arXiv subjects

R. Schwarz

Publications and source records attributed to R. Schwarz.

104 records · Page 6Linked to original sources

arXiv2002

A new two-pole accretion polar: RX J1846.9+5538

We report the discovery of a new, bright (V = 17 mag) AM Her system as the optical counterpart of the soft ROSAT All-Sky-Survey source RX J1846.9+5538 (= 1RXS J184659.4+553834). Optical photometric and spectroscopic follow-up observations reveal a single period of 128.7 min, consistent with a high degree of spin-orbit synchronization, and a low polar field strength ($B<20$ MG) of the primary accretion region. The system was observed in optical intermediate and high states that differ by about 1 mag. These brightness variations were accompanied by a correlated change of the optical light curve, which we interpret as a switch between one- and two-pole accretion. This explanation is also supported by the X-ray light curves, which at two different epochs display emission from two equally bright accretion regions separated by 160 degrees. Both spots possess distinct spectral X-ray properties as seen from the X-ray hardness ratio, where the secondary accretion region appears significantly softer, thus probably indicating a higher field strength compared to the primary region. In all ROSAT pointings a deep dip is present during the primary flux maxima, very likely caused by absorption in one of the accretion streams.

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arXiv2001

An XMM-Newton timing analysis of the eclipsing polar DP Leo

We present an analysis of the X-ray light curves of the magnetic cataclysmic variable DP Leo using recently performed XMM-Newton EPIC and archival ROSAT PSPC observations. We determine the eclipse length at X-ray wavelengths to be 235 +-5 s, slightly longer than at ultra-violet wavelengths, where it lasts 225s. The implied inclination and mass ratio for an assumed 0.6 M(sun) white dwarf are i=79.7 degrees and Q = M(wd)/M2 = 6.7. We determine a new linear X-ray eclipse and orbital ephemeris which connects the more than 120000 binary cycles covered since 1979. Over the last twenty years, the optical and X-ray bright phases display a continuous shift with respect to the eclipse center by \~2.1 degr/yr. Over the last 8.5 years the shift of the X-ray bright phase is \~2.5 degr/yr. We interpret this as evidence of an asynchronously rotating white dwarf although synchronization oscillations cannot be ruled out completely. If the observed phase shift continues, a fundamental rearrangement of the accretion geometry must occur on a time-scale of some ten years. DP Leo is marginally detected at eclipse phase. The upper limit eclipse flux is consistent with an origin on the late-type secondary, L_X ~ 2.5 x 10**(29) ergs/s (0.20-7.55 keV}), at a distance of 400 pc.

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arXiv2001

BZ Cam during its 1999/2000 optical low state

We report optical observations of the VY Scl star BZ Cam during its previous optical low state in 1999/2000. We find drastic variations in the line profiles. Narrow-band imaging observations show that its nebula extends farther than previously known and seems to be composed of two components. We determine the [OIII] line intensity of BZ Cam's nebula to 4.8 * 10^-13 erg/cm^2/s. We discover a proper motion of BZ Cam of 25+-2 mas/yr which together with the systemic radial velocity yields a space velocity of 125 km/s. We re-interpret the nebula as being photo-ionized by hypothesized transient, luminous, supersoft X-ray emission during optical low states, and shaped by the transverse motion of BZ Cam.

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arXiv2001

System parameters of the long-period polar V1309 Ori

Based on high-resolution optical spectroscopy in the blue and the near infra-red spectral range, we derived velocity images (Doppler tomograms) of the mass-donating secondary star and the accretion stream of the long-period eclipsing polar V1309 Ori (RX J0515.41+0104.6). Combined with HST-spectroscopy of high time resolution and optical photometry we were able to derive the main system parameters and to determine the accretion geometry of the binary. The length of the eclipse of the white dwarf is Delta t_{ecl} = 2418 +/- 60 s, the mass ratio Q = 1.37 - 1.63, and the orbital inclination i=76.6 - 78.9 degrees. The surface of the secondary star could be resolved in the Doppler image of NaI absorption lines, where it shows a marked depletion on the X-ray irradiated side. The accretion geometry proposed by us with a nearly aligned rotator, co-latitude delta ~ 10 degrees, tilted away from the ballistic stream, azimuth chi ~ -35 degrees, explains the shape of the emission-line Doppler tomograms and the shape of optical/UV eclipse light curves.

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arXiv2000

RX J1420.4+5334 - another tidal disruption event?

