A series of photographic, photoelectric, and spectroscopic observations beginning June 1, 1972 has led to the optical identification of Her X-1 (2U 1705 + 34), a pulsed X-ray source in an eclipsing binary system, with the thirteenth magnitude blue variable star HZ Herculis. The detection of optical pulses at the frequency of the X-ray pulsar on three nights makes the identification conclusive and establishes HZ Her as the second known optical pulsar. The strength of the optical pulses may be correlated with the orbital phase but is not obviously related to the high or low intensity states of the X-ray source

Davidsen, A.

Henry, J. P.

Middleditch, J.

Smith, H. E.

English

NASA Technical Reports Server

N73
 1*824
IDENTIFICATION OF THE X-RAY PULSAR IN HERCULES:
A NEW OPTICAL PULSAR
Arthur Davidsen"*"*, J. Patrick Henry5*,
John Middleditch**, and Harding E. Smith1"
*
Space Sciences Laboratory, University of California, Berkeley
Department of Astronomy, University of California, Berkeley
§Department of Physics, University of California, Berkeley
Lawrence Berkeley Laboratory
https://ntrs.nasa.gov/search.jsp?R=19730010097 2020-03-11T19:49:07+00:00Z
ABSTRACT
A series of photographic, photoelectric, and spectroscopic observations
beginning 1972 June 1 has led to the optical identification of Her X-l
(2U 1705 + 34), a pulsed X-ray source in an eclipsing binary system, with
the thirteenth magnitude blue variable star HZ Herculis. The detection
of optical pulses at the frequency of the X-ray pulsar on three nights
makes the identification conclusive and establishes HZ Her as the second
known optical pulsar. The strength of the optical pulses may be correlated
with the orbital phase but is not obviously related to the high or low
intensity states of the X-ray source.
I. INTRODUCTION
The remarkable discovery by Uhuru (Schreier et al. 1972, Tananbaum
et al. 1972) of a class of eclipsing binary systems containing pulsed
X-ray sources promises to yield a wealth of new information on stellar
evolution. We present here a report of our successful search for the
optical counterpart of one of these objects, Her X-l (2U 1705 + 34). The
thirteenth magnitude blue variable star HZ Herculis, which lies near the
center of the latest error box of the X-ray source (Clark et al. 1972)
shows 1.8 magnitude variations in B consistent with the 1.70017 day orbital
period of the X-ray source. Minimum light occurs close to the center of
the X-ray eclipse. More conclusively, we have detected optical pulses from
HZ Her with a period of 1.2379 ± 0.0002 seconds, which is identical with the
period'of"the X-ray pulsar. The power in the optical pulses varies, and
appears to be correlated with orbital phase. A maximum pulsed fraction of 0.2%
was observed on 1972 August 6. A tentative detection of optical pulses from
HZ Her has been reported previously by Lamb and Sorvari (1972). Spectra
obtained with the 120-inch telescope at Lick Observatory show strong
Balmer and He I absorption lines suggesting a B type spectrum. The light
curve, pulsations, and spectra are discussed separately in the following
sections. .
II. PHOTOMETRIC OBSERVATIONS
Following the revision of the location error box of Her X-l
(Clark et al. 1972) we began a search for an optical identification of
the source. Plates were obtained with the 30-inch (76-cm) telescope of
the Leuschner Observatory of the University of California, Berkeley, be-
ginning 1972 June 1. Exposures•on .Kodak 103 aO emulsions with Schott
GG-13 and UG-2 filters revealed a thirteenth magnitude star with a sub-
stantial ultraviolet excess, which was immediately identified with the
catalogued variable star HZ Her (Kukarkin et al. 1969). Variability was
first reported by Hoffmeister (1941), who rioted that HZ Her was bluer
than neighboring stars and concluded it was an irregular variable of the
RW Aurigae type.
Photographic and photoelectric observations were undertaken in June
and July to detect an effect which might be related to the turn-on or
turn-off of the X-ray source, which was reported to have a 36-day period
(Tananbaum et al. 1972). A series of good photoelectric observations on
three successive nights was finally obtained between 13 and 15 July, and
marked variability in times as short as ten minutes was observed. On
