Acquisition and Reduction of BATSE Data
2.3 Detector Response
• Background data. These consist of the background DISCLA and CONT products, which are available continuously. The 4-channel LAD discriminator rates (DISCLA data) are sampled every 1.024 s. The 128-channel high energy resolution (HER) spectra are mapped into medium-resolution 16-channel spectra every 2.048 s, called continuous (CONT) data. The mapping of HER channels to CONT channels is programmable, and is occasionally changed temporarily to optimize the tradeoff between energy and time resolutions for a particular science investigation. The typical energy channel boundaries for the DISCLA and CONT data are given in Table 2.1.
• Housekeeping data. The HKG product contains information on the spacecraft ori- entation as well as its geocentric position at 2.048 s intervals. These positions are accurate to '"" 6 km (3o"). The QUAL product contains diagnostic information on data quality, identifying intervals which should be excluded from data analysis due to telemetry errors, spacecraft reorientations, etc. We also use the QUAL information to exclude intervals containing gamma-ray bursts from pulsar timing analysis.
• Scheduled data. There are several data products which can be specially scheduled and provide high time or energy resolution. Several pulsar modes are available which can provide time resolution as short 16 ms for limited observation intervals.
• Burst trigger data. A variety of special data products are activated with high time and energy resolution when triggered by on-board identification of a gamma-ray burst in the BATSE LAD data stream. These data products are not relevant for pulsar studies.
,.----...
N
E
u'-..__./
0
Q) L
0
Q)
>
10000
1000
u
Q)100
4 - 4 -
w
BATSE LAD, normal incidence
- -
--
/
'
total response' ' ' ' '
' ' '
' full energy
'
depositionI I
' ' '
' ' ' ' ' '
' '
'
10~~~~~~~~~~~~~~~~~~
1 0 100 1000 10000
Energy (keV)
Figure 2.3: Effective area of a BATSE large area detector at normal incidence. The solid curve denotes the total response of the detector, including interactions where the incident photon energy is only partially deposited. The dashed curve denotes the response for full energy deposition in the detector. The feature near 30 keV is due to the iodine K edge.
Adapted from Fishman et al. (1989).
al. (1995). The dominant effect governing this response is the projected detector area along the line of sight to the source, which varies as cos(} (where (} is the viewing angle between the detector normal and the source). However, while the response to 100 keV photons is approximately cos(}, the response at energies both above and below 100 ke V diverge from this for different reasons. At higher energies, the photon attenuation length in Nal is comparable to the thickness of the crystal, so any interactions tend to occur deep in the crystal. The decrease in projected geometric area with increasing() is partially offset by the increase in path length through the detector, both of which have a cos(} dependence. This results in a relatively fl.at angular response for (} ~ 50° at high energies. The response does not fall to zero at 90° incidence due to the finite thickness of the crystal.
0.8
Q) (f)
c
0
Q_
0.6
(f) Q) L
Q)
>
0.4
+--'
0
Q)
0::::
0.2
0 15 30 45 60 75 90
Viewing angle 8 (degrees)
Figure 2.4: Angular response of a BATSE large area detector to photons of various energies.
For comparison, a cos() response is indicated by the dotted curve. The secondary maximum in the 20 keV response at large angles is due to a gap in the detector support structure at the edge of the LADs. Computed using the detector response matrices of Pendleton et al.
(1995).
At low energies, the attenuation length is very short and the interactions occur near the surface of the crystal, making its effective thickness irrelevant. However, the path length through the shielding in this case increases with(), and the resulting attenuation of flux incident on the LAD causes the response to fall more steeply than cos 0. The secondary maximum in the large-angle response for low energies is due to the geometry of the support structure at the edge of the LADs. There is a gap through which low energy photons in that restricted angular range can reach the Nal crystal without passing through most of the shielding. This feature is poorly calibrated and may not be azimuthally symmetric.
A consequence of the variation of angular response with energy is that the inte- grated angular response to a source is dependent upon its intrinsic energy spectrum. Typical
1 .0
Incident spectrum
0.8 ' d N / d E "' E-r
Q)
' '
(/)
c
0
Q_
0.6
(/) Q) L
Q) >
0.4
oo 15'-+-' C9
0 ..2-~
Q) ~
0:::::
0.2
..2-~"s '
'0 15 30 45 60 75 90
Viewing angle 8 (degrees)
Figure 2.5: BATSE LAD angular response to an incident photon power-law spectrum dN / dE ex: E-', for 'Y = 2 and "'( = 5. For comparison, cos() response (dotted line) and cos2 () response (dashed line) are also shown.
accreting pulsar spectra in the 20-100 ke V range can be modeled by a photon power-law of the form dN / dE ex E-1 with photon index 2
<
'Y<
5. Figure 2.5 shows the angular response to a 20-75 ke V photon power law spectrum for the extreme cases of 'Y=
2 and 'Y=
5. For comparison, cos() and cos2 ()responses are also plotted. We see that the predicted response falls off more quickly than cos() because the large number of incident low-energy photons dominate despite the attenuation by the shield. As expected, this effect is more pronounced for the steeper power-law index, where the proportion of incident high-energy photons is even lower. By comparison, Brock et al. (1991) predict an approximately cos() response for gamma-ray bursts, which they model as having a 50-300 keV photon power law with a low-energy cutoff. In general, the integrated response varies approximately as cos() for small angles (0;:; 25°), independent of photon index. At larger angles, cos2 () is amore conservative general assumption if the source spectrum is unknown.