Gamma-ray Burst 170424A

(All information courtesy of the instrument teams.)

Previous IAU Circulars

none

Results of Observations

GCN Circular #21032
N. Omodei, G.Vianello (Stanford University), report on behalf of the Fermi-LAT team:
At 10:12:30.75 on April, 24, 2017 Fermi-LAT detected high-energy emission from GRB 170424A, which was also detected by Fermi-GBM (trigger 514721555 / 170424425).
The best LAT on-ground location is found to be
RA, Dec = 343.7, -45.12 (J2000)
with an error radius of 0.45 deg (90 % containment, statistical error only).
This was 57 deg from the LAT boresight at the time of the trigger.
The data from the Fermi-LAT show a significant increase in the event rate that is spatially and temporally correlated with the trigger with high significance.
The highest-energy photon is a 1 GeV event which is observed 56 seconds after the GBM trigger.
The GRB was also detected using LAT Low Energy (LLE) data selection, with a significance of 6 sigma.
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.
GCN Circular #21033
M. Stanbro (UAH), A. von Kienlin (MPE) and C. Meegan (UAH) report on behalf of the Fermi GBM Team:
"At 10:12:30.75 UT on 24 April 2017, the Fermi Gamma-Ray Burst Monitor triggered and located GRB 170424A (trigger 514721555 / 170424425) which was also detected by the Fermi-LAT (N. Omodei et al. 2017, GCN 21032). The GBM on-ground location is consistent with the Fermi-LAT position.
This burst was also independently detected by INTEGRAL SPI-ACS.
The GBM light curve consists of one episode with a duration (T90) of about 52 s (50-300 keV). The time-averaged spectrum from T0+1.54 s to T0+55.81 s is best fit by a Band function with Epeak = 464 +/- 39 keV, alpha = -0.85 +/- 0.03, and beta = -2.00 +/- 0.09.
The event fluence (10-1000 keV) in this time interval is (3.07 +/- 0.05)E-05 erg/cm^2. The 1-sec peak photon flux measured starting from T0+23.5 s in the 10-1000 keV band is 4.8 +/- 0.2 ph/s/cm^2.
A power law function with an exponential high-energy cutoff fits the spectrum equally well. The power law index is -0.89 +/- 0.02 and the cutoff energy, parameterized as Epeak, is 570 +/- 33 keV.
The spectral analysis results presented above are preliminary; final results will be published in the GBM GRB Catalog."
GCN Circular #21034
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 170424A. Automated analysis of the XRT data will be presented online at http://www.swift.ac.uk/xrt_products/TILED_GRB00066
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.
GCN Circular #21036
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 GRB 170424A (Fermi-LAT detection: Omodei & Vianello, GCN 21032; Fermi GBM detection: Stanbro, von Kienlin & Meegan, GCN 21033) triggered Konus-Wind at T0=36769.094 s UT (10:12:49.094).
The burst light curve shows a single pulse which started at ~T0-20 s and lasted until ~T0+42 s (the total duration is ~62 s). The emission is seen up to ~5 MeV.
As observed by Konus-Wind, the burst had a fluence of 3.46(-0.32,+0.44)x10^-5 erg/cm2, and a 64-ms peak energy flux, measured from T0-0.046 s, of 2.81(-1.05,+1.11)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+41.216 s) is best fit in the 20 keV - 18 MeV range by a power law with exponential cutoff model: dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep) with alpha = -0.91(-0.19,+0.23), and Ep = 353(-70,+111) keV (chi2 = 82/98 dof). Fitting by the GRB (Band) model yields the same alpha and Ep, and an upper limit on the high energy photon index: beta < -2.01 (chi2 = 82/97 dof).
The spectrum near the peak count rate (measured from T0 to T0+8.448 s) is best fit in the 20 keV - 18 MeV range by the GRB (Band) model with the following parameters: the low-energy photon index alpha = -0.77(-0.23,+0.33), the high energy photon index beta = -3.30(-6.70,+1.17), the peak energy Ep = 446(-127,+170) keV, chi2 = 108/97 dof.
The Konus-Wind light curve of this GRB is available at http://www.ioffe.ru/LEA/GRBs/GRB170424_T36769/
All the quoted errors are at the 90% confidence level. All the quoted values are preliminary.
GCN Circular #21037
A. Cucchiara, D. Morris (U. of Virgin Islands), C. Guidorzi (U. Ferrara), B. Gendre (U. of Virgin Islands), reports on behalf of a larger collaboration:
"On April 25.12 UT (T_0 +14.5h) we began observing the center of the field of LAT GRB 170424A (Omodei et al. GCN #21032) using the Las Cumbres Observatory 1m Sutherland facility. We covered 30% of the LAT error region and we obtained a series of observations in R and I band for a total of 6 minutes on sky in each filter.
We identified no optical counterpart within the center of Swift-XRT tile #20753 (Evans et al. GCN #21034), out to a radius of ~13 arcmin, at the following 3-sigma limits:
```
R > 21.2 mag
I > 20.8 mag
```
These magnitude are calibrated against nearby USNO-B1 sources, and are not corrected for Galactic Extinction."
```
Antonino Cucchiara, PhD
Assistant Professor
Physics and Astronomy
University of the Virgin Islands

Phone: 340-693-1688
College of Science and Mathematics
#2 Brewers Bay road
St. Thomas, USVI 00802
```
GCN Circular #21074
B. Sbarufatti (INAF-OAB/PSU), P. A. Evans (U. Leicester) report on behalf of the Swift XRT team.
We report on Swift XRT observations of the field of the Fermi-LAT discovered GRB 170424A (N. Omodei et al. 2017, GCN 21032). Swift performed an initial tiled observation of the LAT error circle (27 arcmin) using a 7-tile pattern (7-tile pattern 100% coverage radius: 24 arcmins, maximal coverage radius: 33 arcmins), 700 s of exposure/tile, on April 25th (P. A. Evans 2017, GCN 21034).
Three uncatalogued sources were detected:
```
Source# RA             Dec          Rate              Exposure time 
2       22h 52m 53.04s -45d10m43.5s 1.5+/-0.5E-2 cps  684s
3       22h 53m 10.51s -45d29m11.5s 1.9+/-0.6E-2 cps  677s
5       22h 52m 09.64s -45d02m43.3s 1.5+/-0.7E-2 cps  684s
```
On May 3rd and 4th further observations were performed to check for fading of the sources. Source 2 was still detected without showing any evidence of fading. Sources 3 and 5 were no more detected, but the 3-sigma upper limits are consistent with the earlier detections, within errors.
```
Source# Rate              Exposure time 
2       1.1+/-0.2E-2 cps  2.8 ks
3       < 1.4E-2  cps     3.1 ks      
5       < 7.1E-3  cps     3.1 ks
```
We conclude that Source 2 is not the GRB afterglow, while the nature of Sources 3 and 5 remains uncertain.
All the quoted errors are at the 90% confidence level.
This circular is an official product of the Swift-XRT team.

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Jochen Greiner, last update: 10-Jun-2017 [Disclaimer]