Simultaneous observations of the AM Her type variable H0538+608 made with IUE, EXOSAT, and a 1.3 m ground based telescope, and subsequent optical spectrophotometry at high and low resolution are discussed. The X-ray and optical data show clear evidence of a 3.30 + or - 0.03 hr period. Three SWP spectra were taken outside of eclipse and during overlapping phase intervals. The UV spectra contain strong emission lines characteristic of this class of objects and a flat continuum which appears to be deficient, given the brightness of source at optical and X-ray wavelengths. There is evidence for intensity variations in emission lines, particularly C IV. The X-ray light curves for H0538+608 reveal behavior which may be related to irregularities in its accretion flow

Lamb, D. Q.

Mcclintock, J. E.

Remillard, R.

Shrader, C. R.

Silber, A.

English

NASA Technical Reports Server

c_3,,
N89 - 107 0 313 
SI1VRJLTANEOUS IUE, EXOSAT, AND OPTICAL OBSERVATIONS
OF THE UNUSUAL AM HER TYPE VARIABLE H0538+608
C.R. Shrader
Astronomy Program, Computer Sciences Corporation
and Goddard Space Fltght Center
R. Remlllard and A. Sllber
Center for Space Research, M.I.T.
J.E. McClintock
Harvard-Smlthsontan Center for Astrophysics
and
D.Q. Lamb
University of Chicago
ABSqRAcF
We report on slrr_ltaneous observations of the
AM Her type variable H0538+608 made with IUE,
EXOSAT, and the 1.3 m McGraw-Ihll Observatory
telescope. Subsequent optical spectrophotcanetry at
hxgh and 1_, resolution was also performed. The
X-ray and optical data show clear evidence of a
3.30_0.03_ hour period. Three _ spectra were taken
outside of eclipse and during overlapping phase
xntervals. The UV spectra contaxn strong emission
lines characteristic of this class of objects and a
flat continuum whlch appears to be deficient, g_ven
the brightness of source at optical and X-ray
wavelengths. There is evidence for intensity
variations in ermssion lines, particularly CIV. The
X-ray light curves for H0538+608 reveal some
interesting behavior which may be related to
irregularittes in its accretion flow.
Keyvmrds: X-ray Binaries, Cataclysmic Variables
1. INTR_ION
AM Her type objects are widely believed to
consist of a magnettc white dwarf in synchronous
rotation wath a late type secondary and with
magnetically channeled accretion flow onto the white
dwarf. They are characterized by substantial
optical polarization modulated at the orbital
period, rapid optical flickering as well as periodic
photometric varzations, strong emission lines, UV
excess and X-ray emission. They tend to be faint
with periods are on the order of hours (Ref 1).
Most are not phase resolvable with IUE.
The X-ray source H0538+608 (also 4U0541+60,
Ref. 2 and 1}D53+607, Ref. 3) was identified w_th
a V=14.6 blue (U-B=-0.81) object ustng positional
inforn_atlon fr_n the HEAOA-3modulating colllrmtor
experiment (Rel. 4). Its optical spectrum was
found to contain broad emission lines of 1f, HeI and
HelI. Photcrnetrac variations were modulated at the
nominal 3 hour period as were measurements of
circular polarization. These properties are all
characteristic of AM Her type CVs.
On day 281-2 (UI'), 1985 a comprehensive
program of optical, X-ray and UV rr_nitorlng of
H0538+608 was carried out. 'l_is report is a
presentation of the data obtained w_th IUE and a
preliminary dtscussion of its implications. Parts
of the optical and X-ray are also presented.
2. OBSERVATIONS AND I_kTAANALYSIS
Since the cataloged X-ray intensities for
110538+608 are comparable to AM tfer itself and sxnce
we expected the UV continuum to scale to optical and
X-ray fluxes in a sJmllar rrmnner we expected to be
able to obtain well exposed S'VvP spectra in
30 - 45 minute integrations, le. rr_ch less than the
three hour period. Vvhat we found however, was that
this object re as deficient both in its soft X-ray and
UV intensity. Figures 1 and 2 show the 1150 -2000 A
(S_) spectra obtained for H0538+608 and a composite
X-ray, UV and optxcal spectrum. The integration
times for the SWP spectra are 70, 100 and 50
minutes. The optical and X-ray points on figure 2
are for approximately quarter phase; the UV points
are based on integrations outside of eclipse.
E_I __ i F J r
) 3
E-13
I
i_,_
n
$ E-13
-i
S_4P26B_8 _ 26 UT do_ 28Z 19B5
PhQte g I - 9 4
A Deeade of UV Astronomy wtth IUE, Proc. Celebratory Sympomum, GSFC, Greenbelt, USA, 12-15 April 1988, ESA SP-281, Vol. 1 (June 1988)
https://ntrs.nasa.gov/search.jsp?R=19890001332 2020-03-20T05:58:09+00:00Z
138 C R SCHRADER ET AL
