The Burst and Transient Source Experiment (BATSE) on board the Compton Gamma Ray Observatory (CGRO) monitored the entire sky from 1991-2000. I will review highlights of BATSE observations including gamma ray bursts, black hole candidates, accreting pulsars, and active galaxies. On 2008 June 11, the Fermi Gamma Ray Space Telescope was launched. The Gamma ray Burst Monitor (GBM) on board Fermi continues the all-sky monitoring legacy started with BATSE. I will review early results and planned observations with GBM

Wilson-Hodge, Colleen A.

NASA Technical Reports Server

All Sky Observations with BATSE and GBM
Colleen A. Wilson-Hodge1 for the GBM and BATSE teams
1 NASA/MSFC, Huntsville, AL 35812, USA
E-mail(CAWH): Colleen.Wilson@nasa.gov
Abstract
The Burst and Transient Source Experiment (BATSE) on board the Compton Gamma Ray Observatory
(CGRO) monitored the entire sky from 1991-2000. I will review highlights of BATSE observations including
gamma ray bursts, black hole candidates, accreting pulsars, and active galaxies. On 2008 June 11, the
Fermi Gamma Ray Space Telescope was launched. The Gamma ray Burst Monitor (GBM) on board
Fermi continues the all-sky monitoring legacy started with BATSE. I will review early results and planned
observations with GBM.
Key words: Fermi; GBM; Gamma Ray Bursts; X-ray binaries; Pulsars; Accretion; Earth occultation
1. Instrument Descriptions
1.1. BATSE
BATSE consisted of 8 detector modules positioned on
the corners of the CGRO spacecraft. Each module con-
sisted of two detectors, a large area detector (LAD) and
a spectrocopy detector (SD), both of which were sodium
idodide (NaI) scintillation crystals. Each LAD had a ge-
ometric area of 2025 cm2, a thickness of 1.27 cm, and
was sensitive from about 25 keV to 1.8 MeV. Each SD
had a geometric area of 126 cm2, a thickness of 7.62 cm,
and depending on the gain settings, was sensitive from 8
keV - 10 MeV. The BATSE detectors viewed the entire
unocculted sky at all times (Paciesas et al 1999).
1.2. GBM
GBM consists of 14 detector modules, 12 low-energy (8
keV-1 MeV) NaI detectors, and two high energy (150
keV- 30 MeV) bismuth germanate (BGO) detectors.
Each NaI detector has a geometric area of 126 cm2 and
a thickness of 1.27 cm, while each BGO detector has a
geometric area of 126 cm2 and a thickness of 12.7 cm.
The NaI detectors are arranged on the Fermi spacecraft
to produce a 9 steradian field of view. Two detectors
are positioned opposite each other looking 20 deg from
the spacecraft zenith. Four detectors are evenly spaced
around the spacecraft looking 45 deg from the spacecraft
zenith. The remaining six detectors are evenly spaced
around the spacecraft and are looking 90 deg from the
spacecraft zenith. BGO detectors are positioned on the
+X and -X sides of the spacecraft (Meegan et al 2007).
+90
-90
-180+180
2704 BATSE Gamma-Ray Bursts
Fig. 1. The BATSE sky map, showing 2704 triggered gamma ray
bursts over the 9 year mission, clearly illustrating the isotropy of
gamma ray bursts.
2. Gamma Ray Burst Observations
2.1. BATSE
Before CGRO was launched, gamma ray bursts (GRBs)
were believed to be galactic by many in the astronom-
ical community. BATSE showed that these bursts had
an isotropic angular distribution (Fig. 1), while the in-
tensity distribution showed fewer weak bursts than ex-
pected from a homogeneous distribution of sources in
Euclidean space. Because no observed galactic compo-
nent had these spatial properties, BATSE provided the
first clear indication that gamma ray bursts were of cos-
mological origin (Meegan et al. 1992).
BATSE triggered when it observed a change in count
rate above a specified threshold (typically 5.5 σ) usu-
ally in the 50-300 keV band in either 64 ms, 256 ms,
or 1.024 s timescales. In triggered mode, high rate data
were accumulated for 573.4 s. During its 9 year mis-
sion, BATSE triggered a total of 8021 times. These
https://ntrs.nasa.gov/search.jsp?R=20110004375 2019-08-30T14:21:51+00:00Z
Fig. 2. The TRIGDAT light curve for a GRB observed with GBM on
2008 August 6 at 21:29:40 UT shows the combined count rates
in all energy channels from GBM NaI detectors 1 and 2.
triggers included: 2704 GRBs, 1192 solar flares, 185
soft gamma repeaters, 2003 transient sources, 78 terres-
trial gamma flashes, 1717 magnetospheric events, and
142 triggers that were phosphorescence spikes, acciden-
tal triggers, unknown events, or undetermined due to
insufficient data (Paciesas et al. 1999, Briggs et al. in
preparation).
