arXiv subjects

J. Greiner

Publications and source records attributed to J. Greiner.

At least 343 records · Page 19Linked to original sources

arXiv1996

The UV/X-ray emission of the symbiotic star AG Draconis during quiescence and the 1994/1995 outbursts

We present the results of an extensive campaign of coordinated X-ray (ROSAT) and UV (IUE) observations of the symbiotic star AG Dra during a long period of quiescence (1990-1993) followed by two optical outbursts in 1994 and 1995. The hot component (i.e. X-ray emitting compact object) turns out to be very luminous during the quiescent phase: (9.5+/-1.5)*10^36 (D/2.5 kpc)^2 erg/s. This suggests that the compact object is burning hydrogen-rich matter on its surface even in the quiescent (as defined optically) state at a rate of (3.2+/-0.5)*10^-8 (D/2.5 kpc)^2 M_sun/yr. Assuming a steady state, i.e. burning at precisely the accretion supply rate, this high rate suggests a Roche lobe filling cool companion though Bondi-Hoyle accretion from the companion wind cannot be excluded. We have discovered a remarkable decrease of the X-ray flux during both optical maxima. In the UV these events were characterized by a large increase of the emission line and continuum fluxes. The anticorrelation of X-ray/UV flux and optical brightness evolution is very likely due to a temperature decrease of the hot component which could be the result of an increased mass transfer to the burning compact object, causing it to slowly expand to about twice its original size during each optical outburst.

astro-ph↗

arXiv1996

RXTE observations of GRS 1915+105

The galactic superluminal motion source GRS 1915+105 was observed with RXTE at several occasions during its ongoing active state. The observed X-ray intensity changes drastically on a variety of time scales ranging from sub-seconds to days. In particular, the source exhibits quasi-periodic brightness sputters with varying duration and repetition time scale. These episodes occur occasionally, while the more common X-ray intensity variations are faster with much smaller amplitudes. The spectrum during the brightness sputters is remarkably different from the spectrum of the mean high state emission. We argue that such sputtering episodes are possibly caused by a major accretion disk instability. Based on the coincidence in time of two radio flares following the observed X-ray sputtering episodes we speculate that superluminal ejections (as observed from GRS 1915+105 during earlier activity periods) are related to episodes of large amplitude X-ray variations.

astro-ph↗

arXiv1996

RX J0719.2+6557: A New Eclipsing Polar

A new magnetic, eclipsing cataclysmic variable is identified as the counterpart of the X-ray source RX J0719.2+6557 detected during the ROSAT all-sky survey. The relative phasing of photometric and spectroscopic periods indicates a self-eclipsing system. Doppler tomography points to the heated surface of the secondary as a strong source of emission and of diskless accretion. Near-infrared spectroscopy revealed two unusual strong emission features originating on the heated side of the secondary.

astro-ph↗

arXiv1996

X-Ray Observations of GRS 1915+105

A comparison of ROSAT observations with simultaneous BATSE monitoring data shows a different intensity evolution in the soft and hard X-ray band of GRS 1915+105 since 1992. In addition, fits of simultaneous ROSAT/BATSE spectra require more than one spectral component. A short description of results of RXTE pointed observations of GRS 1915+105 since April 1996 is also presented.

astro-ph↗

arXiv1996

ROSAT observation of GRS 1739-278

We have observed GRS 1739-278 with the ROSAT HRI on March 31, 1996. The improved X-ray position proves the identification with an earlier discovered variable radio source. We derive an extinction of A_V = 14+/-2 from the dust scattering halo, which implies a distance of d approx. 6-8.5 kpc. Thus, GRS 1739-278 radiates at least near the Eddington limit for a 1 M_sun compact object. The existence of the dust scattering halo necessitates a prolonged X-ray activity period prior to the ROSAT observation. Combined with the non-detection by Granat during an earlier galactic center observation, we speculate that the X-ray turn-on of GRS 1739-278 happened in Nov./Dec. 1995.

astro-ph↗

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.

