GRB 070714B - Discovery of the Highest Spectroscopically Confirmed Short Burst Redshift
ABSTRACT Gemini Nod & Shuffle spectroscopy on the host of the short GRB 070714B shows a single emission line at 7167 angstroms which, based on a grizJHK photometric redshift, we conclude is the 3727 angstrom [O II] line. This places the host at a redshift of z=.923 exceeding the previous record for the highest spectroscopically confirmed short burst redshift of z=.546 held by GRB 051221. This dramatically moves back the time at which we know short bursts were being formed, and suggests that the present evidence for an old progenitor population may be observationally biased. Comment: Conference procedings for Gamma Ray Bursts 2007 November 5-9, 2007 Santa Fe, New Mexico (4 pages, 2 figures)
GRB 070714B - Discovery of the Highest
Spectroscopically Confirmed Short Burst
J. F. Graham∗,†, A. S. Fruchter∗, A. J. Levan∗∗, M. Nysewander∗, N. R.
Tanvir‡, T. Dahlen∗, D. Bersier§and A. Pe’er∗
∗Space Science Telescope Institute
†Johns Hopkins University
∗∗University of Warwick
‡University of Leicester
§Liverpool John Moores University
Abstract. Gemini Nod & Shuffle spectroscopy on the host of the short GRB 070714B shows a
single emission line at 7167 Å which, based on a grizJHK photometric redshift, we conclude is the
3727 Å [O II] line. This places the host at a redshift of z=.923 exceeding the previous record for
the highest spectroscopically confirmed short burst redshift of z=.546 held by GRB 051221. This
dramatically moves back the time at which we know short bursts were being formed, and suggests
that the present evidence for an old progenitor population may be observationally biased.
Keywords: Gamma-Ray Bursts, Short Gamma-Ray Bursts, Gamma-Ray Burst Redshifts, Gamma-
Ray Burst Host Galaxies
GRB 070714B was initially detected by the Burst Alert Telescope (BAT) on the
NASA SWIFT spacecraft. The gamma ray emission consisted of several short spikes
with a collective duration of 3 seconds followed approximately twenty seconds later by
fifty seconds of softer emission. The main component also shows a small spectral lag
(Norris et al. GCN 6631 ). This emission is similar to previous short bursts including
GRB 050724 and places this burst securely in the short category.
Rapid follow observations with the 2m Liverpool Telescope and the 4m William Her-
schel Telescope found a fading optical transient within the XRT error circle. Subsequent
observations detected a host galaxy at the location of the optical transient.
Here we report on photometry and spectroscopy of the host galaxy, which lead us to
conclude that the host is a moderately star-forming galaxy at a redshift of z=0.923. The
spectroscopy and redshift discussed here was originally reported in GCN 6836 (Graham
et al. 2007 ).
arXiv:0802.1346v1 [astro-ph] 10 Feb 2008
Host Galaxy Imaging
Optical imaging was obtained in g, r, i, and z bands with the GMOS instrument on the
Gemini North Telescope. Due to a narrow widow between the object rising and dawn,
observations were preformed in non-photometric conditions shortly before and often
extending slightly past astronomical twilight, at airmasses just below and sometimes
slightly above 2, and had to be spread across several nights. Photometric calibration
observations were subsequently preformed via observations in photometric conditions
of the object field and selected area 95 of Landolt (1992) . Science and calibration
observations were scaled from field star fluxes. Absoulte flux was determined from
SDSS magnitudes of the Landolt stars. Magnitudes are listed in table 1.
North on the nights of July 24th, 25thand 26th, 2007. Single exposures were taken
totaling one minute each in coadds of 5 x 12s, 6 x 10s and 5 x 12s, in J, H and K
respectively. The images were taken under photometric conditions and low airmass.
mag ± error
25.79 ± .34
24.90 ± .21
23.97 ± .12
24.01 ± .13
22.27 ± .12
22.28 ± .20
21.13 ± .13
Photometric magnitudes of the host galaxy in each observed band.