July 14 the B magnitude of HZ Her declined steadily, the total change
observed in five hours amounting to 0.9 magnitudes. On the other two nights
its behavior was more erratic, showing flare-ups of 0.2 to 0.3 magnitudes
in a few minutes, superposed on a generally rising light level.
Bahcall and Bahcall (1972) and Liller (1972) have reported that HZ Her
varies with the 1.7 day eclipse cycle of Her X-l. Although our observations
are not sufficient to establish the periodic behavior, they are consistent
(-*•
with that result. The light curve obtained by assuming a 1.70017 day period
is shown in Figure 1, which includes photographic arid photoelectric measure-
ments made on 12 nights in June, July, and August.
A schematic representation of the X-ray light curve is shown for
comparison. The most remarkable contrast is the broadness of the optical
minimum which begins at least 0.2 in phase before the X-ray eclipse. The
total variation of B is 1.8 magnitude, while the X-ray minimum is at
least a factor of 20 below the high intensity level (Tananbaum, 1972).
Several photoelectric measurements of U were also obtained, and the
variation of (U-B) with phase is also displayed in Figure 1. At maximum
light (U-B) * -0.7 while at minimum (U-B) w +0.3. Reliable measurements
of V were not obtained because of the sky brightness at Leuschner.
III. OBSERVATIONS OF OPTICAL PULSES
We have searched for optical pulses from HZ Her on ten nights in July
and August, 1972, with an EMI 9658 photomultiplier tube mounted at the
focus of the 24-inch (61 cm) telescope at Lick Observatory. The photopulses
were converted to logic pulses by an amplifier discriminator and then used
for synchronous signal-averaging with the electronics described previously
by Nelson et al. (1970). In parallel with the signal-averaging the pulses
were integrated-for 10 ms and passed through a divider, subtracter, tape
interface, Kennedy 1600 incremental tape recorder and onto a seven track
digital tape for later Fourier analysis. A General Radio 1120 frequency
counter which displayed the photon counting rate was used to assist in the
offset guiding and monitoring of HZ Her. Typical counting rates were 5000
counts/sec in the high state and 2000 counts/sec in eclipse of which 900
counts/sec were from the sky.
The major analysis in the pulsation detection experiment is an efficient
version of the Cooley-Tukey Fourier transform which has been calibrated pre-
viously on the Crab Pulsar on 1971 April 22, October 13, and 1972 February
28, and August 11 by simultaneous synchronous signal averaging for run
lengths of 218 x 4 ms, 220 x 4 ms, 221 x 1 ms, and 216 x 10 ms respectively.
By a summation of harmonics of the 100 to 150 most significant spikes the
transform recovered at least 85% of the signal/noise ratio obtained from
signal averaging.
For additional accurate calibration, a Monte-Carlo program generated
a. run with 50 counts/sec underneath a 30 Hz sine-wave form and an 8000 count/sec
background for a run of 218 x 4 ms which produced a peak of 11 to 12 a
in the transform. The pulsation levels given in Figure 2 were derived
from the Monte Carlo simulation and from the fact that the percentage of
pulsation with respect to a reference light level varies inversely as the
strength of that light level and as the square root of the ratio of the
length of run to background counting rate. Transform lengths of 218 -220
x 10 ms were typical in the reduction of the data. A reference level of
4000 counts/sec was chosen because this represents both HZ Her at its
brightest phase and the mean light level seen from the star plus sky in
the 17" diaphragm used.
Coherent optical pulsations, lasting from 45 to 90 minutes were detected
on the nights of August 4, 6, and 11 with confidence levels of la, 5a, and
5cr respectively. The nights of July 21, 22, 28, and August 3, 5, and 8
gave negative results down to less than 2o. A marginal result of 2.5a was
obtained on the night of August 7. The data are plotted versus orbital
phase in Figure 2.
The August 4 pulsations occurred during the second half of three hours
of observations and clearly showed the fundamental and first two higher
harmonics. The power in the three frequencies decreased somewhat in the
