The eight uncollimated BATSE Large Area Detectors (LAD's) provide the ability to monitor pulsed hard x ray sources on a nearly continuous basis. Using data from the LAD's, the pulse timing and pulsed flux of the 4.8 second period binary x ray pulsar Centaurus X-3 was analyzed over a two month period. The methods and initial results of this analysis, which includes both data folded onboard GRO and 1.024 second resolution discriminator rates folded on the ground, are presented

Finger, Mark H.

Fishman, Gerald J.

Meegan, Charles A.

Paciesas, William S.

Wilson, Robert B.

English

NASA Technical Reports Server

N92-2i896
Monitoring Cen X-3 with BATSE
Mark H. Finger, 1 Robert B. Wilson, 2
Charles A. Meegan 2, William S. Paciesas, 3 and Gerald J. Fishman
Abstract
The eight uncollimated BATSE Large Area Detectors (LAD's) provide the
ability to monitor pulsed hard x-ray sources on a nearly continuous ba-
sis. Using data from the LAD's, the pulse timing and pulsed flux of the
4.8 second period binary x-ray pulsar Centaurus X-3 has been analysed
over a two month period. The methods and initial results of this analysis,
which includes both data folded on-board GRO and 1.024 second resolution
discriminator rates folded on the ground, are presented.
1. Introduction
One of the objectives of the Compton Gamma ray Observatory's Burst and Transient
Source Experiment (BATSE) is the long-term monitoring of x-ray pulsars. As a explo-
ration of BATSE's pulsar monitoring capabilities, this paper presents the initial results
of a timing analysis of the x-ray pulsar Centaurus X-3.
Pulsations from Cen X-3 with a period of 4.8 seconds were first observed in Uhuru
data in 1971 [1]. Further analysis of the Uhuru data revealed that the source was a
member of a binary system [2]. This was demonstrated both by 2.087 day period doppler
signatures in the detected pulse period, and eclipses with the same period. Since that
time a considerable body of observations has accumulated, including observations by
Uhuru [31, Cos-B [4], SAS a [53, Heao 1 [6], Hakucho [7], Tenma [81, Ginga [9], R6ntgen
on Kvant [10], and Art-P on Granat [11]. To this body of observations BATSE promises
to add a nearly continuous data set spanning several years.
BATSE has eight uncollimated Large Area Detectors (LAD's) placed on the corners of
the Compton Gamma Ray Observatory spacecraft [12]. Cen X-3 (or any other source)
is continuously viewed by four of these detectors during the portion of each orbit that it
is unocculted by the Earth. Each LAD consists of a .5 inch thick by 20 inch diameter
NaI(T1) scintillator coupled by a light integration chamber to three 5 inch photo-multiplier
tubes. The useful energy range of the LAD's is from 20 keV to 1 MeV. The data types
from these detectors relevant to pulsar analysis are summarized in Table 1.
Astronomy Programs, Computer Sciences Corporation
2 Space Science Laboratory, NASA/Marshall Space Flight Center
3 Department of Physics and Center for Space Plasma and Aeronomic Research, University of Alabama
in Huntsville
185
https://ntrs.nasa.gov/search.jsp?R=19920012653 2020-03-17T13:16:14+00:00Z
Data Type
Folded On-Board
DISCLA
CONT
Energy
Channels
16
4
16
Time
Resolution
Period/64
1.024 see
2.048 see
Accumulation
Time per day
5000-10000 sec
50000-86400 see
50000-86400 sec
Table 1. Data from the BATSE LAD's that is useful for pulsar analysis.
The BATSE hardware can fold data on-board for periods specified in integral numbers of
microseconds. The result of this folding is 64 phase resolved count spectra each with 16
energy channels. Due to scheduling competition with other pulsars, and due to telemetry
competition with burst data, typically only 5000 to 10000 seconds of this data type is
acquired per day for Cen X-3. The DISCLA data contains the four discriminator rates
for each of the LADS with 1.024 second time resolution. The CONT data contains 16
channel count spectra for each of the LADS with 2.048 second time resolution. Useful
data of each type is available at all times the source is visiMe, except for times of SAA
passage, solar flares, and other interfering events.
In the analysis presented here we begin by studying the lowest DISCLA energy channel
(20-60 keV). This data is used to establish the history of Cen X-3's intensity and rotational
phase. High intensity portions of the folded on board data are then examined using the
phase ephemeris developed from the DISCLA data. The CONT data, which contains
additional spectral information, has not yet been analysed.
2. DISCLA Analysis
The DISCLA data used in this analysis consists of the counting rates from the eight
