GRB 150206B

L. P. Singer (NASA/GSFC), M. M. Kasliwal (Carnegie Observatories/Princeton),
and S. B. Cenko (NASA/GSFC) report on behalf of the intermediate Palomar
Transient Factory (iPTF) collaboration:

Fermi detected GRB 150206B (Fermi trigger 444908790 / bn150206407) at
2015-02-06 09:46:27.48. At 2015-02-06 10:23:45, 37 minutes after the
burst, we began searching for optical counterparts using the Palomar
48-inch Oschin telescope (P48).

We imaged 20 fields covering an area of 144 deg^2 mostly inside the
1-sigma statistical+systematic region of the final Fermi GBM localization.
We estimate a 51% prior probability that these fields contain the true
location of the source. Sifting through candidate variable sources using
image subtraction and standard iPTF vetting procedures, we detected the
following optical transient candidates:

iPTF15gz, at the coordinates:
  RA(J2000)  =  13h 47m 18.31s (206.826292 deg)
  Dec(J2000) = +55d 38' 33.7"  (+55.642694 deg)
No source is visible at this position in archival SDSS and iPTF images.
The following P48 photometry is suggestive of fading:
  +116.7 min:   R = 20.25 +/- 0.13
  +160.7 min:   R = 20.54 +/- 0.19

iPTF15hb, at the coordinates:
  RA(J2000)  =  14h 07m 09.34s (211.788915 deg)
  Dec(J2000) = +63d 02' 20.6"  (+63.039047 deg)
This position is near the galaxy SDSS J140709.19+630218.3, with photoZ ~ 0.2
suggesting an absolute magnitude of M_R ~ -21, consistent with typical
optical afterglows. We report the following photometry:
  +98.20 min:   R = 18.94 +/- 0.05
  +142.2 min:   R = 18.87 +/- 0.04
  +186.3 min:   R = 19.00 +/- 0.04

iPTF15gv, at the coordinates:
  RA(J2000)  =  14h 15m 28.31s (213.867960 deg)
  Dec(J2000) = +58d 24' 47.8"  (+58.413284 deg)
This coincides with the galaxy SDSS J141527.96+582447.9, with photoZ ~ 0.085,
suggesting M_R ~ -18. Although this is fainter than a typical optical
afterglow, the source is rapidly fading:
  +125.3 min:   19.89 +/- 0.07
  +169.3 min:   20.33 +/- 0.11
which fits a power law relative to the time of the burst, F_nu ~ t^-alpha,
with alpha = 1.3 +/- 0.4.

Times are relative to the GBM trigger. Magnitudes are in the Mould R
filter and in the AB system, calibrated with respect to point sources in
SDSS as described in Ofek et al. (2012, http://dx.doi.org/10.1086/664065
Loading...
 
 ).

There were several other optical candidates in these fields; all of the
candidates listed below are coincident with galaxies of known redshift in
SDSS, by which association we infer absolute magnitudes of M_R ~ -19,
typical of type Ia supernovae but not long GRB optical afterglows (see
Section 2.4 of Singer et al. 2015, http://arxiv.org/abs/1501.00495
Loading...
 
 , and
references therein). These are:

iPTF15ha, at the coordinates:
  RA(J2000)  =  14h 03m 11.73s (210.798855 deg)
  Dec(J2000) = +64d 22' 19.3"  (+64.372029 deg)

iPTF15gu, at the coordinates:
  RA(J2000)  =  14h 29m 11.95s (217.299810 deg)
  Dec(J2000) = +56d 59' 21.4"  (+56.989264 deg)

iPTF15gy, at the coordinates:
  RA(J2000)  =  14h 47m 18.26s (221.826083 deg)
  Dec(J2000) = +54d 36' 30.6"  (+54.608492 deg)

iPTF15gx, at the coordinates:
  RA(J2000)  =  15h 03m 42.64s (225.927684 deg)
  Dec(J2000) = +54d 48' 03.1"  (+54.800849 deg)

Further observations of iPTF15gz, iPTF15hb, and iPTF15gv, are encouraged
to determine if one of the three is associated with the GRB.

The diagram http://www.its.caltech.edu/~lsinger/iptf/Fermi444908790.pdf
shows the locations of our candidates and the P48 fields in relation to
the Fermi GBM 1- and 2-sigma statistical+systematic contours.

E. Burns (UAH) and H.-F Yu (MPE)
report on behalf of the Fermi GBM Team:

"At 09:46:27.48 UT on 06 February 2015, the Fermi Gamma-Ray Burst Monitor
triggered and located GRB 150206B (trigger 444908790 / 150206407).

The on-ground calculated location, using the GBM trigger
data, is RA = 220.6, DEC = +57.5 (J2000 degrees,
equivalent to 14 h 42 m, 57 d 30 '), with an uncertainty
of 6.8 degrees (radius, 1-sigma containment,
statistical only; there is additionally a systematic
error which is currently estimated to be 2 to 3 degrees).
The angle from the Fermi LAT boresight is 76 degrees.

Early follow-up observations of the GBM location error box by the
intermediate Palomar Transient Factory yielded three possible optical
counterparts (Singer et al. 2015, GCN 17415
Loading...
 
 ).

The GBM light curve consists of a single main peak
with a duration (T90) of about 5 s (50-300 keV).
The time-averaged spectrum from T0-1.024 s to T0+4.096 s is
best fit by a power law function with an exponential
high-energy cutoff.  The power law index is -0.50 +/- 0.15 and
the cutoff energy, parameterized as Epeak, is 72 +/- 4 keV.

The event fluence (10-1000 keV) in this time interval is
(1.1 +/- 0.05)E-06 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+0.576 s in the 10-1000 keV band
is 5.2 +/- 0.3 ph/s/cm^2.

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