second 45 minute interval after the appearance of pulsations, when the
orbital phase of Her X-l passed "from the 0.10 to 0.12 range to the 0.12 to
0.14 range. The apparent frequencies of 0.8079 and 0.8083 ± 0.0002 Hz from
the two 45-minute data sections were both consistent with the progression
of doppler shift of the Her X-l pulsar according to the ephemeris published
•by Tananbaum et al. (1972). The ratio of power in the three frequencies
was roughly 7:2:3.
The August 6 pulsations occurred mostly in the first hour of a three-houx
observation (when the Her X-l orbital phase was 0.24 to 0.26) and have only
slight power beyond the fundamental frequency. The intensity of pulsation
during this period was uniform, but the pulsation frequency was measured
to be lower than the expected doppler-shifted frequency of 0.8083 Hz with
the power falling in the frequency bin corresponding to the zero-velocity
average of 0.8079 * 0.0002 Hz for the binary system. We know of no instru-
mental effect which could account for this frequency difference. Five nights
later, with the binary again near the same phase (0.18 to 0.22) pulsations
were detected for a 90 minute period with the predicted doppler-shifted
frequency of 0.8083 ±0.0002 Hz. Three transforms in the first half, middle,
and second half of this period indicate that the intensity varied from high
to low, and then to high again. The three hours following this period (phase
0.22 to 0.29) showed no evidence of pulsations.
The marginal result of August 7 occurred during the second quarter of
a three hour observation and may have been diminished by bad weather. The
binary phase was 0.87 and the pulsation frequency of 0.8075 ±0.0002 Hz
agrees well with the doppler shift expected for Her X-l.
The complex phases obtained from the sub-sections of data for all
three nights with substantial power show that the source of the pulsations
is coherent.
IV. SPECTROSCOPIC OBSERVATIONS
Spectra of HZ Her were obtained on 1972 June 15 and August 5, 6, and 7
when the phases of Her X-l were 0.70, 0.65, 0.23, and 0.82, respectively.
The observations were made using the Lick Observatory 120 inch (305 cm)
telescope and the Image-Dissector Scanner (IDS) (Wampler and Robinson, 1972).
Eight minute integrations were usually sufficient to yield more than 2000 counts
per channel across most of the 2000 A interval scanned. Three of the spectra
o o
were centered at 5700 A and the remaining one was centered at 7100 A. Spectra
were not obtained in the blue because of the requirements of concurrent
research with the IDS.
. All the spectra are qualitatively similar. Each shows absorption at
HB, Hel AX4922, 5016, 5876, 6678, and Ha. Figure 3 'shows a representative
observation. The shape of the continuum is artificial, and is created by
a quartz lamp used as a normalization to remove the instrumental"response.
No trace of emission has been found on any of the spectra. The observation
of 7 August, obtained only three hours after the X-ray source made a tran-
sition from low to high intensity states (Tananbaum, 1972), is of lower
quality than the others due to poor weather. However, it shows no signi-
ficant differences from the previous spectra obtained under good conditions.
On the basis of the helium line strengths, we tentatively classify
HZ Her as an early B star, at least during the phases at which we have
o
observed it. The equivalent width of Hel A4922 is approximately 1.0 A
which implies HZ Her is near the Hel maximum at B2. We obtain equivalent
o o
widths of 9.5 A and 5.5 A for H$ and Ha, -respectively. More precise
classification is not possible at present because the IDS has not yet been
used to observe large numbers of standard stars.
Although the ability of the IDS to measure stellar radial velocities
is as yet untested, we see no convincing evidence for large velocities as
suggested by Bopp et al. (1972). All of our spectra give essentially zero
radial velocity with a maximum error of ±100 km/sec. If large radial
velocities are sometimes observed, it is probably due to gas streaming
in this close binary system.
V. SUMMARY
To summarize our principal observations we find:
1. HZ Her sometimes emits optical pulses at the frequency of the
X-ray pulsar Her X-l. In the best case about 0.2% of the maximum light