E-IS
E-I_
E_I I
fl
E-i}L
4_L
S_P26899 ,Z O, UT day Z_Z 19,5
Phase , 5 - 1 8
1300
E-I_
o
0
-I
E-I_=
E_I S
S_Z6_ "4 05 UT doY Z"Z 198S
PholI 0 2 - " S
_$IrDl$
e
/ 3
c E-IS
I
2
/ 2
E-13
c
/
o
n
__-_
t
r
©
m
8
-!
E-l_z00 t L13,, 1400 15_ 16_ 171_K3 181fig
_$ trelt
Figure 1. _ spectra of H0538+608, (a) day 282,
1985 phase 0.1-0.4, (b) day 282, 1985
phase 0.5-1.0, (c) day 282, 1985 phase
0.2-0.5, (d) day 95, 1988 unknown phase
-25 I
E
_'_ -27
v
-2B
-/
-29
OPTICAL UV - X-RAY
SPECTRUM
-30 , , _ , I , ,_ t I , , r _ , , , , I , , , ,
14 15 16 17 I8 19
LOG FREQUENCY (H7}
Figure 2. Composite X-ray, UV and optical spectrum
for H0538+608
We had hoped to use the shape of the UV
continuum as a function of phase to test the
hypothesis that the net flux is a combination of
cyclotron emission from the accretion colunm and the
Rayleigh-Jeans tail of a black body. The later
_uld result from the reemission of hard photons off
the white dwarf surface at the base of the accretion
colunm (Ref. 5). This feature should appear as a
steepening of the $9¢P flux as seen for example, in
AM Her itself (Ref. 6) when the base of the
accretion colurrm is visible.
Since we do not have phase resolved spectra
and owing to the low S/N of the continuum in each of
the spectra we obtained, we cannot unambiguously
rule out the presence of a Rayleigh-Jeans component.
However, a power law in vcavelengthwas fitted to t-_
of the three spectra of day 282, 1985 by binning
portions of the continuum containing no apparent
features. The resulting power law indices were
-1.1!0.5 and -1.2+0.6. This is consistent with the
notion that the Rayleigh-Jeans component is absent.
It also reflects the overall (flat) nature of the
composite spectrum (figure 2).
As is the case with other Am Her objects
observed by IUE the S_ spectra have a number of
strong emission l_nes. The lines identified are
NV(1240A), S1111(1299A), SilV(1394A), CIV(1549A),
HelI(1640A) and NIV(1718A); table 1. So_ of the
emission lines are variable in intensity and some
line ratios are variable. In particular, the CIV is
nearly absent in one of the three _ spectra of day
282, 1985 (S'_¢P26900). This spectrum was the result
of a 50minute integration starting at about phase
0.2. Another spectrum, SVvP26898, also covers this
phase _nterval, but the CIV is clearly present. CIV
is generally weak relative to the other lines when
compared to other AM Her objects observed with IUE.
Another notable feature of the S'_¢P spectra" of
H0538+608 is the ancmalusly large (relative)
strength of the NV feature, as first pointed out by
Bonnet-Bidaud and Mouchet (Ref 7). This is more
pronounced for the day 282, 1985 spectra than for
the April 1988 spectra. The HeII feature appears
broadened towards its blue w_ng on s(mae of the
spectra. In SVvP26900, there is marginal evidence
for a P-Cygni profile in Hell, although the data
quality is poor.
IUE,EXOSAT&OPTICALOBSERVATIONSOFH0538+608 139
The lYe)SAT light curves (figure 3) have
several notable features. The soft X-ray flux (IA_;
0.2 - 1.0 keV) clearly shows Intensity modulation
with eclipsing at the 3.3 hour period. There are
also appear to be slight intensity dips at around
phase 0.5. The duration of the eclipses seems to
vary. The hard X-ray (/V_; 1.0 - 8.0 keV) Intensity
does not show eclipsing. This type of behavior is _ os
similar to that seen in EF Eri, for which the
eclipses are presumed due to obscurations of the u
magnetic polar cap (Refs. 8, 9). However, the ME
light curve is rather erratic, particularly on day o
281. It clearly shows aperiodic variation with
surges by up to a factor of =6. It is apparent fr_n
figure 3 that these are uncorrelated with the LE
light curve. The ME hardness ratio does not reflect
the intensity surges. TheME light curve of day 282
varies less dramatically than on day 281.
EB 30
t
I"XObAT l,l:' (0_ 10 l<cV) _
32 31 36 38
I]L, URb (UI) 198S DAY 2U;_
m
05
o
- O5
m 3
o e
' I ' I ,7 _ _ _ 1---' '-" I
EXOSAT LE (02 - 10 keV
4 6 B 10 12
II()UR$ (U1), 1985 DAY dfll
' I ' '--'--1 --_-_ ' I
EXOSAT ME (1 - 8 keY)
4 6 8
flOURS (UT}, 1985 DAY 281
10 12
Table 1
.........................................................
Net Flux (/ 10 -12 erg/cm2/sec) and
Equivalent Width (angstroms) of promlnlent eml$Jion llne_
.........................................................
N_ CIV Hell NIV
..........................................
3.7 0,65 1 4 0 36SWP26898
160 37 107 35
SWP26899 3 7 0 65 1.3 0 16
158 43 105 12
SWP26900 4.0 0 24 1.4 ---
252 16 115
