Hercules X-1 is a well known bright binary X ray pulsator. It has a 1.70 day orbital period, a pulsation period of 1.24 second, and a 35 day semiperiodic variability. The discovery is reported of a new 144 second periodicity in the X ray emission from Her X-1. The periodicity is seen in X ray observations of Her X-1 by the LAC instrument onboard the Ginga satellite during Aug. to Sep. 1988. The periodic flux variations occur during the time of X ray turnon at the beginning of a high state of Her X-1, in the same time that a pre-eclipse dip also occurs. An analysis of the LAC spectra of Her X-1 during this period is also presented. Large changes in spectral shape occur associated with the dip

Kawai, N.

Leahy, D. A.

Matsuoka, M.

Yoshida, A.

English

NASA Technical Reports Server

N92-21897
144 SECOND PERIODIC FLUX VARIATIONS DURING X-RAY TURN-ON OF
HERCULES X-I
D.A. Leahy I, A. Yoshida 2, N. Kawai 2, M. Matsuoka 2
iDept, of Physics and Astronomy,
University of Calgary, Calgary, Canada T2N IN4
2Institute of Physical and Chemical Research
Hirosawa, Wako, Saitama 351-01 Japan
ABSTRACT
Hercules X-I is a well known bright binary X-ray pulsator. It has
a 1.70-day orbital period, a pulsation period of 1.24 second, and
a 35 day semi-periodic variability. Here we report the discovery
of a new 144 second periodicity in the X-ray emission from Her X-
i. The periodicity is seen in X-ray observations of Her X-I by
the LAC instrument on board the Ginga satellite during Aug.-Sept.
1988. The periodic flux variations occur during the time of X-ray
turn-on at the beginning of a high-state of Her X-l, in the same
time that a pre-eclipse dip also occurs. An analysis of the LAC
spectra of Her X-i during this period is also presented. Large
changes in spectral shape occur associated with the dip.
i. INTRODUCTION
The 1.24 s X-ray pulsar Hercules X-I was discovered by UHURU
(Tananbaum et al. 1972). It is an eclipsing system with a 1.70-
day orbit. The mass ratio and inclination have been determined by
analysis of the optical pulsations (Middleditch and Nelson 1976).
The orbital parameters have been accurately determined by
analysis of arrival times of the X-ray pulses (e.g. see Deeter et
al., 1991 and references therein). In addition the X-ray flux
exhibits a regular variation with a 35 day period. This period is
not well defined, but whether the variations are due to white
noise acting on a precise period, or due to an inherent
instability in the clock mechanism is not yet known (Ogelman,
1987).
Her X-I has been studied in X rays with several previous
satellite instruments. Pravdo et al. (1977) and Pravdo et al.
(1978) study the OSO-8 high-state spectrum. Becker et al. (1977)
find evidence in the 0S0-8 spectra for variations in absorption
during the turn-on, which are also seen in Tenma observations
(Ohashi et al. 1984, Ushimaru et al. 1989). Soong et al. (1990)
193
https://ntrs.nasa.gov/search.jsp?R=19920012654 2020-03-17T13:16:16+00:00Z
report on HEAO-I observations. Mihara et al. (1990) give
unambiguous evidence for the cyclotron absorption feature near 35
keV. Mihara et al. (1991) study the low-state spectrum of Her X-I
and find a two component emission model is needed. Leahy et al.
(1991) find a two component model is needed to represent the high
state spectrum also.
We report a new periodic flux variation seen from Her X-I during
dipping periods. The results are based on analysis of
observations by the GINGA satellite (Makino and the Astro-C Team,
1987) with the Large Area proportional Counter (LAC) instrument
(Turner et al. 1989). We also report on variations in the
spectrum of Her X-I during the dipping period. The LAC had a
total effective area of 4000 cm 2 and low intrinsic background,
thus provided much superior data regarding the details of the X-
ray time variability and spectrum of Her X-I than previous
experiments.
2. OBSERVATIONS AND ANALYSIS
The analysis presented here is based on observations of Her X-I
by the LAC on GINGA during the period 1988 Aug. 17 to Sept. i.
Her X-I was in a low state during the Aug. 17 and 19
observations, and in the high state for the Aug. 28 and Sept. 1
observations. During the Aug. 28 observations extensive dipping
behaviour was seen.
The LAC can observe in several data modes. We use modes MPC2h,
which records 48 channel pulse-height spectra with 0.0625s time
resolution and MPC3h, which records 12 channel pulse-height