2.2. GBM
GBM together with the Large Area Telescope (LAT) on-
board Fermi will enable GRB observations over up to
seven decades in energy. GBM continues the BATSE
legacy and ties bursts observed with Fermi back to the
well studied BATSE sample. GBM has 128 possible trig-
ger algorithms, of which 62 are currently used. These
algorithms contain combinations of energy bands and
timescales from 16 ms - 4 s. Each trigger algorithm has
an independently adjustable trigger threshold defined in
terms of significance above background. GBM triggers
when it detects a change in count rate above a speci-
fied threshold (typically 4.5-5.5 σ) in at least two NaI
detectors. Thereafter, it immediately alerts the TDRS
satellites to send information about the trigger, called
TRIGDAT, to the ground to facilitate observations with
other observatories through GCN notices. This infor-
mation includes the triggered detectors, triggered algo-
rithm, on-board sky location, the on-board classification
(e.g. GRB, Solar Flare, SGR, etc.), the flux, fluence, and
significance of the trigger. In addition, an immediate sig-
nal is sent to the LAT along with a repoint recommenda-
tion if the burst fits pre-defined intensity and hardness
criteria. During trigger mode, time tagged event data
are produced for 300 seconds, the 8 channel continuous
timing (CTIME) data are sped up from nominal 256 ms
to 64 ms resolution, the 128 channel continuous spectra
(CSPEC) are sped up from nominal 4.096 s to 1.024 s,
Fig. 3. From Paciesas et al. (1996). Single step Earth occultation
measurements of GRS 1915+105 with BATSE in the 20-100 keV
band during early May 1992. The high points on the plot are
bright flares from GRS 1915+105 detected as large Earth occul-
tation steps. For comparison, the flux from the Crab Nebula is
∼ 0.3 ph cm−2s−1 in this band
and a ring buffer of 500,000 pre-trigger events is trans-
mitted (Meegan et al 2007). An example TRIGDAT
light curve from a burst observed with GBM in 2008
August is shown in Figure 2.
From 2008 July 13 (when triggers were enabled) until
2008 October 6, GBM triggered a total of 129 times. Of
these triggers, 63 are identified as Gamma Ray Bursts,
33 as soft gamma repeaters or anomalous X-ray pulsars,
four galactic X-ray binaries, and four terrestrial gamma
flashes. The remaining 25 triggers are either too weak
to identify, due to background fluctuations, or magneto-
spheric events.
3. Earth Occultation
The intensity of a galactic or extra-galactic source can
be measured by calculating the difference in observed
count rate before and after the source is occulted by the
Earth. This measurement is most easily made using a
small window of data surrounding the calculated time
when the source of interest enters or exits Earth occul-
tation. Projections of the Earth’s limb on the sky at the
time of these occultation “steps” can be used to locate
new astrophysical sources. This technique is required for
BATSE and GBM because neither instrument has direct
imaging capabilities.
3.1. BATSE
With BATSE, we made extensive use of the Earth occul-
tation technique. We monitored a catalog of 179 sources
including primarily X-ray binaries, plus representative
samples of active galaxies, X-ray emitting stars, and su-
pernova remnants. From this catalog, we detected 74
X-ray binaries, three supernova remnants, and six active
Fig. 4. From Paciesas et al. (1996). A flaring episode from GRS
1915+105 on 1992 May 15. Shown are background subtracted
BATSE count rates with 8.192 s resolution in the 20-100 keV
band summed over two detectors. GR, GS and CR, CS denote
GRS 1915+105 and Cyg X-1 rise and set times, respectively. In
this band, the Crab Nebula count rate is ∼ 300 cts s−1.
galactic nuclei at 10 σ or better either over the entire
9 year mission or in at least one identifiable outburst.
Our limiting sensitivity was 3.5-20 mCrab, due to sky
dependent systematic errors, over the 20-430 keV band
(Harmon et al. 2004).