S'_P33205 2 4 0 49 0 61 0 25
114 53 S7 27
_h_33206 2 3 0 47 0 75 0 45
163 35 260 27
Figure 3. X-ray light curves
Most of the planned phot_netric montoring of
H0538+608 concurrent with the I_OSAT and IUE
observations was lost to clouds. However, low
resolution optical spectra taken during November,
1985 clearly show spectral modulation at one half of
the orbital period (1.67 hours). The spectra
alternate between a reddish continuum with weak
emission lines and a bluish one with strong lines.
November 1986 observations showed similar
modulations, but at the full 3.3 hour period. This
could be an indication that accretion to a second
magnetic pole undergoes transitions between a high
and low state.
Our preliminary Interpretation of the X-ray
light curves is that H0538+608 exhibits accretion
flow variability on time scales less than the
orbital period. These instabilities could be a
variable accretion rate or distortions of the flow
gecrnetry. The latter case would Involve complex
rmgneto-hydrodynanuc effects with an entangling of
the gas stream and magnetic f_eld lines. One might
reasonably expect these type of phenc_nena to be
evident in the UV. The broad emission lines seen by
IUE are believed to originate in the base of the
accretion colurrm (eg. ref. 5). The UV continuum
is probably also variable, but it Is beyond the
capability of IUE to study this system on the
required t_rne scale. However, several possibly
related effects are seen in the IUE data.
The emission line intensities for 1-t0538+608,
particularly CIV are variable. Line ratios as well
as individual intensities show variability. It has
been pointed out by Bonnet-Bidaud and IV_uchet (ref.
7) that the NV feature is anc_nalously strong
relative to other lines. They suggest that th_s may
be indicative of non-solar abundances such as
produced In nova-type outbursts. HO538+608may be
representative of scrne short term evolutionary stage
for this class of objects. The classical nova V1500
Cyg has been identified as an AMHer type CV with
non-synchronous rotation (ref. 10). It is possible
that non-synchronisity could cause accretion
instability if the angle if the angle between the
magnetic and spin axes is sufficiently large.
Although there is no direct evidence of asynchronous
rotation in H0538+608 , we cite the possibility of a
connection between its erratic high energy behavior,
its an_mlous abundances and the systemV1500 Cyg.
140 CRSCHRADERET AL
3. DIS(EJSSION
H0538+608 appears to have several unique
characteristics among AM Her type objects studied.
It has a high X-ray to optical lumanosity ratio, yet
it has a relatively flat spectrum and surprisingly
Iow UV continuum luminosity. The emission lines
observed by IUE are variable, with CIV being
virtually undetected in one _ spectrum. Its X-ray
light curves display an erratic behavior which may
be indicative of either a variable accretion rate or
distortions of the accretion gec_aetry on time scales
less than the orbit period. Optical data taken
about one month later show a spectral rr_dulation as
well as intensity rr_dulation at one half of the
orbital period. Later optical monitoring shows
modulation at the orbital period indicating that one
pole may exhibit high/low states.
The X-ray light curves and the optical data of
November, 1985 are atypical of AM Her objects. The
S_P spectra also have unusual characteristics.
Could H0538+608 be representative of s_ne transient
evolutionary state? We speculate on a possible link
between the type of behavior seen in H0538+608 and
the system V1500 Cyg. In subsequent _rk, using
additional data we plan to address this issue in
more detail.
REFERENTS
1) Cordova, F. and Mason, K.
1983 in Accretion Driven Stellar X-ray Sources,
ed.W. Lewin and van den Heuvel
(Cambridge University Press).
2) Forman, W. et al.
1978 Ap. J., Supp., 38, 357.
3) Wood, K.S. et al.
1984 Ap. J., Supp., 56, 507.
4) Remillard, R. et al.
1986 Ap. J., Lett., 302, Lll.
5) Lamb, D.Q.
1985 in Cataclysmic Variables and Low-Mass X-Ray
Binaries ed. D.Q. Lamb and J. Patterson (Reidel).
6) Raymond, J. et al.
1979 Ap. J. Lett., 230, L95
7) Bonnet-Bidaud, J.M. and/vlouchet, M.
1987 As tr. Ap., 188, 89
8) Patterson, J.,W'llliams, Glen and Hiltner, W.A.
1981 Ap. J., 245, 618
9) Beuerman, K., Stella L., and Patterson J.
1987 Ap. J., 316, 360
lO) Stockman. et al.
1987 Bull. AAS, 19, 1058


Simultaneous IUE, EXOSAT and optical observations of the unusual AM Her type variable H058+608

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server