We have discovered a transient X-ray source, RX J1420.4+5334, which displays a ROSAT flux variation of >150 between the ROSAT All-Sky-Survey in 1990 and a preceding pointed ROSAT observations in July 1990. Optical observations suggest a non-active galaxy as the only visible counterpart. We therefore tentatively identify RX J1420.4+5334 as a tidal disruption event in a non-active galaxy.

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arXiv2000

The ROSAT Bright Survey: II. Catalogue of all high-galactic latitude RASS sources with PSPC countrate CR > 0.2 s-1

We present a summary of an identification program of the more than 2000 X-ray sources detected during the ROSAT All-Sky Survey (Voges et al. 1999) at high galactic latitude, |b| > 30 degr, with countrate above 0.2 s-1. This program, termed the ROSAT Bright Survey RBS, is to more than 99.5% complete. A sub-sample of 931 sources with countrate above 0.2 s-1 in the hard spectral band between 0.5 and 2.0 keV is to 100% identified. The total survey area comprises 20391 sq.deg at a flux limit of 2.4 \times 10^{-12} erg/cmsq/s in the 0.5 -- 2.0 keV band. About 1500 sources of the complete sample could be identified by correlating the RBS with SIMBAD and the NED. The remaining 500 sources were identified by low-resolution optical spectroscopy and CCD imaging utilizing telescopes at La Silla, Calar Alto, Zelenchukskaya and Mauna Kea. Apart from completely untouched sources, catalogued clusters and galaxies without published redshift as well as catalogued galaxies with unusual high X-ray luminosity were included in the spectroscopic identification program. Details of the observations with an on-line presentation of the finding charts and the optical spectra will be published separately. Here we summarize our identifications in a table which contains optical and X-ray information for each source. As a result we present the most massive complete sample of X-ray selected AGNs with a total of 669 members and a well populated X-ray selected sample of 302 clusters of galaxies with redshifts up to 0.70. Three fields studied by us remain without optical counterpart (RBS0378, RBS1223, RBS1556). While the first is a possible X-ray transient, the two latter are isolated neutron star candidates (Motch et al. 1999, Schwope et al. 1999).

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arXiv1999

RX J0537.7-7034: The shortest-period supersoft X-ray source

We present new photometric and spectroscopic observations of the transient supersoft X-ray source RX J0537.7-7034 and find a periodicity of approximately 3.5 hrs. This establishes RX J0537.7-7034 as the supersoft X-ray source with the shortest orbital period. We furthermore derive an inclination of the binary system of 45<i<70 deg, and the masses of the two binary components: M_accretor = 0.6+-0.2 Msun, M_donor = 0.35+-0.02 Msun. This implies that the standard scenario of supersoft X-ray sources, in which the donor is thought to be more massive than the accreting white dwarf to ensure high mass transfer rates on a thermal timescale (van den Heuvel et al 1992), is not applicable for this system. We discuss alternative interpretations of this source as a former nova in which the thermonuclear flashes have become mild and most accreted mass is retained by the white dwarf (SMC 13 type systems), or as a self-sustained wind-driven system.

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arXiv1999

Search for X-ray Afterglows from Gamma-Ray Bursts in the RASS

We report on a search for X-ray afterglows from gamma-ray bursts using the ROSAT all-sky survey (RASS) data. If the emission in the soft X-ray band is significantly less beamed than in the gamma-ray band, we expect to detect many afterglows in the RASS. Our search procedure generated 23 afterglow candidates, where about 4 detections are predicted. Follow-up spectroscopy of several counterpart candidates strongly suggests a flare star origin of the RASS events in many, if not all, cases. Given the small number of events we conclude that the data are consistent with comparable beaming angles in the X-ray and gamma-ray bands. Models predicting a large amount of energy emerging as a nearly isotropic X-ray component, and a so far undetected class of ``dirty fireballs'' and re-bursts are constrained.

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arXiv1999

Search for GRB X-ray afterglows in the ROSAT all-sky survey

We describe a search for X-ray afterglows from gamma-ray bursts using the ROSAT all-sky survey (RASS) data. If the emission in the soft X-ray band is significantly less beamed than in the gamma-ray band, we expect to detect many afterglows in the RASS. We found 23 afterglow candidates, where about 4 detections are predicted. However, follow-up spectroscopy of several counterpart candidates strongly suggests a flare star origin of the RASS events in many cases. We conclude that the RASS data are consistent with comparable beaming angles in the X-ray (within a few hours of the burst) and gamma-ray bands. However, models predicting a large amount of energy emerging as a nearly isotropic X-ray component of the early afterglow are severely constrained by the ROSAT data. In particular, a so far undetected class of ``dirty fireballs'' and delayed ``rebursts'' are constrained.

