Hard X-ray observations of the binary system AM Her were coincident with soft X-ray and ground-based optical measurements. In the 2-60 KeV band, variability was detected with an eclipse during phases 0.5 to 0.7 with respect to the 0. d 12892 period optical minima, synchronous with the known soft X-ray eclipse. The 2-60 KeV uneclipsed flux was 9.5 x 10 to the minus 10th power erg sq cm/sec, of which 86% lies above 10 keV. Thus AM Her contains a hard source located near the similarly eclipsed soft X-ray source. The X-ray data are interpreted in terms of thermal bremsstrahlung from accretion onto a white dwarf

Boldt, E.

Holt, S. S.

Lampton, M.

Serlemitsos, P. J.

Swank, J. H.

English

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X 7/ C3
OSO-8 X-RAY OBSERVATIONS
OF AM HERCULIS
(NASA-TN-X-71313) OSO-8 I- BAY OBSERVATIONS
	 M77-27050
OF AN HERCULIS (NASA) 16 p HC A02/NF A01
CSCL 03A
usclas
G3/89 37025
J. SWANK
M. LAMPTON
E. BOLDT
S. HOLT
P. SERLEMITSOS
MAY 1977	 JUL 1977
RECEIVED
NASA STI FACIUlY
INPUT BRANCH
GODDARD SPACE FLIGHT CENTER
GREENBELT, MARYLAND
OSO-8 X-Ray Observations of AM Herculis
J. Swank* , M. Lampton+ , E. Boldt, S. Holt, and P. Serlemitsos
Laboratory for High Energy Astrophysics
NASA/Goddard Space Flight Center
Greenbelt, Maryland 20771
ABSTRACT
We report the results of hard X-ray observations
of the binary system AM Her, which were coincident with
soft X-ray and ground-based optical measurements. In the
2-60 keV band, variability was detected with an eclipse
during phases 0.5 to 0.7 with respect to the 0.12892 period
optical minima, synchronous with the known soft X-ray eclipse.
The 2-60 keV uneclipsed flux was 9.5 x 10-10 erg cm2
 sec,
of which 86% lies above 10 keV, Thus AM Her contains a
hard source located near the similarly eclipsed soft X-
ray source. The X-ray data are interpreted in terms of
thermal bremsstrahlung from accretion onto a white dwarf.
SUBJECT HEADINGS: stars, U Gem; X-rays, sources
*
NAS/NRC Research Associate
+Also Dept. of Physics and Astronomy, Univ, u` Maryland
tV
- 2 -
F
{i
6
I. INTRODUCTION
The binary system AM Herculis is known to exhibit a variety of
periodic and random phenomena. At optical wavelengths, its spectrum
is usually dominated by high excitation emission lines which show radial
velocity variations of several hundred km/s with a 0.12892 period
(Priedhorsky 1977; Cowley and Crampton 1977). Multicolor photometry
has established light curves having primary and secondary minima (Szkody
and Brownlee 1977; Berg and Duthie 1977) throughout May 1976 and primary
minima alone during November 1976 (Priedhorsky and Krzeminski 1976);
in addition, flickering on a time scale of minutes occurs at all phases.
Long term photographic surveys (Dexter et al. 1976) suggest a long period
on-off behavior with P = 670d. Broadband optical polarization measurements
(Tapia 1976a,b; Tapia 1977) show a brief linear polarization event each
0112892 with an amplitude of several percent, and a high degree of circular
polarization, again variable and synchronized with the binary motion.
At X-ray wavelengths, the UHURU object 3U1809+50 has been tentatively
identified with AM Her by Berg and Duthie (1977); this identification
is combatible with the revised UHURU position and with the Ariel V position
for 2A1815+500. A soft X-ray source at the position of Am Her has been
observed by the SAS-3 group (Hearn et al. 1976a,b; Hearn and Richardson
1977) which shows a periodic intensity variation with orbital phase,
with a well-defined minimum at 0 = 0.1 with respect to the linear polar-
ization maximum. There is an increasing volume of evidence that these
"periodic" phenomena are themselves variable; for instance, the linear
polarization events are substantially less pronounced in 1977 than in
r
,.