Although this technique is less sensitive than imag-
ing instruments, it has the advantage of monitoring the
entire sky all of the time. This allowed us to observe
unpredictable events such as black hole state changes in
Cyg X-1, 1E 1740–29, GX 339-4, and Cyg X-3 (Har-
mon et al. 2004 and references therein) and to find
pre-discovery emission in archival data. We discovered
extremely bright flares, shown in Figures 3 and 4, from
GRS 1915+105 from data prior to its discovery (Paciesas
et al. 1996). In addition, BATSE data were very useful
for studying long-term trends in source behaviors and
were important components in several multiwavelength
campaigns (e.g. Cyg X-3, McCollough et al 1999). With
the BATSE Earth occultation technique, we discovered
three new black hole transient systems, GRO J0422+32,
GRO J1655–40, and GRO J1719–24=GRS 1716–249.
3.2. GBM
We have a guest investigation program (P.I. Wilson-
Hodge) in place to implement the Earth occultation tech-
nique with GBM. Implementation of the technique is in
progress. For GBM, the technique is complicated by the
fact that Fermi is scanning the sky, rather than being
intertially pointed like CGRO. This means the detector
response is changing and must be taken into account.
Figure 5 shows a rising Earth occultation step from Sco
X-1 in GBM NaI #2. Figure 6 shows the estimated
GBM sensitivity taking into account the expected back-
grounds, which are consistent with observed rates. GBM
Fig. 5. An example Earth occultation step observed with GBM on
2008 September 21 from Sco X-1 in the 10-25 keV band. The
diamonds denote the raw CTIME count rates measured with GBM
plotted at 2 s resolution. The solid line denotes the fitting model
which consists of a quadratic background plus an atmospheric
transmission function convolved with predicted count rates for Sco
X-1. The predicted count rates result from folding an assumed
spectral model through the changing detector response function
as the spacecraft moves.
Fig. 6. The estimated 1-day Earth occultation sensitivity limits for
two GBM NaI detectors and one GBM BGO detector for a source
at moderate viewing angles. The BATSE sensitivity limit for
2 LADs is shown for comparison. Also shown are 1 Crab and
100mCrab spectra, as well as the spectrum of the accreting X-ray
pulsar EXO 2030+375.
Fig. 7. (top): Barycentered, orbit corrected spin-frequency for EXO
2030+375 at each periastron passage where BATSE detected pul-
sations. (bottom): Corresponding 20-50 keV pulsed flux for these
observations measured with BATSE.
has considerably better sensitivity from about 8-25 keV
than BATSE because the GBMNaI detectors have only a
Be window instead of the plastic scintillator, aluminum
honeycomb, and aluminum window located in front of
the BATSE detectors. The 1-day sensitivity limit for
the Earth occultation technique with GBM assuming a
source is viewed by two NaI detectors is about 90 mCrab
below 100 keV (Case et al. 2007). The Japanese Monitor
of All-Sky X-ray Image (MAXI), described in detail else-
where in these proceedings and scheduled to be attached
to the space station in 2009, has much better sensitivity,
but only extends up to 30 keV. Earth occultation moni-
toring with GBM will provide the only all-sky capability
above about 50 keV. Hence GBM will provide impor-
tant complementary data for bright events observed with
MAXI.
3.3. Accreting Pulsar Observations
Accreting X-ray pulsars consist of a magnetized neutron
star (B ∼ 1012 G) orbiting a companion. Accretion in
these systems occurs via either Roche lobe overflow, a
stellar wind, or interaction with the circumstellar disk of
a Be star near periastron passage. Recently, INTEGRAL
has discovered a number of likely highly absorbed wind
accreting X-ray pulsars (Walter et al. 2006).
3.4. BATSE
A large number of accreting pulsars were monitored
daily with BATSE. For example, Figure 7 shows the
spin-frequency history and the pulsed flux measured for
the accreting Be/X-ray pulsar EXO 2030+375. Each
point corresponds to a periastron passage in the system,
when the source was typically detectable for 4-12 days.
Monitoring of this particular pulsar led to the discovery
that the infrared magnitudes showed a decline shortly
before the pulsar began to spin down and the pulsed
Fig. 8. An example of an accreting X-ray pulsar in the GBM data.
