A&A 452, 955-958 (2006)
DOI: 10.1051/0004-6361:20054748
A. D. Schwope1 - M. R. Schreiber1 - P. Szkody2
1 - Astrophysikalisches Institut Potsdam, An der Sternwarte 16,
14482 Potsdam, Germany
2 -
Dept. of Astronomy, Box 351580, University of Washington, Seattle, WA 98195, USA
Received 22 December 2005 / Accepted 28 February 2006
Abstract
We report the detection of a 110 MG cyclotron harmonic in the
SDSS-spectrum of the magnetic cataclysmic variable (MCV) RX J1554.2+2721. This feature
was noted earlier by others but remained unexplained. The wide spectral
coverage of the new spectrum together with the earlier detection of a Zeeman
split Ly line in a field of 144 MG makes
the identification almost unambiguous.
We propose to explain the non-conforming
UV-optical photospheric temperature of the white dwarf
by an as yet unobserved cyclotron component
in the ultraviolet which also could significantly contribute to the overall
energy balance of the accretion process.
Key words: stars: novae, cataclysmic variables - stars: individual: RX J1554.2+2721- radiation mechanisms: thermal - accretion, accretion disks - while dwarfs - binaries: close
RX J1554.2+2721 was identified as a CV in the Hamburg Quasar Survey (Jiang et al. 2000)
and independently by Tovmassian et al. (2001, henceforth TEA01) as the
optical counterpart of a soft
RASS source. Its magnetic nature was suggested by Tovmassian et al. who
also tentatively identified low frequency flux variations as cyclotron
harmonics in a field of 30 MG. They also recognized its period in the
CV period gap. Thorstensen & Fenton (2002, henceforth TF02) determined an
accurate orbital
period of
min right in the centre of the period gap
and
estimated the distance to the system on the basis of spectral features of the
secondary star to be roughly 210 pc. They also noted a pronounced
double-humped structure of the I-band light curve due to ellipsoidal
modulation of the secondary and a spectral hump of possible magnetic origin at
around 5000 Å. Finally, Gänsicke et al. (2004, henceforth GEA04)
reported the detection
of Zeeman split Ly
absorption lines in a HST-STIS snapshot
spectrum. The UV data were successfully modeled with a centered dipole model
with polar field strength 144 MG and photospheric temperature of 17 500 K,
thus making RX J1554.2+2721 only the third polar with a field strength in excess of
100 MG. The combined optical/UV spectral energy distribution, however,
suggested a significantly higher temperature of about 23 000 K.
![]() |
Figure 1: SDSS-spectrum of RX J1554.2+2721 obtained May 8, 2005. Also shown is the ugriz photometry obtained June 22, 2003. |
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Our review on RX J1554.2+2721 is motivated by a single spectrum
taken by the SDSS on May 8, 2005. The spectrum of the object with designation
SDSS J155412.33+272152.4, obtained with an exposure time of 46 min,
is reproduced in Fig. 1. Also shown in the figure is the SDSS
photometry obtained June 22, 2003, with
,
which shows very little deviation from the
spectroscopy. For details of the SDSS project the reader is refered to
Adelman-McCarthy et al. (2006),
Fukugita et al. (1996),
Gunn et al. (2006), and
York et al. (2000).
The accumulated phase uncertainty at the time of the spectral observations,
i.e. after 13 456 cycles according to the
ephemeris of TF02 is
,
hence we cannot assign a proper
orbital phase to the spectrum.
The main features of the spectrum are unchanged with
respect to the observations by TF02 and TEA01, the prevalence of the M4
secondary in the red spectral range, a hump centered on
5000 Å,
prominent high-state emission lines of H and He and a spectral upturn at the
blue end.
The new SDSS spectrum has a continuum flux level very similar to the March 2001 low-state spectrum by TF02 (their Fig. 3b) but a very different emission
line spectrum containing even lines of ionized helium. The low-state spectrum
of TF02 has 20%
less flux in the range 5000-7000 Å, 10% more flux at 4600 Å and
matches exactly at 7600 Å, where mainly the secondary contributes.