I- 3 -
1976 (Tapia 1977, private communication).
In this letter we report the OSO-8 observations of AM Her which
were conducted 11-12 October 1975, when the system was observed for
0173 by the GSFC 2-60 keV and U. Wisconsin 0.15-25 keV X-ray experiments.
This time interval is sufficient for nearly complete coverage of the
binary period in spite of the 0.062 period of the spacecraft orbit.
Accordingly we are able to construct light curves of AM Her in the energy
bands of these experiments. The fact that the AM Her binary period
clearly emerges from these data conclusively establishes that AM Her
is the hard X-ray source. The high energy (> 2 keV) data are of particular
interest because they set a lower limit to the temperature of the emitting
region, or alternatively establish, for the first time, the presence
of an energetic nonthermal component within the AM Her system.
II. ANALYSIS OF TIMING DATA
The GSFC OSO-8 X-ray experiment: has been described previously
(Serlemitsos et al. 1976; Pravdo et al. 1976). Data on AM Her were
collected using detector A, a xenon-filled proportional counter which
views a 5 0 FWHM circular patch of sky offset 1750 from the spacecraft
spin axis. To remove the effects of spacecraft rotation and spin axis
motion, the background count rates were first determined from off-source
portions of each rotation and subtracted. The resulting count rates
were then corrected with an exposure factor determined from the known
position of AM Her and the spacecraft aspect. The corrected data were
combined by a weighted average into 1000s intervals. Light curves for
the 2-60 keV and 10-60 keV bands were constructed by folding these means
- 4 -
at the known optical period. A convincing level of modulation is seen
in both energy bands (see Figure 1) with an essentially complete eclipse
centered at JD 2442696.977, which is at phase 0.58 with respect to
'	 the primary B-band optical minimum measured that same night (Berg, private
3
communication 1977). Simultaneous soft X-ray observations by the Wisconsin
group (Burner 1977) reveal that the 0.15-0.28 keV flux has the same
temporal behavior as seen at higher energies, with a phase difference
not exceeding 0.05 cycles. Polarimetry was not available during our
observations, so the phase (or indeed the existence) of a periodic optical
polarization pulse is unknown. However, the relative timing of the
X-ray and optical minima appears stable: 223 days later, the phase
:
of the SAS-3 minimum with respect to Berg's 1976 photometric minima
is 0.59.
III. THE X-RAY SPECTRUM
The GSFC OSO-8 experiment provides 63 channel pulse height distri-
butions accumulated over time intervals corresponding to 0.25 spacecraft
t	 rotations. For AM Her these data were grouped into 21 bins and compared
=	 i
i
with count distributions expected for power law and optically thin thermal
bremsstrahlung functions. Background data were obtained from adjacent
rotation sectors. The best fitting power law had an energy index of
-0.4 and an associated absorption parameter of N H 4 x 1022 CM 2.
The best fitting bremsstrahlung temperature was 2 x 10 9 K with NH
3 x 1022 cm 2 . These spectra are surprisingly hard; about half the
observed 2-60 keV energy lies above 28 keV. However, the fit quality
is in neither case statistically satisfactory. The models give
X2 = 35 and 36, respectively, for 18 degrees of freedom, indicating
that the spectrum is more complex than these simple models. Complicated
X-ray spectra are theoretically expected in geometries having appreciable
stellar X-ray albedo contributions. For example, the albedo calculations
s
of Feldsteiner and Opher (1976) indicate that about 30% of the X-rays
intercepted by a star will be reemitted after modification by multiple
Compton scattering, atomic fluorescence, and photoelectric absorption.
The 2-60 keV energy flux, averaged over the entire light curve,
was 7.6 x 10 10 erg cm2 sec. Essentially none of this flux arrives
during phases 0.5-0.7. Our time averaged 2-6 keV flux, 5.2 x 10 11
ergs cm 2-1 , lies between the 1970-1971 value of 8.7 x 1 11 ergs c 2s	 0	 m
-1 (Giacconi et al. 1974) and the 1971-1973 value of 3 x 10 11s	 ergs
cm2s-1 (Murray and Ulmer, 1976) for 3U1809+50. The 2-10 keV flux,
1.05 x 10 10 ergs cm 2 s-1 , is 98 + 10% of the 1975 intensity for 2A1815+500
(Cooke et al. 1977).