BATSE LADS in the 20-60 keV energy deposit range sampled every 1.024 seconds. The
data set analysed covers the period from Truncated Julian Date (TJD) 8369 to TJD 8433
(23rd April to 26th June 1991). These rates are typically between 1000 and 2000 counts
per second. In this energy range the sky is brighter than the Earth's atmosphere. This
contrast imposes a modulation at the spacecraft orbital period (5600 seconds) of 500-
1000 counts per second. Excursions from these typical rates are caused by solar flares,
gamma-ray bursts, and atmospheric electron events. And of course, buried in these rates
is the few counts per second periodic signature of Cen X-3.
The first step taken in the DISCLA processing was to filter the data and form a linear
combination of the rates from individual detectors. Data was rejected from periods
when Cen X-3 was occulted by the Earth. Data was also rejected during periods when
solar flares, gamma ray bursts, or other events caused rapid variations in the counting
rates. The rates were then summed over the detectors exposed to Cen X-3, with a
weighting for each detector given by the cosine of the angle between the detector's normal
and fhe source direction. This weighting roughly mimics the detector response, and
produces a linear combination that is fairly insensitive to reorientations of the Compton
GRO spacecraft. In this first step of processing the time-tags associated with each rate
186
were reduced to the geocenter,thereby eliminating any further need for the spacecraft
ephemeris.
The linearly combined rates werethen "pulse folded" using a technique simular to that
employed by Deeter et al. [13] in timing analysesof Vela X-1. The data were divided
into segments300 secondslong which wereindividually fit to modelsof the form:
3
R(t) = Bo+ B,t + B2t 2 + Z[Ckcos27rk¢+ Sksin27rk¢] (1)
k=l
where R is the model rate, the quadratic in time t represents the background rate, ¢ is a
provisional model of the pulsar's rotational phase, and the pulsar light curve is parame-
terized by the first three harmonics of the observed pulse shape. Due to the 1.024 second
resolution of the DISCLA rates, little additional information could be obtain by including
higher harmonics. These X _ fits to this nine parameter model used variances calculated
from the individual detector rates by assuming Poisson statistics. Data segment.s with a
X2 per degree of freedom greater than 1.4 were rejected from further analysis. For the
remaining segments sufficient information was recorded to allow the calculation of the X_
associated with the model in equation (1) for an arbitrary set of parameters.
A further step was then employed that resulted in a X 2 fit to the remaining data over
intervals of approximately 10000 seconds. The resulting fit was to a model of the form
3
R = B(t) + (2)
k=l
where B(t) is the background model and 5¢ is a correction to the model of the pulsar's
rotational phase applied above. The pulsar light curve is assumed to be constant over
each interval. For each 300 second segment within an interval the background was again
modeled as a quadratic,
B(t)= B; + B[t + B_t 2 for r'<t<r '+_ (3)
where r i are the segment boundaries. The model however was constrained to be contin-
uous in value and slope at the boundaries of adjoining segments. These interval fits were
obtained from the previous segment fits using a sequential estimation technique known
as Square-Root Information Filtering (SRIF) [14]. Only the estimates of the pulsar light
curve harmonic amplitudes and their associated errors were retained from these interval
fits.
The parameterized light curves obtained from the interval fits were then correlated with
a template to obtain a Cen X-3 pulse intensity and phase offset for each interval. The
template used is shown in Figure 1. This resulted from folding data from TJD 8369.9
to 8370.5, subtracting the mean, and normalizing to obtain a mean square value of one.
187
2.0
1.5
1.o
0.5
Eo_ 0.0
< -0.5
-1.0
! ! I " " I
• • i . • . l i | I | i i | J I * I
0.2 0.4 0.6 0.8 I .0
Pulse Phose
Figure 1. The pulse template used for Cen X-3 timing This was derived from data in
the interval TJD 8569-8574.
3. Results
In the calculation of pulsar phases for the folding of the DISCLA data the planetary
ephemeris DE200 was used in conjunction with the Cen X-3 binary system ephemeris
given in Table 2. This binary ephemeris was based on orbital elements found by Murakami
et al. [7] and the orbital period derivative derive by Kelley et al. [5]. It was found
necessary to make an adjustment to the orbital phase. This adjustment, which was
consistent with the quoted ephemeris errors, was determined from data in the period