spectra with 0.0078s time resolution. The data were selected to
pass several selection criteria. Then the detector background
count rates in the different pulse height channels were
determined using the method of Hayashida (Hayashida et al.,
1989).
These background subtracted data were used to construct light
curves in four energy bands and also a plot of softness ratio vs.
time. The energy bands chosen were 0- 4,65 keY (low energy band),
4.65- 9.63 keV (iron line and iron edge band), 9.3- 14.0 keV
(medium energy band), and 14.0- 37.2 keV (high energy band).
The light curves and softness ratio curves are shown in Figure 1
for the dipping state on Aug. 28. Light curves for parts of the
low state data on Aug. 17 and of the high state data on Sept. 1
are shown in Figures 2a and 2b. The constancy of the count rates
and of the softness ratio is apparent for the high state and low
state data. This is in great contrast to the strong variability
seen during the dipping period.
194
Inspection of Figure 1 suggests a regular variation of intensity.
Visually there appears to be a period of about 70 s. The most
significant variations are seen in the 0-4.65 and 4.65-9.3 keV
energy bands. The entire data for Aug. 28 was combined and
subjected to a period search using the epoch folding technique.
This data spans the time period 16 hr 8 min 16.2 s to 19 hr 38
min 4.3 s UT (12588.1 s), with some large data gaps, giving a net
observation time of 2383.5 s.
The resulting folded light curves from the epoch folding of the
Aug. 28 data in the four energy bands are shown in Figure 3. The
period of 143.6 seconds is highly significant- the reduced
chisquared values for the four energy bands are 44.3, 53.1, 21.3
and 9.8, in order of low to high energy. There is a clear double
peak in the folded light curves. The 143.6 s period is the period
of maximum chisquared for both of the higher energy bands. For
the two lower energy bands there is a nearby period (140.8 s)
with slightly higher chisquared. However, the chisquared vs.
period is affected by both the longer timescale dip modulation,
and the presence of data gaps. The two higher energy bands are
less affected by the dip modulation. The chisquared vs. period
plots for these high energy bands more clearly indicate the
correct period (see Figures 4a and b), giving strong evidence
that the correct period for the flux variations is 143.6 s.
The data from the low state and from the high state were also
epoch-folded. There were significant data gaps for the 10w-state
and high-state data similar to those in the dipping-state data.
There is no statistically significant period found for either the
low-state or high state data. The chisquared vs. period curve for
the low state data is shown in Figure 4c.
A spectral analysis for selected periods during the observation
was also carried out. As reported by Leahy et al. (1991) only one
two-continuum spectral model was found to adequately fit the
data. The model consists of: an unabsorbed continuum (power-law-
plus-cyclotron-absorption-line); an absorbed continuum of the
same form; an iron emission line. The model has I0 parameters (PI
to PI0):
POWL(I,2)*CYAB(3,4,5)*(I+PI0*ABSM(6))+GAUS(7,8,9)
Here POWL and CYAB are given by:
POWL=PI E -P2 CYAB= exp{-P3 E 2 /((E-P4)2+p52) }
CYAB is the function used to represent the cyclotron absorption
feature seen in the spectrum of Her X-I (Mihara et al. 1990, and
references therein), however we use only the first cyclotron
harmonic. ABSM is absorption by neutral gas of cosmic abundance
with log of column density given by P6, and using the cross-
section given by Morrison and McCammon (1983). GAUS is a Gaussian
195
function with area P7, center energy P8 and width (a) P9. Figure
5 shows the results of fitting the two component model to the dip
onset periods A, B and C of Figure i.
3. DISCUSSION
The phenomenon of pre-eclipse dips for Her X-I has been long
established. The Ginga light curves, with their superior
statistics, show the dipping phenomenon to be much more complex
than apparent in previous observations. The well known dips
correspond to periods B and C and period D and E in Figure I,
where the total count rate drops by factor of 4 or more, and
lasting tens of minutes. However the Ginga data resolves new
"minidips", which last about one minute. Two of these are clearly
seen during period A, more are seen in periods G and H. Periods