During the GSFC observations data were also acquired by the Wisconsin
soft X-ray experiment. Analysis of these data (Burner 1977) confirms
the presence of the hard component reported above, and establishes the
simultaneous presence of a steep spectrum soft component with an intensity
of 1.6 x 1010 ergs cm2 s 1 in the 0.19-0.28 keV band. Thus the soft X-
ray flux appears to have been several times that measured by SAS-3 (Hearn
et al. 1976a,b; Hearn and Richardson 1977) during a low optical state
in 1976. The soft flux observed by the Wisconsin experiment is satis-
factorily fit by a simple exponential spectrum having kT - .03 keV and
an absorption parameter NH < 1020 cm2 , in contrast to the hard flux
which, if thermal, has a temperature > 1000 times higher.
rf	
--_—
i- 6 -
The hard and soft X-ray measurements reported here have been plotted
in Figure 2, along with Perry's simultaneously acquired V and B photometry
(Berg and Duthie 1977). This photometry shows AM Her to have been in
its bright state. For contrast we have also indicated the range of
optical variability by plotting one of Tapia's (1977) low state UBV
measurements, taken 237 days later. The recent infrared measurements
by Rieke and Merrill and the optical continuum (see Stuckman e +. al.,
1977) are also shown. From these observations, we note that a substantial
amount of the luminosity of AM Her appears in the hard X-ray band:
Lx (2-60 keV) - 9.1 x 10 32 (D/100 pc) 2 erg/sec. The soft X-ray (0.19-
0.28 keV), visible (0.3-lum), and infrared (1-10 um) bands have respectively
2 x 1032 , 1.8 x 1032 , and 1.2 x 10 32 erg/sec at 100 pc distance. Since
the X-ray spectrum above 60 keV has not been measured, the total hard
X-ray luminosity may considerably exceed the 2-60 keV value. For a
thermal bremsstrahlung source with T > 10 9K, the Gaunt factor varies
approximately as (hv)
-0 ' 3
 and Lx (> 2 keV) - 1.9 x 1033 (kT/200 keV)0.7
(D/100pc) 2 e-a/sec.
IV. DISCUSSION
Models of AM Her have largely been based on the similarity of its
optical variability to that of the cataclysmic variable systems, specifi-
cally those of the U Geminorum (Bond and Tiff't 1974) or Z Camelopardalis
type (Berg and Duthie 1977). In such systems maLe rial is transferred
from a dwarf companion to a degenerate primary star by means of a disk
or ring surrounding the latter. 	 Double emission lines characteristic
of disks are frequently seen in such systems. Their absence in AM Her
t
g
- 7 -
can be explained by supposing that light from a single hot spot dominates
the emission from the disk (Szkody and Brownlee 1977; Cowley and Crampton
1977) or that the disk i-- absent and material is transported to the
primary in a direct stream (Chanmugam and Wagner 1977; Stockman et al.
1977) which is presumably guided by the magnetic field of the primary.
These models also explain the observed emission line radial velocities
being maximum at minimum light (Priedhorsky 1977; Cowley and Crampton
1977). The delay of the U-band minimum with respect to the V and R
minima can be explained by supposing that the secundary star is hotter
towards the primary, so that its light contributes appreciably to the
V and R bands, while t:e hot spot or stream trails the primary (Szkody
1976; Cowley and Crampton 1977; Chanmugam and Wagner 1977). Attempts
to model the spectrum and circular polarization of optical continuum
have invoked cyclotron emission in a 10 8 gauss field at the surface
of the co-rotating primary (Stockman et al. 1977); in this way one
0
may also explain the 7690 A absorption feature as the quadratic Zeeman
turning point of Ha (Angel et al. 1976).
Here, we point out that the hard X-ray emission can be understood
in terms of thermal bremsstrahlung radiation from material transferred
to the degenerate primary and heated by its infall. Models of this
kind have been developed by Hoshi (1973), Aizu (1973), Fabian et al.