TJD 8370-8374, and is reflected in the mid eclipse epoch given in Table 2.
Epoch of mid Eclipse
Binary Period
Period rate of change
Projected semi-major axis
"_7r/2= JD 2448370.1470 + .0005
Porb = 2.0870686 days
Porb = -1.02"10 -s
x = 39.664 sec
Table 2. Cen X-3 binary ephemeris adopted from Murakami et al. and Kelly et al. with
an adjustment in orbital phase.
The data was initially folded with a pulse frequency of 0.20747 Hz. Phase residuals from
the template correlation were then fit and the interval fits recalculated. After several
iterations the following total phase model was arrived at:
188
¢ + (_¢ =- -0.2350 + 17925.701442At - 0.03355982At 2 + 0.0003579274At a
where At is the time elapsed in days since JD 2448370.5 in the pulsar timescale.
(4)
• i i i
,---_ 0
0
03 8370 8375 8580 8585
03
t
8390
8415 8420 8425 8430 8435
Truncated Julian Day
Figure 2. The Intensity history of Cen X-3 obtained by correlation of the folded rates
with a pulse template.
189
The intensity history of Cen X-3, asdeterminedfrom the 10000secondinterval template
correlations, is show in Figure 2. Severalhigh intensity periods are visible, the most
notable being from TJD 8369to 8384,from TJD 8399to 8407and from TJD 8413to TJD
8430. During theseperiods the eclipsesare clearly visible. The phaseresiduals relative
0.4
0.2
0.0
-0.2
-0.4
_' f ' t
,, t +P 'I
f tl ,l
• .... l .... , , , , | , i i i |
8370 83858375 8380 8390
0.4
_3
•_ 0.2
r_ 0.0
-0.2
_3
[.-
n -0.4
+l t' ' I ' "'t" 11
t
tto , I tt
• . . l .... i .... i + I I i I I i
8400 84058395 8410
0.4 } "
02 +'o.o * __"_*'_
_ O.2 11t,_.l_+_"lllPt'+" il
-0.4 . 1....... }
- i | i i I , i i *
8415 8420 8425 8430 8435
Truncated Julian Day
Figure 3. The phase residuals of Cen X-3 relative to the model given in equation (4)
obtained by correlation of the folded rates with a pulse template.
190
to the model in equation (4) are shownin Figure 3. In order to eliminate the majority
of intervals when no signal was detected, phase points are only shown if the intensity to
intensity error ratios is greater than 1.5. The connectivity of the phase points is clear in
the intervals of high intensity. In the intervals from TJD 8388 to 8390 and from TJD
8408 to 8413 however, it appear that no signal from Cen X-3 has been detected, and it
is possible that the phase slips by a cycle. During the interval from TJD 8399 to 8407
the phase residuals appear to have a periodic signature which suggest a possible need for
orbital parameter corrections. However the same signature is not present in the other
intense intervals. A possible explanation that needs to be investigated is a variation of
pulse shape with orbital phase within this or other intervals. The total observed phase
in these three period of high intensity were fit with a quadratic model resulting in the
periods and period derivatives given in Table 3.
Epoch (JD) Period (sec) Period Rate (10 -'° sec/sec)
2448372.0 4.8199422(10) 2.06(29)
2448403.5 4.8201772(5) -0.01(6)
2448421.0 4.8200583(3) - 1.00(3)
Table 3. Cen X-3 pulse period and derivative at selected epochs. Errors on the last digit
are given in parentheses.
C)
(D
O9
03
c-
0
O
(D
C_
-D
©
03
EL
I I I I
6
4-
2
o 1
-2
4
g
0.0 0.2 0.4 0.6 0.8 1.0
Phase
Figure 4. The light curve of Cen X-3 in the 20 to 60 keV energy band for the period
TJD 8399-8407 based on folded-on-board data.
191
Folded-on-board Data Analysis
The folded-on-board data analysis is described by Wilson et al. in these proceedings.
The analysis of the folded-on-board data from Cen X-3 is still in its early stages. An
example of the light curves obtained from the folded on board data is shown in Figure 4.
For this light curve data in the 20 to 60 keV range from the high intensity period from
TJD 8399 to 8407 was folded using the period and period derivative given in Table 3.
Data was included only for Cen X-3 orbital phases in the range from .1 to .9. The total
accumulation time of the included data was 23369 seconds. Significantly greater pulse
shape information is available in the folded-on-board data than in the DISCLA. However
due to counting statistic the full hardware resolution of 64 phase bins is not useful and
is not shown in the figure.
Conclusion
The uniqueness of BATSE from the standpoint of pulsar observation is its ability to view
in all directions continuously. This ability makes BATSE an excellent instrument for the
long term monitoring of x-ray pulsars. This is particularly true for the longer period
pulsars which may be studied exclusively with the continuously available DISCLA and
CONT data.
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