from this system appears in these pulses. The pulses were detected during
both high and low intensity states of the X-ray source. Our three strongest
detections of optical pulses occurred between 0.1 and 0.3 in orbital
phase. We also note that the .observations of Lamb and Sorvari (1972) and the
negative results of Crampton and Morbey (1972) are consistent with this
possible correlation between pulsations and phase.
2. The light curve or HZ Her is consistent with the 1.70017 day
eclipse period of the X-ray source. Minimum light coincides with the
X-ray eclipse, but the transition from maximum to minimum takes much
longer in the optical region.
3. Flares of 0.2 to 0.3 magnitudes in times shorter than an hour
are superposed on the periodic variation.
4. The ultraviolet excess of HZ Her is greatest at maximum light.
The total variation of U-B is ~1.0 magnitude.
5. The spectrum appears to be early type B, at least near phases 0.25
and 0.75. No emission lines have been observed. A spectrum obtained
shortly after the X-ray source turned-on showed no differences from those
obtained earlier.
The correlated optical and X-ray variations at 1.2378 seconds and
1.70017 days establish the identity of HZ Her and Her X-l conclusively.
With the exception of the Crab pulsar and several supernova remnants, this
is. the first absolutely certain identification of a galactic X-ray source.
ACKNOWLEDGEMENTS
We wish to record our thanks to H. Spinrad for his interest in this
work and his generous contribution of 120-inch time, to W. Liller for
communicating data prior to publication, to J. Faulkner, S. Faber, and
to their students for donation of 24-inch time, and to all the observers at
Lick who were inconvenienced by the requirements of our observing schedule.
We thank J. Saarloos for his assistance and J. Nelson for his valuable
advice on the pulsar observations. One of us (HES) v?ishes to thank J. Wampler
and L. Robinson for instruction in the use of the IDS system. We are indebted
to B. Margon for stimulating our interest in the optical identification of
X-ray sources. Special thanks are due to S. Bowyer for his enthusiastic
support and extremely helpful discussions throughout the course of this work.
This work has been supported in part by NASA Grant NCR 05-003-450 and by the
U. S. Atomic Energy Commission.
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REFERENCES
Bahcall, J. M., and Bahcall, N. A., 1972, I.A.U. Circ. No. 2427.
Bopp, B. W., Grupsmith, G. and Vanden Bout, P., 1972, I.A.U. Circ. No. 2424.
Clark, G. W., Bradt, H. V., Lewin, W. H. G., Markert, T. H., Schnopper, H. W.,
and Sprott, G., 1972, preprint.
Crampton, D. and Morbey, C. L. , 1972, I.A.U. Circ. No. 2428.
Hoffmeister, C., 1941, Kleinere Veroffentlichung der Universitats - Sternwarte
zu Berlin-Babelsberg, Nr. 24.
Kukarkin, B. V., Pavenago, P. P., Efremov, Y. I., and Kholopov, P. N., 1969,
General Catalogue of Variable Stars (Second edition; Moscow: Academy of
Sciences.) .
Lamb, D. Q., and Sorvari, J. M., 1972, I.A.U. Circ. No. 2422.
Liller, W. , 1972, I.A.U. Circ. No. 2427.
Nelson, J., Hills, R. , Cudaback, D., and Wampler, J., 1970, Ap. J.3 (Letters)3
172, L79.
Tananbaum, H., Gursky, H., Kellogg, E. M., Levinson, R., Schreier, E., and
Giacconi, R., 1972, Ap. J.3 (Letters), 174, L143.
Tananbaum, H., 1972, private communication.
Wampler, J., and Robinson, L., 1972, P.A.S.P., 84, 161.
10
FIGURE CAPTIONS
Fig. I Light curve of HZ Her for P = 1.70017 days, TQ = J.D. 2441329.5772.
B magnitude is displayed in the upper portion, (U-B) in the center,
and the .X-ray variation at the bottom. Phase 0.0 corresponds to
the center of the X-ray eclipse. Mean errors are 0.05 in B and
0.07 in U-B. A photographic observations, (^photoelectric
observations.
Fig. 2 Pulsed fraction of the maximum light from HZ Her vs. phase.
Error bars and upper limits are *1 a. Positive detections
were obtained on three different nights.
Fig. 3 Spectrum of HZ Her obtained with the Lick 120-inch telescope
and Image Dissector Scanner on 1972 August 6 at 0430 UT. The
shape of the continuum reflects that of a quartz lamp used for
normalization.


Identification of the X-ray pulsar in Hercules: A new optical pulsar

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