Results from the HEAO A-1 instrument which observed X-ray source variability over a wide range of accessible timescales are surveyed. The objects observed include quasars, BL Lacertae, and active galactic nuclei. A high sensitivity search for X-ray pulsars, known black hole candidates, period fluctuation in binary pulsars, and X-ray and gamma ray bursts are among the topics covered

Friedman, H.

English

NASA Technical Reports Server

X-RAY VARIABILITY OBSERVED WITH HEAO A-1 
Herbert Friedman 
Naval Research Laboratory 
The A-1 instrument was designed to extend the range of temporal 
variability that was observed in previous missions. In Figure 1,  we 
indicate the variety of time scales that are covered, 10 msec to 40 sec 
in scanning data on single scans. We could go to microsecond time 
scales and high bit rate data, but only when the spacecraft was over 
a ground station capable of receiving telemetry in real time. High 
ecliptic latitude objects could be scanned for many days. Repeat 
sequences of scan were observed at 6-month intervals, over the 17  
months of the mission. We have in hand data with which to complete an 
all-sky survey for X-ray pulsars in the high frequency range, and we 
can look for the detailed features of the temporal variations of the black 
hole candidates. The burst sources are especially interesting because, 
with the large areas of the A-1 instrument, we can get better profiles 
than previous missions have obtained and we can search them for evi- 
dence of pulsation or other variability intrinsic to the burst itself. 
I shall start with an example of evidence for longer term variation 
in extragalactic objects. Figure 2 shows positional data on Pks 2155-304, 
a radio source, believed to be a BL Lac object, and illustrates how the 
various experiments in the mission combined to refine the error boxes. 
The A-3 experiment produces the diamond-shaped arrays of potential 
error boxes. A-1 and A-2 can then bracket these error boxes and A-1 
particularly can select the one diamond which is the true position of the 
source. Figure 3 is  a sample of flux measurements over a period of 6 
days, which shows the degree of variability on this time scale. In the 
region around day 316, oscillations in the flux are observed on a time 
scale of the order of 6 h r ,  implying that the size of the source is less 
than 6 light hr .  Such dimensions are comparable to the size of the solar 
system. Figure 4 fits a source identified with BL Lac object 3C 371. It 
shows the A-2 and A-1  error boxes; the suspected source lies on the 
edge of the A-1 error box, which is about 1 /10  of a square degree. 
Figure 5 is a sample of the flux variations, which could be followed for 
about 60 days, because the source is  very close to the ecliptic pole. 
Around day 238 we see what looks like a flare lasting on the order of a 
day, superimposed on a rather steady decline in the flux, and then a 
rise again around days 273 to 280. Optically, the source is observed to 
vary by about 1 . 2  visual magnitudes; the evidence here is  for similar 
variations in the X-ray range of the spectrum. Such observations add 
significantly to the information that i s  gained optically for these sources. 
By comparisons of optical information and high energy information we 
can develop a more detailed model of the mechanisms that are operating 
in these very powerfinl sources. 
https://ntrs.nasa.gov/search.jsp?R=19800006719 2020-03-21T20:34:27+00:00Z
So much for samples of what can be done with extragalactic 
sources. If we turn to the sources within the galaxy, perhaps the most 
interest attaches to candidate black holes. The leading candidate for a 
black hole is still Cygnus X-1. Interestingly, it was one of the earliest 
sources picked up in rocket surveys; we observed it in 1964 and again 
in 1965, and found that it had changed i ts  intensity by about a factor 
of four. Uhuru quickly detected the very short term variability charac- 
ter  of the source; it produces its emission in erratic, bursting fashion. 
Our chairman for this session, Marty Weisskopf, has been modelling the 
variability characteristics of this source, and has made a very good case 
for describing the emission as shot noise, random bursts of emission 
which have a typical time characteristic on the order of a fraction of a 
second (0.3 to 0.5 sec). 
The top profile of Figure 6 shows a transit of Cygnus X-1 through 
the A-1 collimator; the passage takes approximately 10 sec from one 
corner of the collimator triangle to the other. Below it is a similar 
transit of GX 339-4, which Uhuru observed as a long term variable, but 
which HEAO shows is almost a duplicate of Cygnus X-1 in its short term 
