GRB 150402A

M. De Pasquale (INAF-IASF Palermo) and H. Krimm (CRESST/GSFC/USRA)

Further 9.9 ks of XRT observations of GRB150402A (Cummings et al., GCN circ. 17661
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 ),
taken between T0+643.2 and T0+673.6 ks, show that the uncatalogued X-ray source
detected in the BAT error region (Krimm et al., GCN circ. 17664
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 ) is still detected
at a count rate of 3.9e-3 ct s^-1, showing no fading with respect to previous observations.
We therefore conclude that this source is not the X-ray afterglow of GRB150402A.
The 3 sigma upper limit calculated at the centre of the BAT error circle is 3.7e-3 ct s^-1.

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H. A. Krimm (CRESST/GSFC/USRA), M. De Pasquale (INAF-IASFPA),
N. P. M. Kuin (UCL-MSSL), J. R. Cummings (CPI), J. Kennea (PSU),
A. Y. Lien (GSFC/UMBC),  K. Page (U. Leicester)
report on behalf of the Swift team

Further observations of GRB 150402A (Cummings and Palmer, GCN 17661
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 ) were 
carried out by the Swift observatory (Evans, GCN 17662
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 ).  We collected 4.8 ks of 
Photon Counting (PC) mode data between T0+108.4 ks and T0+121.6 ks, where T0 is 
the time of the BAT trigger.  One uncatalogued X-ray source has been detected 
inside or close to the Swift/BAT error region, it is below the RASS limit and 
shows no definitive signs of fading. Therefore, at the present time we cannot 
confirm this as the afterglow. Details of this source are given below:

   RA (J2000.0):  173.5883  =  11h 34m 21.19s
   Dec (J2000.0): +40.9398  =  +40 56��� 23.3������
   Error: 4.6 arcsec (radius, 90% conf.)
   Count-rate: (3.5 �� 1.1)e-3 ct s^-1
   Flux: (1.7 �� 0.5)e-13 erg cm^-2 s^-1 (observed, 0.3-10 keV)

One catalogued source was also detected too far from the BAT GRB position
to be a likely afterglow candidate.
     The UVOT observed in the uvw1 filter.  Close to the XRT position a star is 
seen in the DSS, which shows no evidence of an additional fading source 
contributing to the emission. The GRB upper limit is uvw1 > 18.63.
     Revised analysis of the BAT data yields a refined position of

   RA (J2000):  173.604 = 11h 34m 25.1s
   Dec (J2000): +40.988 = 40d 59' 18"

with an estimated uncertainty radius of 3.0 arcmin (90% containment). 
Mask-weighted data is available from T-2.0 to T+8.0 sec, which does not cover 
the entire burst, but we estimate T90 from the un-weighted data to be 14.4 +- 
1.6 sec (estimated error including systematics).  The available light curve 
shows two peaks, each about 4 seconds long. The time-averaged spectrum from 
T+0.0 to T+8.1 sec is best fit by a simple power-law model.  The power law index 
of the time-averaged spectrum is 2.29 +- 0.47.  The fluence in the 15-150 keV 
band is 1.5 +- 0.4 x 10^-7 erg/cm2. All the quoted errors are at the 90% 
confidence level.
     No further observations of this source are currently planned.

Owen Littlejohns (ASU), Nat Butler (ASU), Alan M. Watson (UNAM),
Alexander Kutyrev (GSFC), William H. Lee (UNAM), Michael G. Richer
(UNAM), Chris Klein (UCB), Ori Fox (UCB), J. Xavier Prochaska
(UCSC), Josh Bloom (UCB), Antonino Cucchiara (ORAU/GSFC), Eleonora
Troja (GSFC), Enrico Ramirez-Ruiz (UCSC), José A. de Diego (UNAM),
Leonid Georgiev (UNAM), Jesús González (UNAM), Carlos Román-Zúñiga
(UNAM), Neil Gehrels (GSFC), and Harvey Moseley (GSFC) report:

We observed the field of GRB 150402A (Cummings, et al., GCN 17661
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 )
with the Reionization and Transients Infrared Camera (RATIR;
www.ratir.org) on the 1.5m Harold Johnson Telescope at the
Observatorio Astronómico Nacional on Sierra San Pedro Mártir from
2015/04 3.02 to 2015/04 10.92 UTC (26.39 to 34.29 hours after the
BAT trigger), obtaining a total of 3.16 hours exposure in the r
and i bands.

Currently, there is only one source detected by the Swift/XRT
within the Swift/BAT error circle (Evans, GCN 17662
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 ).

  RA(J2000)  =  11h 34m 34.0s
  Dec(J2000) = +40d 58' 37.8"

This source has an error circle of 6.2 arcseconds. We detect
no uncatalogued transient inside the error circle for this source.
In comparison with the SDSS DR9, we obtain the following upper
limits (3-sigma):

  r     > 22.75
  i     > 22.68

These magnitudes are in the AB system and are not corrected for
Galactic extinction in the direction of the GRB.

We thank the staff of the Observatorio Astronómico Nacional in San
Pedro Mártir.

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

Swift has initiated a ToO observation of the Swift/BAT GRB 150402A. 
Automated analysis of the XRT data will be presented online at
http://www.swift.ac.uk/ToO_GRBs/00020486

Any uncatalogued X-ray sources detected in this analysis will be
reported on this website and via GCN COUNTERPART notices. These are
not necessarily related to the Swift/BAT event. 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.

J. R. Cummings (CPI) and D. M. Palmer (LANL) on behalf of the Swift-BAT science team:

At 00:37:46 Swift-BAT triggered on GRB 150402A (trigger# 636865). Image
attempts were prematurely terminated by a preplanned slew.  A significant
source was found in ground analysis at RA, Dec 173.658, +40.992, which is

RA (J2000)  11h 34m 37.9s
Dec (J2000) 40d 59' 33"

with an estimated uncertainty radius of 2.5 arcmin (90% containment).

The lightcurve shows two symmetrical peaks, each about 5 seconds long, the
second harder and brighter.  The counting rate in BAT of the brighter peak
was about 1500 counts/sec.  T90 was about 14 +- 2 seconds.

A Swift TOO has been requested in order to locate the afterglow.