The pronounced difference in the line spectrum between TF02 low state
and SDSS suggests the
presence of a soft X-ray ionizing source at the time, when the SDSS
spectrum was taken, hence a certain level of
accretion. The SDSS-spectrum was thus not obtained in a low accretion state
although the continuum flux level might be regarded as indicative of a low
state.
The May 2001 high-state spectrum of TF02 has a very similar line spectrum compared to the SDSS spectrum but an enhanced continuum flux by about 20-30%. The red part of the high-state spectrum by TEA01 (their Fig. 4b) matches exactly the new SDSS spectrum while the blue part of their spectrum taken one night later is again about 25% brighter. Taken together, neither the orbital variability nor the rather frequent changes between high and low states seem to have a pronounced influence on the system's brightness (although one must admit that high-state photometry in the blue part of the spectrum is missing).
The atomic emission lines in the SDSS-spectrum seem to be unusually sharp and
peaked. They can be fit with the superposition of a broad and a narrow Gaussian
line with
km s-1 and
250-300 km s-1 (after deconvolution with the instrumental profile).
These narrow lines appear wider than the narrow emission
lines in other polars which are of reprocessed origin from the secondary
(
km s-1) but velocity smearing is likely be responsible for the
relative large width.
The broad line clearly indicates an origin in an accretion stream.
The very feature which makes the spectrum of RX J1554.2+2721 outstanding is the hump
centered
on 4950 Å, which we regard as a cyclotron emission
line, either the cyclotron fundamental or a low cyclotron harmonic.
This feature was
present in the spectra of TEA01 and TF02 too but didn't show up so prominently
due to the shorter wavelength coverage and blending with atomic emission
lines.
![]() |
Figure 2: Cyclotron spectrum of RX J1554.2+2721 computed by sutraction of suitably scaled M-dwarf and white dwarf spectra. The model spectra for 5 and 10 keV are plotted with an offset of -5 units. |
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The difference spectrum is reproduced in Fig. 2.
It shows a single cyclotron harmonic rising sharply at 4600 Å and
dropping of
at 5450 Å. The red continuum has more flux than the blue continuum
between 4150-4600 Å. The use of a cooler 15 000 K white dwarf instead of
the 20 000 K does not cure the problem.
Either there is an emission component in
the red part of the spectrum or the blue part is affected by Zeeman
absorption. Again, on the basis of just one spectrum we cannot decide between
the alternatives but our main conclusions are not affected by these
uncertainties.
The lack of any neighboring cyclotron harmonic excludes any low-field/high-harmonic interpretation of the observed structure. The 144 MG field determined by GEA04 suggests an identification with either the cyclotron fundamental or the second harmonic. If it would be the fundamental then the derived field strength would be 220 MG. This exceeds the implied polar field strength by a large amount and is regarded as the less likely, although not impossible interpretation. If for instance the photospheric spectrum would be obtained under an unfavourable viewing geometry or the field structure deviate from the centered dipole model, as many polars do, the assumed polar field strength can be very much different from the real value. However, unless other evidence is available the identification with the second harmonic seems plausible.
We have modeled the harmonic with isothermal plasma cyclotron radiation using
the code described in previous papers (Schwope et al. 1990). Two model curves
with slightly different field strength B, plasma temperature kT, aspect
angle ,
but the same density parameter
are also
shown in Fig. 2. The density parameter
essentially determines where the spectrum turns from the low-frequency
optically thick to the high-frequency optically thin part.
The angle
is the angle between the line
of sight and the magnetic field.
The narrower line was calculated for kT=5 keV,
B=108 MG,
,
,
the broader line for
10 keV, 113 MG,
,
and
.
The width of individual cyclotron lines is a function of
and T, but a scatter in these values and, in particular,
a scatter in B may further broaden individual lines.
We cannot discern between these different
broadening mechanisms and the models were calculated with the most simple
assumptions, a homogeneous plasma with
adjusted to match the observed
width of the line.