This Figure shows 283-s pulses from Vela X-1 in the 12-52 keV
energy band for GBM NaI 4.
flux abruptly dropped (Wilson et al. 2002). This im-
plied that the density of the disk around the companion
star declined, making less material available for accre-
tion onto the pulsar. Later, after CGRO deorbited, the
infrared magnitudes again brightened accompanied by a
brightening of the X-ray outbursts and a transition to
spin-up observed with the Rossi X-ray Timing Explorer
(Wilson et al 2005).
The techniques used to detect known accreting pulsars
with BATSE are described in detail elsewhere (Bildsten
et al. 1997, Finger et al. 1999). The basic idea is that the
data, when the pulsar of interest was above the horizon,
were fitted with a model consisting of a background plus
a Fourier expansion in the pulse phase model which used
an approximately known spin-frequency. This model was
used to generate pulse profiles for short segments of data,
short enough that the loss of power due to phase drift
was small. These short segment profiles were then shifted
using a grid in pulse frequency and sometimes frequency
derivative. The shifted profiles were combined and the
best frequency (and derivative) was selected from the
grid using a modified Z2
n
statistic. In addition to this
technique, daily fast Fourier transforms of the BATSE
data were performed to look for new periodicities in
the data. Six new accreting pulsars were discovered
with BATSE: GRO J1008–57, GRO J1948+32, GRO
J1744–29, GRO J2058+42, GRO J1750–27, and GRO
J1944+26 = XTE J1946+274.
3.5. GBM
We have a guest investigation program in place to moni-
tor accreting pulsars using GBM (PI Finger). We expect
that GBM should be able to detect most of the X-ray
pulsars observed with BATSE, despite GBMs smaller
area. This is because of the fortunate combination of
a large number of photons in the 8-25 keV range from
Fig. 9. Estimated GBM 3-σ root-mean squared (rms) pulsed flux
sensitivity to accreting X-ray pulsars for 50 ks on-source. For
comparison, we show BATSE’s sensitivity and representative rms
pulsed flux spectra for the accreting X-ray pulsars Vela X-1, EXO
2030+375, A 0535+262, and Cen X-3. One Crab and 20 mCrab
flux units are also shown for reference.
most accreting pulsars combined with GBMs improved
low-energy response over BATSE. For accreting pulsars,
GBM will provide daily monitoring of spin-frequencies,
while MAXI will provide precise flux measurements. To-
gether these results will be excellent for studying cor-
relations between pulsar spin-up rates and total flux.
Figure 8 shows pulses from the 283-s wind-fed accret-
ing X-ray pulsar, Vela X-1, that are visible in the raw
data.
Figure 9 shows the result of our sensitivity calculation
in terms of rms pulsed flux. Details of the calculation
are given in Wilson-Hodge et al. (2007). For reference,
we have also done the same calculation for the BATSE
LADs using actual measured background rates and cor-
responding detector response matrices for two detectors.
Typical rms pulsed flux spectra of 4 accreting pulsars
are also shown that were detectable with BATSE and
are expected to be detectable with GBM.
4. Conclusions
BATSE observed 2704 triggered GRBs and provided
the first clear evidence that GRBs were cosmological.
With BATSE, we monitored the sky using Earth oc-
cultation measurements in the 20-1800 keV band, pro-
viding important contributions to multiwavelength stud-
ies, long-term studies, and observations of unpredictable
events such as state changes in black holes. We pro-
vided the only continuous monitoring of accreting pulsar
spin-frequencies, including transitions from spin-up to
spin-down. Most BATSE data are still available through
NASA’s High Energy Astrophysics Science Archive Re-
search Center (http://heasarc.gsfc.nasa.gov) for archival
studies.
Using GBM we will continue all-sky monitoring obser-
vations. It is still very early in the mission, but results
so far look exciting. GBMs most important contribu-
tions will most likely be for GRBs where, combined with
the LAT, we will provide the broadest spectral coverage
ever achieved. Plus we will complement GRB observa-
tions from other instruments, such as Swift, providing
higher energy coverage with our BGO detectors. Today,
other all-sky monitors are also available such as the Burst
Alert Telescope on-board Swift (15-150 keV, Barthemy
et al. 2005) and in 2009, MAXI (0.5-30 keV). GBM
data will complement these other monitors, providing
additional time coverage in the 8-50 keV band, the only
high energy source monitoring above about 50 keV, and
the only near-continuous monitoring of accreting pulsar
spin-frequencies.
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All Sky Observations with BATSE and GBM

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