G and H also show that the dip can stay at an intermediate count
rate for an extended period of time sometimes, in contrast to the
sharp decrease seen in the transition between periods A and B.
A periodic flux variation has also been found in the Ginga data.
Figure 3 shows this variation at the same 143.6 second period in
all 4 energy bands. At this period a highly significant variation
with a double peaked profile is seen, thus explaining the
apparent 70 second period seen by visual inspection. The two
maxima (and two minima) in the folded light curve are not
separated by 0.5 in phase, consistent with the finding that there
is no real 70 s period.
The 143.6 s periodicity occurs only during the dipping period and
is not present in the other days' data. It is suggested here that
the period is a Keplerian rotation period in the accretion disk
and that the flux variation corresponds to periodic partial
obscuration of the neutron star. For a 1.4 solar mass neutron
star, this period corresponds to a distance of 5xl0Qcm. This
places the matter well within the Roche lobe of the neutron star,
yet far outside the corotation radius or the Alfven radius. The
reason that the disk matter would have an extent enough above the
disk to obscure the neutron star at this particular radial
distance is unclear.
Vrtilek and Halpern (1985) have reported a 108 s during dipping
periods of Her X-I. We do not find any evidence of this
phenomenon in the Ginga data, despite that the Ginga data should
be more sensitive to such time variations.
The spectral analysis shows that, in contrast to previous work
(except Mihara et al. 1991, Leahy et al, 1991), Her X-I requires
a two component spectral model to describe its X-ray spectrum.
During the times when dips occur, the spectrum undergoes complex
changes. However these can all be adequately modeled with the
two-continuum model presented here. Figure 5 shows the spectra
during onset of a major pre-eclipse dip (periods A, B and C of
196
Figure i). The pre-dip spectrum (A) shows weak absorption. As the
dip changes to its full decrease (from B to C) two changes occur:
the unabsorbed component decreases in intensity and the amount of
absorption of the absorbed component increases. Thus the dips are
caused by thick cold matter and which is likely the same matter
which has been detected in both the high-state and low-state
spectra of Her X-I (Leahy et al. 1991).
The pre-dip period A shows small dips which have not previously
been reported, primarily due to the greatly increased sensitivity
of the GINGA LAC compared to previous instruments. These "mini-
dips" are very clear in the lowest energy light curve and in the
plot of softness ratio (Band I/ Band 3, Figure I).
4. SUMMARY
The well known dips of Her X-I have been shown to be more complex
than apparent in previous observations. A new periodicity has
been found which is only present during dipping periods.
The X-ray spectrum of Her X-I has two continuum components- an
unabsorbed component and a highly absorbed component. These two
components are always present: during high, low and dipping
states of Her X-I. The contribution of the absorbed and
unabsorbed components and is highly variable during dips. The
dips are caused by changes in the amount of unabsorbed continuum
and in the amount of absorption.
REFERENCES
Becker, R. et al., 1977, Ap.J., 214, 879.
Deeter, J. et al., 1991, submitted.
Hayashida, K. et al., 1989, Publ. Astron. Soc.Japan, 41, 373.
Leahy, D., Matsuoka, M., Yoshida, A. and Kawai, N. 1991, to
appear in Proceedings of the 22nd International Cosmic Ray
Conferenc_e, Dublin, Aug. 11-23, 1991.
Middleditch, J. and Nelson, J. 1976, Ap.J., 208, 567.
Mihara, T. et al., 1991, to appear in Publ.Astron. Soc.Japan,
Voi.43, no.3.
Mihara, T. et al., 1990, Nature, 346, 250.
Morrison,R and McCammon,D 1983, Ap.J., 270, 119.
Ohashi, T. et al., 1984, Publ.Astron. Soc.Japan, 36, 719.
Ogelman, H. 1987, Astron. Ap., 172, 79.
Pravdo,S., et al., 1977, Ap.J. Lett., 215, L61.
Pravdo,S., et al., 1978, Ap.J., 225, 988.
Soong, Y. et al., 1990, Ap.J., 348, 641.
Tananbaum, H., et al. 1972, Ap.J. Lett, 174, L143.
Ushimaru, N. et al. Publ.Astron. Soc.Japan, 41, 441.
Vrtilek, S. and Halpern, J. 1985, ApJ, 296, 606.
197
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The 144 second periodic flux variations during x ray turn-on of Hercules X-1

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