While RX J1554.2+2721 seems to belong to the normal accreting polars displaying high and low accretion states, the absence of a pronounced increase in the continuum when switching from high to low states is puzzling. One distinct possibility is that the main accretion region is continuously hidden from our view due to an unfavourable geometry. In this model the observed cylotron harmonic would belong to a secondary region with much lower accretion rate (as in VV Pup or UZ For, see Wickramasinghe et al. 1989; Schwope & Beuermann 1997; Schwope et al. 1990). This scenario can be tested by a decent X-ray observation in a high accretion state (indicated by the presence of strong atomic emission lines) which should result in a very low X-ray signal.
Here we discuss shortly an alternative model.
Beuermann (2004) discussed a synthesized spectrum of AM Her composed of a
multi-temperature, multi-density plasma due to contributions from regions with
a wide range of specific mass accretion rates. Following his arguments we may
expect the major part of the cyclotron radiation emitted at higher harmonic
number in the ultraviolet. Assuming a similar scenario for RX J1554.2+2721 we can give a rough estimate of those suspected contributions from the
ultraviolet-to-optical spectral energy distribution shown in
Fig. 3. The two observed spectra shown there are the STIS-spectrum
of GEA04 and the SDSS spectrum. GEA04 noticed that a higher temperature is
needed to match the SED than derived from just fitting the UV-spectrum. We
argue that the inclusion of a cyclotron component with spectral parameters that
one usually encounters in high-state polars solves the discrepancy. We
illustrate this with a composite spectrum consisting of the scaled M-dwarf
GJ 268, the 20 000 K magnetic white dwarf model of GEA04 and two cyclotron
models, one with the extreme small density parameter
,
the
other with the more typical parameter
.
The synthetic
spectrum is not meant to be a proper fit of the data, since there are too
many uncertainties in the spectral decomposition
but merely an illustration of the
possible spectral composition. A proper composed synthetic spectrum
will probably explain the upturn at the blue end of the SDSS-spectrum, where
the third harmonic starts to rise. The integrated flux of our composite
cyclotron spectrum is
erg cm-2 s-1, a factor
30 higher than just the flux in the second harmonic and of the
same order as the ROSAT
X-ray flux in the 0.1-2.4 keV band (TEA01). Neglecting the UV-cyclotron
component would result in a heavily biased energy balance of the accretion
process for this particular source.
![]() |
Figure 3: Combined optical/ultraviolet spectrum of RX J1554.2+2721 using the STIS spectrum of GEA04 and the new SDSS spectrum. The composite model spectrum shown in blue consists of a suitably scaled M-star spectrum (red), two cyclotron model spectra (green) and the 20 000 K white dwarf model of GEA04. |
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At a field strength of 110 MG one can access the very low harmonic numbers usually hidden in the infrared or far infrared. This gives access to those parts of a structured accretion region with very low specific mass accretion rates. We propose to explain the non-conforming temperature estimates from UV and optical spectroscopy by a missed cyclotron component in the ultraviolet, which carries most of the cyclotron luminosity. Neglecting such a component could result in a heavily biased energy balance of the accretion process towards thermal plasma radiation in the X-ray regime. A full decomposition of the spectrum requires low-resolution spectroscopy/spectrophotometry with full phase coverage, both in the optical and the ultraviolet.
Acknowledgements
This project is supported by the Deutsches Zentrum für Luft- und Raumfahrt (DLR) under contract no. FKZ 50 OR 0404 (MRS) and by NSF grant AST 97-30792 (PS).
Funding for the Sloan Digital Sky Survey (SDSS) has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Aeronautics and Space Administration, the National Science Foundation, the US Department of Energy, the Japanese Monbukagakusho, and the Max Planck Society. The SDSS Web site is http://www.sdss.org/
The SDSS is managed by the Astrophysical Research Consortium (ARC) for the Participating Institutions. The Participating Institutions are The University of Chicago, Fermilab, the Institute for Advanced Study, the Japan Participation Group, The Johns Hopkins University, the Korean Scientist Group, Los Alamos National Laboratory, the Max-Planck-Institute for Astronomy (MPIA), the Max-Planck-Institute for Astrophysics (MPA), New Mexico State University, University of Pittsburgh, University of Portsmouth, Princeton University, the United States Naval Observatory, and the University of Washington.