GRB 100401A

J. R. Cummings (GSFC/UMBC), N. Gehrels (NASA/GSFC), F. E. Marshall (NASA/GSFC)
on behalf of the Swift team


At 2010-04-01T07:07:31.9 BAT detected a rate increase (trigger# 417974).
The source was found with insufficient significance onboard to trigger
an automated burst response.  A significant source was found in ground
analysis at RA, Dec 290.813, -8.257, which is:

RA (J2000)   19h 23m 15.5s
Dec (J2000) -08d 15' 25"

with an estimated uncertainty of 2 arcmin radius (90% containment). The
source was 10% coded in the BAT field of view.

As seen in BAT, the burst had a single square-shaped pulse of about
2 seconds duration.

The spectrum from 15 to 150 keV is best fit by a simple power law function
with a photon index of 1.7 +- 0.2.  We note that this is softer than is
typical for "short, hard" GRBs.  The fluence was (3.6 +- 1.2) erg/cm2.  The
1-second peak flux was 2.4 photons/cm2.

Since this burst was not detected with sufficient significance onboard,
there are no automated data products.  A Swift TOO is in progress.

A. Goldstein (UAH) and D. Gruber (MPE) 
report on behalf of the Fermi GBM Team: 

"At 07:07:32.24 UT on 1 April 2010, the Fermi Gamma-Ray Burst Monitor
triggered and located GRB 100401A (trigger 291798454 / 100401297)
which was also detected by the Swift-BAT (J.R. Cummings et al. 2010,
GCN 10567
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 ).  The GBM on-ground location is consistent with the 
Swift-BAT position. The angle from the Fermi LAT boresight is 45 degrees.

The GBM light curve consists of an initial bright pulse at trigger time
lasting ~4 s followed by a second, much softer pulse of ~30 s in duration,
approximately 65 s after trigger time. The total duration (T90) is 
estimated to be about 100 s (8-1000 keV). The time-averaged spectrum of 
the first pulse from T0-1.9 s to T0+3.2 s is best fit by a power law 
function with an exponential high energy cutoff.  The power law index is 
-1.49 +/- 0.17 and the cutoff energy, parameterized as Epeak, is 
144.90 +/- 62.20 keV (chi squared 439 for 483 d.o.f.). The first 
pulse is equally well fit by a simple power law with index 
-1.77 +/- 0.06 (chi squared 444 for 484 d.o.f.).  The 
time-averaged spectrum of the second pulse from T0+74.9 s to 
T0+98.4 s is well fit by a power law function with an exponential
high energy cutoff.  The power law index is -1.52 +/- 0.39 and
the cutoff energy, parameterized as Epeak, is 29.34 +/- 8.96 keV
(chi squared 511 for 483 d.o.f.).  The second pulse is equally well
fit by a simple power law with index -2.16 +/- 0.11 (chi squared 
514 for 484 d.o.f.).

The event fluence (8-1000 keV) over the entire emission is 
(2.39 +/- 0.05)E-06 erg/cm^2. The 1-sec peak photon flux measured 
starting from T0-1.4 s in the 8-1000 keV band 
is 3.61 +/- 0.16 ph/s/cm^2.

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

G. Stratta and V. D'Elia (ASDC) report on behalf of the Swift Team:

Swift began a target of opportunity observation of
GRB 100401A detected on ground (Cummings et al., GCN #10567
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 )
and by the Fermi/GBM (Goldstein and Gruber, GCN #10568
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 ), at
19:33:04 UT on April 2nd, 2010, approximately 36.4 hours after the
Swift/BAT trigger. In about 2 ks of Photon Counting
mode data, no X-ray source is detected inside the BAT
error circle.

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

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M.M. Chester (PSU) and G. Stratta (ASDC) report on behalf of the
Swift/UVOT team:

Swift began a Target of Opportunity observation of GRB 100401A,
detected by Swift/BAT (Cummings et al., GCN Circ. 10567
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 ) and by
Fermi/GBM (Goldstein and Gruber, GCN Circ. 10568
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 ), on April 2nd,
2010 at 19:33:04 UT, approximately 36 hours after the Swift/BAT
trigger.

No optical afterglow consistent with the BAT position is detected
in the UVOT exposure. The 3-sigma upper limit using the UVOT
photometric system (Poole et al. 2008, MNRAS, 383, 627) is:

Filter     T_start(s)   T_stop(s)      Exp(s)      3-sig UL (mag)

u          131128       133123         1963         >20.8

The value quoted above is not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.29 in the direction of the burst
(Schlegel et al. 1998).

SUBJECT: GRB100401A, GROND Upper Limits

A. Updike (Clemson University), A. Rau, P. Afonso and J. Greiner (all
MPE Garching) report on behalf of the GROND team:

We observed the field of GRB 100401A (Cummings et al., GCN #10567
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 ,
Goldstein and Gruber, #GCN 10568)  simultaneously in g'r'i'z'JHK with
GROND (Greiner et al. 2008,PASP 120, 405), mounted at the 2.2m ESO/MPI
telescope at La Silla Observatory (Chile).

Observations started on 2010, April 3, at 09:21 UT, 2.09 days after the
burst, and consisted of 20 min in JHK and 24.6 min in g'r'i'z'. A second
epoch was obtained on 2010, April 4, at 09:09 UT, 2.99 days after the burst.

Image subtraction of the two epochs does not reveal a variable source,
down to the following 1st epoch limiting magnitudes (all in AB system):

g' > 23.7
r' > 23.9
i' > 23.4
z' > 23.4
J  > 21.8
H  > 21.4
K  > 20.3

These magnitudes were obtained using 2MASS field stars and GROND
zero-points as reference, and are not corrected for the Galactic
extinction due to the reddening of E(B-V) = 0.29 mag in the direction of
the burst (Schlegel et al. 1998).