(1976), and Katz (1977). Near the degenerate primary, we suppose that
the accretion flow is magnetically confined to a column of cross sectional
area A - 0 R 2 f where the radius of the star is R = 5 x 10 8 (M /M) cm0
for the Hamada and Salpeter (1971) equation of state. A height h above
6- 8 -
the primary the material is abruptly heated by a shock and radiates
optically thin thermal X-rays for a time interval 4h/Vfreefall before
reaching the stellar surface. If h < R, the infalling material will
be heated to nearly its freefall temperature, 109 (M/M®) 2K. A model
of this kind should approximately describe a real flow provided that
the total luminosity is low enough to make Compton interactions unimportant
(Katz 1977). The model can be made quantitative by adopting s+.;:e dis-
tribution of temperature and density within the shocked materIP1, i.e.,
for R < r < R+h. Following Fabian et al. (1976) we take these functions
to be constant. Combining the mass conservation law kpAVfreefall.
M with the luminosity expression L - GMM/R, we find for the density
of the shocked medium
n - 4 x 1012 
L33 f 1(M/Me)-1 cm 3 .	 (1)
If bremsstrahlung is to account for the X-ray luminosity L, then the
height h of the radiating column must be
h - 4.4 x 1011 L33 f (M/M®) 3 cm9	 (2)
If this model is correct and kT > 60 keV, then M/MO
 > 0.33. Since
M/Ma must be less than Chandrasekhar's limit of 1.40 we adopt M = M e for
the primary. From the observed hard energy flux, Lx ti 1033 0/100 pc)2
erg s-1 . Hence the column height is 4.4 x 10 11
 f (D/100 pc) -2
 cm, which
is less than the primary's radius (making the model self-consistent)
provided that f < 1073 0/100 pc) 2 . An accretion column occupying this
small fraction of the star's surface does not appear to be in conflict
with existing accretion theories, especially in view of the extreme ratio
of magnetic energy density to gas pressure. For a 1-2 M  system the opening
-- 9 -
angle of a dipole field line reaching the inner Lagrangian point would
be less than 5 0 , corresponding to an entire polar cap having f k 2 x 13-3.
We further note that the observed optical and soft X-ray fluxes do not
rI
cause appreciable Compton cooling of the infalling electrons; hence the
bremsstrahlung mechanism for the production of the observed hard X-rays
appears justified.
In this model, the observed orbital variation in hard X-ray intensity
is due to synchronous rotation of the primary and its occultation of
the hot column of material accreting onto it. The height of the column
and the inclination of the primary ' s magnetic pole must combine to give
the required X-ray eclipse duration. If h << R, about half of the hard
radiation is intercepted by the primary, heating it to an effective
temperature which depends upon the long term average X-ray production
rate and upon the fraction of energy which is thermalized. In any
case, Teff ti 41000 L3^3 (R/5 x 108) _" K. This heating could make the
primary appear as bright as mV ti lb at 100 pc, just below the observed
low-state photometric intensity. The combined X-ray and optical radiation
from the primary may result in appreciable differential heating of the
secondary companion star.
A portion of the hard radiation intercepted by the primary may
also be responsible for the soft X-ray emission through the initiation
of intense coronal activity on the white dwarf around the accretion
column. A corona occupying 10% of the star's area and having a base
pressure of ti 
7	 210	 (D/100 pc) dyne /cm at 5 x 10 5 K would supply
the volume emission measure ne2V= 2 x 1056 (D/100 pc) 2 cm73 which we
ii
- 10 -
!1
infer from the I& keV observations using the radiative cooling calcu-
lations of Raymond et al. (1976). This model explains the synchronism	 i
and similarity of the hard and soft X-ray orbital light curves, and
predicts that the hard and soft fluxes will vary together in response
to variations in the accretion flow.
We wish to thank Lars. A. N. Bunner and R. A. Berg for communicating
their data to us in advance of publication, and we wish to acknowledge
useful discussions with Dr. S. M. Lea.
REFERENCES
Aizu, K. 1973, Prog. Theoret, Phys. 49, 1184.
Angel, J.R.P., Stockman, H. S., and Schmidt, G. D. 1976, B.A.A.S. 8,
#4, 510.
Berg, R. A., and Duthie, J. G. 1977, Ap. J. 211, 859.
t
Bond, H. E. and Tifft, W. G. 1974, P.A.S.P. 86, 981
Bunner, A. 1977 (in preparation).
