GRB 160424A

A. Y. Lien (GSFC/UMBC), A. D'Ai (INAF-IASFPA), J. A. Kennea (PSU),
K. L. Page (U Leicester), D. M. Palmer (LANL), T. Sakamoto (AGU) and
M. H. Siegel (PSU) report on behalf of the Swift Team:

At 11:49:07 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 160424A (trigger=683906).  Swift did not immediately slew
to the burst due to an observing constraint. 
The BAT on-board calculated location is 
RA, Dec 319.481, -60.405 which is 
   RA(J2000) = 21h 17m 55s
   Dec(J2000) = -60d 24' 18"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a single-peaked
structure with a duration of about 5 sec.  The peak count rate
was ~5000 counts/sec (15-350 keV), at ~0 sec after the trigger. 

Due to an observing constraint, Swift will not slew until T0+48.7
minutes. There will be no XRT or UVOT data until this time. 

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.)

P.A. Evans (U. Leicester), J.P. Osborne (U. Leicester), D.N. Burrows
(PSU), J.A. Kennea (PSU), M. Perri (ASDC) and G. Tagliaferri (INAF-OAB)
report on behalf of the Swift-XRT team:

The XRT began observing the field of GRB 160424A at 12:40:02.1 UT,
3054.8 seconds after the BAT trigger. Using promptly downlinked data we
find a fading, uncatalogued X-ray source located at RA, Dec 319.48505,
-60.41481 which is equivalent to:
   RA(J2000)  = 21h 17m 56.41s
   Dec(J2000) = -60d 24' 53.3"
with an uncertainty of 3.8 arcseconds (radius, 90% containment). This
location is 36 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 (3.94 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 2.5
(+2.04/-1.82) x 10^21 cm^-2 (90% confidence).

F.E. Marshall (NASA/GSFC) and A. Lien (GSFC/UMBC)
report on behalf of the Swift/UVOT team:

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 3059 seconds after the BAT trigger (Lien et al. GCN Circ. 19335
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 ). 
No credible afterglow candidate
has been found in the initial data products. The 2.7'x2.7' sub-image 
covers 100% of the XRT error region (Evans et al. GCN Circ 19336
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 ).  
Sensitivity is less than usual because of reduced pointing stability.
We estimate an upper limit of 16th mag.
No correction has been made
for the expected extinction corresponding to E(B-V) of 0.03.

H. A. Krimm (GSFC/USRA), S. D. Barthelmy (GSFC), J. R. Cummings (GSFC/UMBC),
N. Gehrels (GSFC), A. Y. Lien (GSFC/UMBC), C. B. Markwardt (GSFC),
D. M. Palmer (LANL), T. Sakamoto (AGU), M. Stamatikos (OSU), T. N. Ukwatta (LANL)
(i.e. the Swift-BAT team):
 
Using the data set from T-61 to T+243 sec from the recent telemetry downlinks,
we report further analysis of BAT GRB 160424A (trigger #683906)
(Lien, et al., GCN Circ. 19335
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 ).  The BAT ground-calculated position is
RA, Dec = 319.482, -60.408 deg, which is 
   RA(J2000)  =  21h 17m 55.7s 
   Dec(J2000) = -60d 24' 29.8" 
with an uncertainty of 1.2 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 33%.
 
The mask-weighted light curve shows a FRED-like peak starting at ~T-0.5 sec,
peaking at ~T+0.4 sec, and ending at ~T+10 sec.  T90 (15-350 keV) is
6.3 +- 1.0 sec (estimated error including systematics).
 
The time-averaged spectrum from T-1.51 to T+6.63 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.40 +- 0.10.  The fluence in the 15-150 keV band is 1.2 +- 0.1 x 10^-6 erg/cm2.
The 1-sec peak photon flux measured from T+0.06 sec in the 15-150 keV band
is 5.1 +- 0.4 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/683906/BA/

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

During the XRT observations of GRB 160424A (Evans et al., GCN Circ. 
19336
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 ) the star trackers onboard Swift were unable to acquire a correct 
lock. As a result of this the attitude solution for the spacecraft is 
uncertain, and visual inspection of the images shows that the position 
of the XRT afterglow is highly uncertain. Our best estimate of the 
position is as given in GCN Circ. 19336
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 , however the uncertainty is  ~50 
arc sec.

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

Hoi-Fung Yu (MPE) reports on behalf of the Fermi GBM Team:

"At 11:49:06.48 UT on 24 April 2016, the Fermi Gamma-Ray Burst Monitor
triggered and located GRB 160424A (trigger 483191350 / 160424492),
which was also detected by the Swift (Lien et al. 2016, GCN 19335
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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 18 degrees.

This burst was also independently detected by INTEGRAL SPI-ACS.

The GBM light curve shows a single pulse with a duration (T90)
of about 7.6 s (50-300 keV). The time-averaged spectrum from
T0+0.003 s to T0+6.528 s is well fit by a power law function
with an exponential high-energy cutoff. The power law index
is -1.1 +/- 0.1 and the cutoff energy, parameterized as Epeak,
is 220 +/- 30 keV.

The event fluence (10-1000 keV) in this time interval is
(2.6 +/- 0.2)E-06 erg/cm^2. The 1.024-sec peak photon flux measured
starting from T0+0.832 s in the 10-1000 keV band is 7.2 +/- 0.3 ph/s/cm^2.

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

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

The long-duration GRB 160424A
(Swift-BAT trigger #683906: Lien et al., GCN Circ. 19335
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 ;
Krimm et al., GCN Circ. 19338
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 ;
Fermi GBM detection: Hoi-Fung Yu, GCN Circ. 19340
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 )
triggered Konus-Wind at T0=42546.397 s UT (11:49:06.397).

The burst light curve shows a single pulse with a total
duration of ~2.3 s. The emission is seen up to 2 MeV.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB160424_T42546/

As observed by Konus-Wind, the burst
had a fluence of 1.59(-0.35,+0.60)x10^-6 erg/cm2,
and a 64-ms peak flux, measured from T0+0.464 s,
of 1.71(-0.62,+0.84)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+8.448 s)
is best fit in the 20 keV - 2 MeV range
by a power law with exponential cutoff model:
dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep)
with  alpha = -1.24(-0.51,+0.66)
and Ep = 133(-40,+134) keV (chi2 = 58/60 dof).
Fitting by a GRB (Band) model yields the same alpha and Ep,
and an upper limit on the high energy photon index: beta < -2.0
(chi2 = 57/59 dof)

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