Detecting high-z GRBs is important for constraining the GRB formation rate,
and tracing the history of re-ionization and metallicity of the universe. Based
on the current sample of GRBs detected by Swift with known redshifts, we
investigated the relationship between red-shift, and spectral and temporal
characteristics, using the BAT event-by-event data. We found red-shift trends
for the peak-flux-normalized temporal width T90, the light curve variance, the
peak flux, and the photon index in simple power-law fit to the BAT event data.
We have constructed criteria for screening GRBs with high red-shifts. This will
enable us to provide a much faster alert to the GRB community of possible
high-z bursts.Comment: 4 pages, 4 figures, to be published in the proceedings of ''Gamma Ray
  Bursts 2007'', Santa Fe, New Mexico, November 5-

David Palmer

Ed Fenimore

K. S. Dhuga

M. Galassi

M. Stamatikos

N. Gehrels

T. N. Ukwatta

T. Sakamoto

English

arXiv

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Screening High-z GRBs with BAT Prompt
Emission Properties
T. N. Ukwatta∗,†, T. Sakamoto†,∗∗, M. Stamatikos†,‡, N. Gehrels† and K. S.
Dhuga∗
∗The George Washington University, Washington, D.C. 20052
†NASA Goddard Space Flight Center, Greenbelt, MD 20771
∗∗The University of Maryland, Baltimore County, Baltimore, MD 21250
‡Oak Ridge Associated Universities, P.O. Box 117, Oak Ridge, Tennessee 37831-0117
Abstract. Detecting high-z GRBs is important for constraining the GRBformation rate, and tracing
the history of re-ionization and metallicity of the univers. Based on the current sample of GRBs
detected by Swift with known redshifts, we investigated therelationship between red-shift, and
spectral and temporal characteristics, using the BAT event-by-event data. We found red-shift trends
for the peak-flux-normalized temporal width T90, the light curve variance, the peak flux, and
the photon index in simple power-law fit to the BAT event data.We have constructed criteria for
screening GRBs with high red-shifts. This will enable us to pr vide a much faster alert to the GRB
community of possible high-z bursts.
Keywords: Gamma-ray Bursts, High-z GRBs
PACS: 98.70.Rz, 98.62.Ai
INTRODUCTION
High-z GRBs offer the potential to probe the early universe into the epoch of re-
ionization. In Big Bang cosmology, the universe went through a "dark age" when matter
had cooled below 3000 Kelvin and became neutral. Re-ionization of matter occurred
once bodies such as quasars started radiating, initiating the ionization of neutral matter,
producing an ionized plasma. Bodies we see now with a red-shift 20> z> 6 (150 million
to one billion years after the Big Bang) [1] emitted their radiation in this re-ionization
period. (The highest-z GRB detected thus far is z=6.29 [2].)
High-z GRBs also have the potential to trace the star formation rate and metallicity
histories of the universe [1, 3]. Moreover, GRBs are 100-1000 times brighter at early
times than are high red-shift quasars. GRBs are expected to occur out to z> 10,
whereas the distribution of quasars drops significantly beyond z=3. Another benefit is
that due to the relatively clean neighborhoods of GRB progenitors, GRB afterglow have
simple power-law spectra with no emission lines. Thus GRBs are clean probes of the
intergalactic medium (IGM), whereas quasars are contaminated by continuous material
emission from the central engine to the quasar’s neighborhood. High-z GRB studies will
probe the IGM less than 1 Gyr after the Big Bang. In this respect, the study of high-z
GRBs provide a unique method to probe the early universe.
Clearly, detecting high-z GRBs is very important for probing the early universe.
Currently, a significant number of GRBs with redshifts around z= 2 have been observed,
but only a handful of events for redshifts greater thanz = 5. To help identify new GRBs
Screening High-z GRBs with BAT Prompt Emission Properties November 28, 2018 1
with high-z, we have developed an early-stage observational filter on bursts, so that
follow-up ground based observatories can be notified, and they can subsequently attempt
to get optical red-shift measurements for these candidate high-z objects.
SCREENING HIGH-Z BURSTS USING BAT DATA
Our observational filter for candidate high-z GRBs is a set ofscreening criteria, taking
advantage of some prompt-emission properties of GRBs whichare analyzable within a
few hours after the burst.
We have found trends between some observational characteristics n the prompt
emission from observed GRBs and their corresponding redshifts (z):
1. The temporal period T90, normalized by the peak photon flux(see Fig. 1);
2. The normalized light curve variance (using T100 duration) (see Fig. 2);
3. The peak photon flux (Fig. 3);
4. The photon index derived from the simple power law fit to theburst spectrum
(Fig. 4).
Each characteristic is not sufficiently discriminatory to screen for high-z GRBs by
their own. However, the combined cuts on these properties enabl us to filter out high-z
bursts with significant probability.
FIGURE 1. T90 normalized by the peak photon flux cut.
DISCUSSION
Using trends in pulse width, variance, peak flux, and photon pwer law, we have for-
mulated automated screening criteria to identify possiblehigh-z GRBs by utilizing the
prompt emission properties of GRBs within a few hours after th burst. This will en-
able more intense ground-based observational campaigns for measuring red-shifts of the
detected GRBs.
Screening High-z GRBs with BAT Prompt Emission Properties November 28, 2018 2
With the limited available sample of GRBs having known high-z, the probability of a
GRB being selected by our criteria with a redshift≥ 5.0 is 45 %, with a redshift≥ 4.0 is
67 % and with a redshift≥ 3.5 is 89 % (Table 1). Based on an analysis of Swift existing
burst data, the criteria we used will generate an alert once amonth on average.
Currently, our screening criteria are implemented in automated scripts and running in
real time. Our filter for high-z has already generated alertson GRB 071021 (GCN 6967),
GRB 071118 (GCN 7109) and GRB 071129 (GCN 7139), which enhanced ground based
follow-up observations.
We are working on implementing these screening criteria to BAT TDRSS data. We
also note that there is an ongoing effort to formulate high-zscreening criteria to include
the XRT and UVOT datasets.
FIGURE 2. Normalized light curve variance cut.
FIGURE 3. BAT peak photon flux cut.
Screening High-z GRBs with BAT Prompt Emission Properties November 28, 2018 3
TABLE 1. Selected GRBs using the high-z criteria
from Swift GRBs with known red-shifts.
Gamma-ray Burst Name Measured Red-shift
GRB 050730 3.967
GRB 050814 5.3
GRB 050904 6.29
GRB 060115 3.53
GRB 060510B 4.90
GRB 060522 5.11
GRB 060605 3.8
GRB 061110B 3.44
GRB 070110 2.352
FIGURE 4. Photon Index cut.
REFERENCES
1. D. Q. Lamb, and D. E. Reichart, ApJ, 2000, 536, 1
2. G. Tagliaferri et al., A&A, 2005, 443, L1
3. M. Gilfanov, R. Sunyeav, and E. Churazov, Proc. of the MPA/ESO/MPE/USM Joint Astronomy
Conference, 2002, 157
Screening High-z GRBs with BAT Prompt Emission Properties November 28, 2018 4