Chanmugam, G. and Wagner, R. L. 1977, Ap. J. (Letters) 213, L13.
Cooke, B e A., Ricketts, M. J. Maccacaro, T., Pye, J. P., Elvis, M.,
Watson, N. G., Griffiths, R. E,, Pounds, K. A., McHardy, I.,
Maccagni, D., Seward, F., Page, C. G., and Turner, M.J	 1977,
(preprint).
Cowley, A. P. and Crampton, D. 1977, Ap. J. (Letters) 212, L121.
Dexter, L. H., Gottlieb, E. W., and Liller, W. 1976, B.A.A.S. 8, A,
511.
Fabian, A. C., Pringle, J. E., and Rees, M. J. 1976, MNRAS 175, 43.
Felsteiner, J., and Opher, R. 1976, Astron. and Ap. 46, 189.
Giacconi, R., Murray, S., Gursky, H., Kellogg, E., Schreier, E.,
Matilsky, T., Koch, D., Tananbaum, H. 1974, Ap. J. Suppl. 237, 37.
Hamada, T. and Salpeter, E. 1961, Ap. J. 134, 683.
Hearn, D. R. and Richardson, J. A. 1977 (preprint).
Hearn, D. R., Richardson, J. A., and Clark, G. W. 1976a, Ap. J. (Letters)
210, L23.
Hearn, D., Richardson, J., and Clark, G. 1976b, B.A.A.S. 8, A, 512.
Hoshi, R. 1973, Prog, Theoret. Phys. 49, 776.
i
.Katz, J. I. 1977 (preprint).
Murray, S. S. and Ulme; M. P. 1976, Ap, J. 210, 230.
Pravdo, S. H., Becker, R. H., Boldt, E. A., Holt, S. S., Rothschild,
R. E., Serlemitsos, P. J., and Swank, J. H. 1976, Ap. J. (Letters)
206, L41.
Priedhorsky, W. C. 1977, Ap. J. (Letters) 212, L117^
Priedhorsky, W. and Krzemin' ►:J, W. 1976, B.A.A.S, 8, A, 510.
Raymond, .1. r., Cox, D. t'., and Smith, B. W., 1976, Ap. J. 204, 290.
Serlemitsos, P. J., Becker, R. H., Boldt, E. A., Holt, S. S., Pravdo, S.,
Rothschild, R. E., and Swank, J. H. 1976, Proc. Symp. X-ray Binaries,
NASA SP-389, p. 67.
Stockman, H. S., Schmidt, G. D., Angel, J. R. P., Liebert, J., and Tapia, S.
1977 (preprint).
Szkody, P. 1976, B.A.A.S., 8, A, 510.
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Tapia, S. 1976x, IAUC 2987.
Tapia, S. 1976b, B.A.A.S. 8, #4, 511.
Tapia, S. 1977, A" J. (Letters) 212, L125.
i
FIGURE CAPTIONS
FIGURE 1 - Light curve for Am Her for 1.7 - 60 keV (lower) and
10 - 60 keV (upper) energy bands, based on an assumed
01128926 period with phase 0.0 at JD2442696.6446
center of primary optical minimum (Berg, private
communication, 1977). The same data are plotted for
two cycles.
FIGURE 2 - Observed spectral energy distribution, F v(erg cm 2sec-1
Hz 1). Crosses: data taken 11-12 October 1975, including
Wisconsin soft X-ray flux ("W") and optical photomet-. l
 by
Perry ("P"). Noncoincident data are: H, soft X-ray
flux by Hearn et al. 1976a; T, UBV photometry
by Tapia (1977); S, continuum spectrophotometry by
Stockman et al. (1977); M, infrared photometr y by Merrill;
R, 10.6 m measurement by Rieke (see Stockman et al., 1977).
r-
fi
i
LL
.01
COUNTS
C M_2 SEC-'
> 10 keV
0
.03
COUNTS .02
CM-2 SEC-'
>1.7 keV	 .01
0L
0 0.5	 0	 0.5 0
PHASE
w-
Oi
O
O
O
a^Y
O
N
4)
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OSO-8 X-ray observations of AM Herculis

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