variability. And there is a third source, Circinus X-1, which also shows 
this type of variability. The persuasive evidence for the black hole 
character of Cygnus X-1  comes out of the dynamics of the binary system 
in which it  occurs. The evidence is that the mass of the compact object, 
Cygnus X - 1 ,  exceeds six solar masses. In theory, if the mass exceeds 
three solar masses, it cannot be a neutron star;  it must be a black hole. 
We do not have similar evidence for G X  339-4 and Circinus X-1. They 
are candidate black holes only because their temporal variations so closely 
resemble Cygnus X- 1. 
Figure 7 shows integral chi-squared distributions for Cyg X-1; in 
the 30 to 300 msec range there is a very large deviation from a Poisson 
distribution. The discrepancy exists all the way down to the 1 to 10 
msec range. Only in the 0.3 to 3 msec range do the data fit a Poisson 
distribution. In Figure 8 we show the autocorrelation; themupper curve 
is  stretched out by a large factor (the bottom scale is 1 sec, while the 
scale at the top is 100 msec) . Toward the origin there is  a strong 
component at around 3 to 4 msec. It appears that ,  in addition to the 
approximately 0.5 sec shot noise pattern which has been confirmed by 
many observers, there is  an additional component around 3 to 4 msec. 
One can speculate about the significance of this pattern in terms of black 
hole models. If Cygnus X-1  i s  a six solar mass black hole, the material 
at the edge of the accretion disc at the innermost stable orbit, would be 
circulating the black hole with a period of 3 or 4 msec. The approxi- 
mately 0.5 sec shots may be associated with turbulence over a large 
portion of the accretion disc. Perhaps the 3 msec component is indica- 
tive of material which is  tearing off the edge of the accretion disc and 
falling into the black hole, surviving less than one Keplerian orbit. 
That is just pure speculation, but the 3 msec effect is worthy of serious 
theoretical interpretation. 
Figure 9 is a tabular summary of the rapid fluctuations in the 3 
to 4 msec range. While the total count rate is approximately 1,000 counts 
per second, the time average of these bursts is approximately 50 counts 
per second; they represent a twentieth of all the luminosity. Individually 
each one of these fast bursts is about as luminous as one of the longer 
300 msec bursts. 
Figure 10 shows samples of data from GX 339-4, three transits 
through the scan collimators with 40 msec bins, Wide fluctuations are 
evident. Over a fraction of a second there are intensity excursions of 
factors of three or four above average. Figure 11 shows the integral 
chi-squared distribution against chi-squared; the solid line represents 
a Poisson distribution. In the ranges from 5 to 50, 40 to 400, 80 to 
800, 160 to 1600 msec, there are very wide deviations revealing the shot 
noise character. We do not have data at the very short time scale end 
of the distribution because these were taken in our 5 msec timing mode 
and not the high bit rate data mode. Figure 1 2  is an attempt to sum- 
marize what we know about the three candidate sources, Cygnus X-1, 
Circinus X-1, and G X  339-4. In the case of Cygnus X-1  and Circinus 
X-1, we have clear evidence for binary orbital periods. A bimodal 
characteristic of the X-ray spectrum appears in all three sources. 
Extended off-periods exist for Circinus X-1 and GX 339-4. The strong 
resemblances in these three sources suggest a common model and they 
will be the object of much more detailed analysis with data that are in 
hand from the HEAO-1 mission. 
We have had a few opportunities in the mission to look at bursters 
with the A-1 instrument. Figure 13 shows a burst from 1728-34, The 
rise time of the burst reveals a great deal of fluctuation. We have tried 
to search the data on this source at high frequencies to see if there i s  
any evidence for rapid periodicity or quasi-periodicity in the source. 
The generally accepted model for a black hole and its accretion disc is 
that hot spots will form in the accretion disc as differential rotation 
winds up magnetic fields and eventually leads to a tearing mode instability 
resembling a solar flare. The flare becomes a hot spot which is carried 
around with the spin of the disc and because of gravitational focussing 
and doppler effects, the signal is likely to be strongly modulated. If 
the hot spot can survive several rotations, we should see the periodicity 
of the Keplerian motion; if it can not survive a few rotations, no perio- 
dicity will be observed. If it survives long enough, we might even 
observe the drift of the material as it  spirals inward toward the black 