Host Galaxy Spectroscopy
Initial spectroscopic observations were obtained on the night of the July 25th. Due
to the drop in detector sensitivity long-ward of 9250 Å a central wavelength of 7250
Å was selected yielding a spectral range of 5250 to 9250 Å. Further spectroscopy was
conducted on the night of September 13thwith the central wavelength shifted out to
7750 Å. The R400 grating offerers a reasonable compromise between spectral resolution
and width of coverage and was used both nights. Due to the abundance of skylines in
the spectral range the Nod & Shuffle method was used. The first and second night of
spectroscopy consisted of four and six 10-minute Nod & Shuffle exposures respectively.
The individual spectroscopic exposures were reduced using the standard Nod &
Shuffle process. This resulted in eight and twelve 300 second images for the 7250 and
7750 Å central wavelengths respectively. To optimize cosmic ray rejection the positive
and negative (once inverted) images of each central wavelength were combined in a
time. Spectral extraction was preformed with IRAF task appall using a 10 pixel wide
aperture in the spatial direction. The continuum was too weak to allow automatic tracing
of the aperture center. However a tracing of a bright star also present in the slit showed
insignificant variation in the spatial location of the continuum along the spectra negating
the need for automatic tracing. A fixed center aperture was thus used.
This yielded a spectrum with a spectral resolution of 1.37 Å per pixel and a spectral
resolution of 0.15 arc seconds per pixel. A single spectral line was observed at 7167
Å with a measured equivalent width of −42.89 ± 4.09 and flux of 2.0 ± .3 ∗ 10−17
erg/s/cm2. Aside from a faint continuum no other spectral features were detected in
the 5150 to 9900 Å spectral range.
Spectroscopic Line Determination
The optical and IR photometry also allows us to constrain the 7167 Å line identity
via a photometric redshift determination. There are three reasonable candidates for the
observed spectral line at 7167 Å; the 3727 Å [O II] line placing the object at a redshift
of .92, the 5007 Å [O III] line placing the object at a redshift of .43, and the 6563 Hα
line placing the object at a redshift of .09.
These possibilities along with the best-fitting photometric redshift (and their respec-
tive best-fitting spectral type for the host galaxy) are calculated using the template-fitting
method. In this method, the observed photometry is matched to synthetic photometry de-
rived using the filter through-puts for the instrument used and a set of library templates
redshifted in the range 0<z<6. The spectral templates used here consists of the E, Sbc,
Scd, and Im templates from Coleman et al. (1980)  , together with two starburst tem-
plates from Kinney et al. (1996) . The calculated a photometric redshift probability
distribution for the host galaxy is show in figure 1.
The detected line is by a factor of ten most likely the 3727 Å [O II] line making the
photometric redshift consistent with a GRB host galaxy at z=0.923. The best fit spectral
energy distribution (-19.6 V magnitude Scd galaxy) with the measured photometry over
plotted is shown in figure 2.
Photometric redshift relative probability distribution with the various line possibilities
the measured photometric values over plotted.
The Spectral Energy Distribution of the z=.923 fit (a -19.6 V magnitude Scd galaxy) with
Spectroscopy of the host of GRB 070714B reveals a single line at 7167 Å. Our
Å [O II] line at a redshift of z=0.923. This nearly doubles the highest spectroscopically
redshift of a short burst with a sub-arsecond position. A number of bursts have shown
no obvious host, thus suggesting that their hosts might be distant and faint (Berger 2007
) Additionally, other authors have determined the redshifts of galaxies in XRT error
circles, and some of these galaxies have had redshifts also around z ∼ 1 .
However, we note that a fairly small XRT error circle (for a short burst) with radius of
5" would have a greater than 60% chance of containing a random galaxy as bright as the
host of GRB 070714B. In cases such as this one where the burst has an optical afterglow,
the host, and thus the redshift, can be determined with very little chance of confusion.
This observation thus moves back the time at which we know short bursts were being
formed to the first half of the age of the universe, and suggests that the present evidence
for an old progenitor population may be, at least partially, observationally biased.
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