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arXiv1999

Zeeman lines and a single cyclotron line in the low-accretion rate polar 1RXS J012851.9-233931 (RBS0206)

We present a low-resolution discovery spectrum and CCD-photometry of the bright X-ray source 1RXS J012851.9-233931 found in the ROSAT All-Sky Survey. These first observations suggest that the source is an AM Herculis star (polar) accreting at a low rate. The optical spectrum is dominated by Zeeman absorption features from the white dwarf, indicating a mean photospheric magnetic field of 36 +- 1 MG. Only weak Balmer line emission was observed. In the near infra-red, a single intense cyclotron hump was observed. The inferred magnetic field strength in the accretion plama is 45 +- 1 MG, the temperature in the plasma is below 2 keV. Likely orbital periods are ~90 min or ~146 min, the latter inside the cataclysmic variable period gap. The system is an ideal target for further detailed investigations of the field structure on a magnetic white dwarf by phase-resolved spectropolarimetry.

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arXiv1998

The isolated neutron star candidate RBS1223 (1RXS J130848.6+212708)

In the ROSAT Bright Survey (RBS) we have almost completely optically identified the brightest ~2000 high-galactic latitude sources from the ROSAT All-Sky Survey Bright Source Catalogue (1RXS). A small number of sources has empty X-ray error circles on optical images. ROSAT HRI follow-up observations of RBS1223 (=1RXS J130848.6+212708), a soft object with extreme X-ray to optical flux ratio, have confirmed a relatively bright X-ray source, whose position could be determined to an accuracy of 1.6 arcsec (90%) due to the presence of a nearby, X-ray detected bright star. Deep Keck R- and B-band images of the field were taken, but the refined X-ray error circle remains empty to a limiting magnitude B ~ 26m. With an X-ray to optical flux ratio of log (f_X/f_opt)>4.1 this object is almost certainly an isolated neutron star, similar to the two so far best-known examples RX J1856.4-3754 and RX J0720.4-3125. We discuss limits on the number of similar objects in the RBS catalogue.

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arXiv1997

Polars -- multisite emission -- multiwavelength observation

We review the main observational characteristics of AM Herculis stars (polars) at X-ray, EUV, UV, IR and optical wavelengths. Particular emphasis is given to multi-epoch, multi-wavelength observations of the eclipsing polar HU Aqr (RX J2107.9-0518). In AM Herculis stars the broad-band spectral energy distribution from X-rays to the IR is governed by only very small structures: the hot accretion regions on the footpoints af accreting field lines. The most extended structures in the binary systems on the other hand, the mass-donating secondary stars and the accretion streams, distinctly appear only as Doppler-shifted emission or absorption lines. They can best be studied by investigating selected narrow spectral features in the optical, ultraviolet or the near infrared. In this contribution both aspects will be highlighted, the structure of the accretion regions as inferred from multi-wavelength observations with low or no spectral resolution, as well as the structure of the secondary stars and the accretion streams as inferred from high-resolution spectral observations and Doppler mapping.

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arXiv1996

The Long-Term X-Ray Lightcurve of RX J0527.8-6954

We present further observational data of the supersoft X-ray source RX J0527.8-6954 exhibiting a continuous decline over the past 5 yrs. With no clear trend of a concordant temperature decrease this might suggest an evolutionary scenario where the white dwarf leaves the steady burning branch and the combined effect of reduced luminosity and cooling at constant radius produces the observed effect. We also use the on-axis ROSAT HRI pointings to derive an improved X-ray position and supply a finding chart for the counterpart search.

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arXiv1996

The long-term X-ray lightcurve of RX J0527.8--6954

Supersoft X-ray sources are commonly believed to be stably burning white dwarfs. However, the observations of some supersoft sources show dramatic variability of their X-ray flux on timescales ranging from days to years. Here, we present further observational data of the supersoft X-ray source RX J0527.8--6954 exhibiting a continuous decline over the past 5 yrs. With no clear trend of a concordant temperature decrease this might suggest a evolutionary scenario where the WD leaves the steady burning branch and the combined effect of reduced luminosity and cooling at constant radius produces the observed effect.

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