hole with decreasing Keplerian period. The evidence for 1728-34 has 
been very tantalizing. Although the statistical evidence is marginal, 
there appears to be a periodicity around 1 2  msec, in one strong burst. 
Off the burst ,  this source has a steady component of emission with no 
evidence of periodicity. Three other bursts have been observed with 
less statistical significance. In one of them there is a hint of pulsation 
closer to 10 msec, enough to make one wonder whether a black hole 
model for the burster ought to be considered seriously in competition 
with all the evidence for bursters being neutron stars. Of course, a 
fast spinning accretion disk model for a neutron star burster is not 
excluded. 
These are some samples of the kind of information that we have 
from the mission and what we are trying to do with the data. I would 
like to end with some historical comments about the program. D r .  Lucas 
has already mentioned how far back the effort at Marshall goes, but it 
goes back considerably further than that. In 1965, at a meeting of the 
Space Science Board at Woods Hole, Riccardo Giacconi and I were the 
senior scientists (I wonder what we are now), and I pushed hard for 
large area detectors while Riccardo pressed for a mirror. Now we have 
seen both these approaches accomplished in the HEAO program. Here at 
Huntsville, Ernst Stuhlinger was my principal contact in the late sixties, 
and serious plans were made to carry very large arrays of detectors on 
left-over Apollo hardware. With Saturn V hardware it would have been 
possible to put a big fence of detectors on the moon (Fig. 14) looking 
over a lunar horizon to observe occultations of X-ray sources at the 
horizon. These drawings were made here at Huntsville. Figure 15 is a 
sketch of how one might put 100 ft2 of detector modules on the LEM, or 
put a large folded array that could be deployed Like solar panels on the 
service module (Fig. 16) .  You all know the history of what happened 
to the Apollo hardware, and how the HEAO concept was taken away from 
the manned missions and run as an unmanned program, how it went 
through one drastic reduction around 1973, and how it finally came to 
the successful launches of HEAO-1 and HEAO-2 in the last couple of 
years. 
A-1 SURVEY O F  X-RAY SOURCE VARIABILITY 
a UTILIZE LARGE COLLECTING AREA AND WIDE 
RANGE OF  ACCESSIBLE TIMESCALES 
a 10 msec - 40 sec IN SCANNING DATA, SINGLE 
SCANS. LONGER TIMESCALES (UP T O  1 yr) BY 
COMPARING SCANS 
a MICROSECOND TIMESCALES IN HIGH B I T  RATE 
DATA 
a QUASARS, BL LACERTAE, ACTIVE GALACTIC 
NUCLEI: 1 hr TO 1 yr 
a HIGH-SENSITIVITY SEARCH FOR X-RAY PULSARS 
a DETAILED STUDY OF KNOWN BLACK HOLE 
CANDIDATES 
PERIODICITY SEARCH AT SHORT TIMESCALES 
SHOT NOISE MODELING 
SEARCH FOR SHORT DURATION SPIKES 
a PERIOD FLUCTUATION STUDY IN BINARY PULSARS 
X-RAY AND GAMMA-RAY BURSTS 
Figure 1 
A-2 ERROR @OX FROM 
IiOFT X-RAY OATA 
v 
RIGHT ASCENSION (1950) DEGREES 
Figure 2 
A A I 
3lÃ SIT %la Sl# 520 
J.0-2443843.3 
Figure 3 
Figure 4 
r v  r .  T r r l , V T +  
1 
3C311 
P 
k t -LIGHT CURVE - 
t tt+++++t - 7 t - 
L 
L +++I t T-T 
1-T . 
2 I . . l l l l . , . I l l l , ( l l , , ~ . , l * ~ , l l l ~  -- - 
2 30 240 250  260 270 -280 290 
Figure 5 
1 
out 
QsD 
Figure 6 
?NTEG.RAL DISTRIBUTION 
Or' CHI-SCUASED 
* 30-300 ms 
. - 
THEORETICAL CHI-SQUARED 
D l  STRIBUTION FOR RANDOM 
-. .. 
Figure 7 
CYGMJS X-1 
AUTOCORRELATION FUNCTION 
Figure 8 
CYGNUS X-1 RAPID FLUCTUATIONS 
STATISTICALLY SIGNIFICANT EXCESS OF  EVENTS 
ON TIMESCALES 1 - 10 msec 
FREQUENT SMALL-AMPLITUDE EVENTS RATHER 
THAN A SMALL NUMBER O F  LARGE EVENTS 
AUTOCORRELATION FUNCTION GIVES MINIMUM 
CHARACTERISTIC TIMESCALE: 3-4 ms 
RATE O F  OCCURRENCE: - > 10 EVENTS: /s 
EVENT TIME-AVERAGE 
LUMINOSITY 
COUNT RATE 1500 cts /s  50 c ts /s  
(TOTAL SOURCE COUNT RATE I S  1000 cts/s) 
F i g u r e  9 
TIME (sec) 
Figure 10 
\- POISSON 
x 219 
Figure 11 
CHARACTERISTICS O F  BLACK HOLE CANDIDATES 
1. VARIABILITY 
ORBITAL PERIOD 
BIMODAL X-RAY SPECTRUM 
EXTENDED "OFF" STATES 
CORRELATION TIME 
SHORTEST TIME SCALE 
2. OPTICAL COMPANION1 
EMISSION LINES 
3 .  MASS FUNCTION 
M 
X 
4. DISTANCE 
5. MAXIMUM LUMINOSITY 
ergs /s 
6. OTHER EMISSION 
CYG X-1 CIR X-1 
5 .60 d 16 .6  d 
YES YES 
NO YES 
0 .5  s VARIABLE 
21 m s  21  m s  
HD 226868 FAINT HIGHLY REDDENED 
09.7 lab m = 16 R 
MV = 8.9 STRONG He AND HEI 
0.22 M, - 
2 10 M, - 
-2.5 kpc >8 kpc 14  
.., 
-2 x 10  3 7 
RADIO, IR RADIO 
YES 
YES 
0 .3  - 0.6 s 
210 m s  
>4 - kpc 
Figure 12 
500 
GOO 
BOO 
00 
Figure 13 
Figure 14 
Figure 15 
Figure 16 


X-ray variability observed with HEAO A-1

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