Aim:
This catalog comprises an up-to-date (December 1999) list of luminous
(>1036 erg/s), binary supersoft X-ray sources.
This Web-version supersedes the printed version of Greiner (1996).
Introduction:
After the discovery of supersoft X-ray sources with Einstein Observatory
observations, the ROSAT satellite with its PSPC detector has discovered about
four dozen new supersoft sources and has thus established luminous
supersoft X-ray sources (SSS) as a new class of objects. Though many different
classes of objects emit supersoft X-ray radiation (defined here as emission
dominantly below 0.5 keV which corresponds to effective temperatures
of the emitting objects of <50 eV), we consider here sources with
bolometric luminosities in the range 1036-1038 erg/s.
Optical observations have revealed the binary nature
of several of these objects.
A white dwarf (WD) model, the so-called close-binary supersoft source (CBSS)
model, is perhaps the most promising (van den Heuvel et al. 1992;
Rappaport, DiStefano, Smith 1994). It invokes steady-nuclear burning on the
surface of an accreting WD as the generator of these systems' prodigious flux.
Indeed, SSS temperatures and luminosities as derived from the X-ray data
suggest an effective radius comparable to that of WDs.
Eight SSSs have orbital periods between approximately 4 hrs and 3.5 days.
These are the candidates for the CBSS model.
Mass transfer rates derived from the CBSS model are in the right range for
steady nuclear burning of the accreted matter.
This catalog comprises an up-to-date (December 1999) list of luminous
(>1036 erg/s) supersoft X-ray sources.
We include in this catalog accreting binary sources of high luminosity which
are thought to be in a state of (steady or recurrent) hydrogen burning.
Since CAL 83, the prototype, is known to have an ionisation nebula
(Pakull and Motch 1989), and
further supersoft binaries are expected to also have one, we include also
sources associated with very luminous planetary nebulae.
Not included are the low-luminosity objects like single (i.e. non-interacting)
white dwarfs and magnetic cataclysmic variables, and
PG 1159 stars which reach similar luminosities but form a rather distinct class
(e.g. Dreizler et al. 1995).
Excluded are also supersoft active galactic nuclei which reach luminosities
up to 1045 erg/s, and the recently found examples of
large-amplitude outbursts of supersoft X-ray emission which have been
interpreted as tidal disruption events (e.g. Komossa & Greiner 1999).
Since most of the new sources are X-ray discoveries, the final inclusion in
the group of luminous close binary supersoft sources has to await the optical
identification. Only then a distinction is possible among the various and
quite different types of objects which show a supersoft X-ray spectrum
(i.e. emission only below 0.5 keV) but have different luminosities.
Due to this fact of necessary follow-up optical observations, it can well
happen that a source is included in an early version of the catalog but later
turns out to be of a different type.
An example is RX J0122.9-7521 which has long been thought
to be a SMC supersoft source (Kahabka et al. 1994), but has been identified
as a galactic PG 1159 star (Cowley et al. 1995, Werner et al. 1996),
and therefore has been removed from this catalog.
Einstein, ROSAT and beyond...
The two most famous supersoft X-ray sources, CAL 83 and CAL 87
(Long et al. 1981), have been discovered with Einstein satellite observations.
ROSAT observations established these sources as a distinct class in the early
nineties, and
the majority of the X-ray measurements have been performed with the ROSAT
position sensitive proportional counter (PSPC) during 1990-1995,
yielding a source position accuracy of about 25''.
The PSPC with its spectral resolution of about 50% below 1 keV
has been used in nearly all cases to discover the supersoft X-ray spectrum.
During the years 1995-1999 the high-resolution imager (HRI) on ROSAT has been
used to improve the coordinates of
the newly detected sources down to typically 10''
and to monitor the long-term X-ray intensity.
At these soft energies, the HRI count rates are typically a factor of 7.5-8
smaller than those of the PSPC (David et al. 1994, Greiner et al. 1996a).
Since 1997, some of the brightest supersoft X-ray sources have been also
observed with the low-energy concentrator spectrometer (LECS) onboard
BeppoSAX. As of the time of this writing, there is only one SSS (U Sco)
which has not been observed by ROSAT, and consequently has no entry in
the "ROSAT count rate" field.
With the Chandra and XMM missions starting regular observations, a wealth
of new information on the X-ray properties of these supersoft sources
can be expected, as well as
new discoveries. In particular, the better energy resolution, through-put
and location accuracy will improve our understanding. Therefore, this will
most probably not be the last version of a SSS catalog.
Organisation of the catalog:
The catalog is organised as follows:
The catalog consist of four major parts:
a master table,
a bibliography,
a query form,
and the individual source pages. While the master table
should provide some basic numbers, the
individual source pages are thought to provide some more parameters as well as
the links to the details behind these numbers like graphs and references.
- The
master table gives an overview of
all sources with their main
characteristics, including ROSAT PSPC count rate, temperature, luminosity,
type of binary and orbital period (if known).
- The
bibliography contains relevant papers,
sorted chronologically, and alphabetically within each year. It is supposed
to be complete for the CBSS type sources, while for the other sources
only those papers where included which provided input for the source
parameters. An attempt has been made to provide a direct link to all papers
which are electronically available (either directly from the journal pages
or through ADS).
- The
query form allows you to interactively
create new tables for a sub-sample of sources and arbitrary parameters
of your choice.
- The individual source tables provide information on a variety of source
parameters, including figures (e.g. of spectra or light curves) and links
to references. All source related numbers appear in red, and links in blue.
Blue "numbers" correspond to the reference with the same number at the bottom
of the page, and the underlying link points to the bibliography.
