in the 10-1000 keV band
is 10.2 +/- 0.2 ph/s/cm^2.


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

V. Sharma, V. Bhalerao (IITB) and D. Bhattacharya (IUCAA), A. R. Rao (TIFR) and S. Vadawale (PRL) report on behalf of the AstroSat CZTI collaboration:

Analysis of AstroSat CZTI data showed clear detection of GRB170228A (Fermi-LAT detection: E. Bissaldi et al., GCN Circ. 20785
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 ) in the 40-200 keV energy range. The light curve shows single peak structure with main peak at 19:03:02.71 UT, 2 s after the Fermi trigger. The measured peak count rate is 541.9 counts/sec above the background in combined data of four quadrants, with a total 3569 counts. The local mean background count rate was 349.1 counts/s. Using cumulative rates, we measure a T90 of 14.1 s.

It was clearly detected in the CsI anticoincidence detector (Veto) also as bright detection in the 100-500 keV energy range.

CZTI GRB detections are reported regularly on the payload site at http://astrosat.iucaa.in/czti/?q=grb. CZTI is built by a TIFR-led consortium of institutes across India, including VSSC, ISAC, IUCAA, SAC and PRL. The Indian Space Research Organisation funded, managed and facilitated the project.

A. Tsvetkova, S. Golenetskii, R.Aptekar, D. Frederiks, P. Oleynik,
M. Ulanov, D. Svinkin, A. Lysenko, A. Kozlova and T. Cline
on behalf of the Konus-Wind team, report:

The long-duration, hard-spectrum GRB 170228A
(Fermi-LAT detection: Bissaldi et al., GCN 20785
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 ;
Fermi GBM observation: Veres et al., GCN 20786
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 ;
AstroSat CZTI detection: Sharma et al., GCN 20788
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 )
triggered Konus-Wind at T0=68584.788 s UT (19:03:04.788).

The burst light curve starts with a bright FRED-like pulse having a
duration of ~20 s, followed by a weaker emission,
which is detectable until ~T0+100 s.
The emission is seen up to ~3 MeV.

As observed by Konus-Wind, the burst
had a fluence of 4.99(-0.99,+1.51)x10^-5 erg/cm2,
and a 64-ms peak energy flux, measured from T0+0.316 s,
of 8.97(-2.47,+2.57)x10^-6 erg/cm2/s
(both in the 20 keV - 10 MeV energy range).

The time-integrated spectrum of the burst
(measured from T0 to T0+98.560 s)
is best fit in the 20 keV - 15 MeV range
by a power law with exponential cutoff model:
dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep)
with  alpha = -1.03(-0.18,+0.22),
and Ep = 1017(-317,+657) keV (chi2 = 97/98 dof).
Fitting by the GRB (Band) model yields the same alpha and Ep,
and an upper limit on the high energy photon index:
beta < -1.98 (chi2 = 97/97 dof).

The spectrum near the peak count rate
(measured from T0 to T0+8.448 s)
is best fit in the 20 keV - 5 MeV range
by the GRB (Band) model with the following parameters:
the low-energy photon index alpha = -0.43(-0.14,+0.17),
the high energy photon index beta = -2.78(-2.28,+0.48),
the peak energy Ep = 675(-92,+112) keV,
chi2 = 95/76 dof.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB170228_T68584/

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