Blue "words" contain links to figures which will open in a separate window
(this separate window is used for all subsequent links too).
The data are organized as follows:
- The top part contains the name, coordinates (equinox 2000.0),
the type of binary, the ROSAT count rate, a link to the discovery paper and
a finding chart. The "Discovery" reference refers to the first paper
which realised the luminous supersoft X-ray emission. Some of the sources
have been known already for decades at this time, so there may be many
references listed which appeared earlier than the "Discovery" reference.
If a source is not optically identified, the R.A./Dec. numbers are
the X-ray positions with uncertainties as given in the above section of ROSAT
related issues. Otherwise, the optical positions are given with typical
uncertainties of 1''. All coordinates are equinox 2000.0.
- The next (table) block contains general data of the system like distance,
orbital period, brightness and color. The radial velocity amplitude
is given for the HeII 4686 A line; note that it is common in CBSS that
other emission lines as well as the Balmer absorption lines have quite
different velocity amplitudes!
The right column provides links to spectra in all wavelength ranges.
For sources supposed to belong to an external galaxy, the galaxy name
is given
instead of the presently known distance. This is motivated by recent changes
in the distance determination of the LMC.
This in turn also affects the
distance to M31 because the distance ratio of LMC and M31 is more accurately
known than the corresponding absolute distances.
Note that different authors may have used different distances for sources
within the same galaxy, so that a simple comparison of the luminosities
may be misleading!
The galactic absorbing column (NHgal) is taken from
Dickey and Lockman 1990) and
is given for comparison with the values derived from the X-ray fits.
- The next table block provides details on the X-ray spectral fitting. Here,
a distinction is made between the use of a simple blackbody model versus
more sophisticated white dwarf atmosphere models. While in general the
blackbody overestimates the bolometric luminosity by a large factor (10-100),
it should be noted that there are a variety of white dwarf atmosphere models
on the market which have been used by different authors for different sources.
Thus, special care is needed when comparing these parameters among
different sources!
- The next table block gives hints on the variability of the source,
separated into orbital modulation and non-orbital, intrinsic variability.
The comments are short, so please use the links to check out the original
papers to get a complete picture about the source's behaviour.
- The final table block applies to only a few sources, for which
optical measurements of an ionisation nebula or bipolar outflow or
radio measurements have been conducted. The radio fluxes (limits)
are given for 3.5 cm.
- Finally, the references are given in full with all co-authors, and
stating the important pieces of new information (this reflects purely my
subjective view, and you should contact me if something is
missing or wrong.) The references are sorted in time of appearance, so that the
numbers in the data blocks will not change when the catalog is updated.
Warning and Request:
I would like to emphasise that every user of this catalog should spare no pains
to consult the original papers in order to avoid propagation of my errors in
the literature. I will keep this catalog updated, and would appreciate
(1) being informed on any errors/omissions users might discover and
(2) getting reprints of papers on supersoft sources to be included in
the next version.
Availability and
Reference:
This catalogue is freely available for everybody, but should not be copied
or sold to other parties.
It is an unfortunate aspect of modern science funding that impersonal and
statistical measures are used to assess the productivity and usefulness of
persons/programs. Therefore,
if the catalogue was helpful for your research work,
a reference to the published version
J. Greiner, 2000, New Astr. 5, 137
would be appreciated.
Acknowledgements:
I appreciate the help of many collegues who sent preprints and reprints of
their work. Special thanks to Rosanne Di Stefano for her steady
encouragement
to produce this catalog and for extensive discussion on its content.
This Web-version would not have been possible without the enormous
help by Arne Rau and Robert Schwarz who wrote the Perl and Java scripts
for the generation of the source pages.
I apologise to anyone whose paper slipped through the literature search.
The ROSAT project was supported by the German Bundesministerium für
Bildung, Forschung, Wissenschaft und Technik (BMBW/DARA) and the
Max-Planck-Society. This research has made use of the Simbad database,
operated at CDS, Strasbourg, France.
References:
- Cowley A.P., Schmidtke P.C., Hutchings J.B., Crampton D., 1995,
PASP 107, 927
- David L.P., Harnden F.R., Kearns K.E., Zombeck M.V., 1994,
The ROSAT HRI manual, GSFC
- Dickey J.M., Lockman F.J., 1990,
ARAA, 28, 215
- Dreizler S., Werner K., Heber U., 1995,
in White Dwarfs, eds. D. Köster and K. Werner, Lecture Notes in
Physics 443, Springer, Berlin, p. 160
- Greiner J., 1996,
Supersoft X-ray Sources,
Proc. of workshop held in Garching (Germany), Feb. 28 - Mar 1 1996,
Lecture Notes in Physics, vol. 472, Springer, Berlin
- Greiner J., Schwarz R., Hasinger G., Orio M., 1996,
A&A 312, 88
- Kahabka P., Pietsch W., Hasinger G., 1994,
A&A 288, 538
- Komossa S., Greiner J., 1999,
A&A 349, L45
- Pakull M.W., Motch C., 1989,
in Extranuclear Activity in Galaxies,
ed. E.J.A. Meurs, R.A.E. Fosbury, (Garching, ESO), p. 285
- Rappaport S., DiStefano R., Smith J.D., 1994,
ApJ 426, 692
- van den Heuvel E.P.J., Bhattacharya D., Nomoto K.,
Rappaport S.A., 1992,
A&A 262, 97
- Werner K., Wolff B., Cowley A.P., Schmidtke P.C.,
Hutchings J.B., Crampton D., 1996, in Supersoft X-ray Sources,
ed. J. Greiner,
Lect. Notes
in Phys. 472, Springer, p.
131
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This page is maintained by Jochen Greiner
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last edited: May 11, 2000