astro-ph↗

arXiv1996

Supersoft X-Ray Sources in M31

The nearby Andromeda galaxy (M31) has been observed with the ROSAT PSPC in a mosaic of 6 pointings with 25 ksec each. In the paper describing the results on the total sample of detected sources, Supper et al. (1996) also report the positions for 15 supersoft X-ray sources and the blackbody fit results for the brightest of these sources. We report here in more detail on the X-ray spectral characteristics of all these 15 supersoft X-ray sources. Optical multi-colour photometric data obtained in 1990 at the Michigan- Dartmouth-MIT Observatory at Kitt Peak were used originally in the selection process of the supersoft sources, and are the basis for the finding charts given here for most selected X-ray sources.

astro-ph↗

arXiv1996

UV and X-Ray Monitoring of AG Draconis During the 1994/1995 Outbursts

The recent 1994-1995 active phase of AG Draconis has given us for the first time the opportunity to follow the full X-ray behaviour of a symbiotic star during two successive outbursts and to compare with its quiescence X-ray emission. With \ros observations we have discovered a remarkable decrease of the X-ray flux during both optical maxima, followed by a gradual recovering to the pre-outburst flux. In the UV the events were characterized by a large increase of the emission line and continuum fluxes, comparable to the behaviour of AG Dra during the 1980-81 active phase. The anticorrelation of X-ray/UV flux and optical brightness evolution is shown to very likely be due to a temperature decrease of the hot component. Such a temperature decrease could be produced by an increased mass transfer to the burning compact object, causing it to slowly expand to about twice its original size.

astro-ph↗

arXiv1996

A Systematic Search for Supersoft X-Ray Sources in the ROSAT All-Sky Survey

We have conducted a systematic search for supersoft X-ray sources using the ROSAT all-sky survey data. With the optical identification of the selected sources being almost complete, we discuss the statistics of the various source classes and their observability. Besides supersoft close binary sources this search also can be used to estimate the number of isolated neutron stars in the Galaxy, such as those described by Stocke et al. 1995 and Walter et al. 1996.

astro-ph↗

arXiv1996

Catalog of Luminous Supersoft X-ray Sources

This catalog comprises an up-to-date list of luminous (>10^36 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, and further supersoft binaries are expected to also have one, we include also sources associated with very luminous planetary nebulae. Not included are PG 1159 stars which reach similar magnitudes but form a rather distinct class. Excluded are also supersoft active galactic nuclei which reach luminosities up to 10^45 erg/s.

astro-ph↗

arXiv1996

Rapid follow-up ROSAT observation of GRB 940301

The strong GRB of March 1, 1994 was imaged by COMPTEL and found to have an identical location within the errors as a burst which occurred on July 4, 1993. This location coincidence had prompted speculations on a possible single source origin for both bursts. We have performed a ROSAT PSPC mosaic observation within four weeks of GRB 940301. The results of these observations and the comparison of the intensities of the detected sources with those detected during the ROSAT all-sky-survey in September 1990 are presented. We neither find a flaring X-ray source in the April 1994 observation nor any variability of the X-ray sources detected in the all-sky-survey. We discuss the consequences of our negative result on both, the possibility of the location coincidence being due to a repeating burst source as well as due to two independent sources. In the former case the source could either be a Soft Gamma Repeater similar to SGR 1806-20 and SGR 0525-66, or a sofar unknown classical burst repeater. We conclude that a quiescent luminous X-ray source as is found for the above mentioned Soft Gamma Repeaters is very unlikely to be present in the case of GRB 930704 / GRB 940301.

astro-ph↗

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.

astro-ph↗

arXiv1995

Four new active galaxies with steep soft X-ray spectra

We have discovered four AGN in the ROSAT all-sky-survey data with very steep X-ray spectra. We apply several models to these X-ray spectra with emphasis on warm absorber models which give an adequate description of the data. We report on the follow-up optical and radio observations which allow the identification of three of these objects as Narrow Line Seyfert 1 galaxies, and the fourth as BL Lac object. We have measured small-FWHM Hbeta lines, strong FeII emission and weak [OIII] emission in the three Narrow Line Seyfert 1 galaxies, in line with known correlations with respect to the steepness of the X-ray spectra. We have discovered strong optical variability in the BL Lac object and two of the Seyfert galaxies using photographic plates of the Sonneberg Observatory field patrol. We finally discuss the statistical implications of our search algorithm on the expected number density of soft X-ray selected AGN and conclude that up to 30% of X-ray selected AGN might have supersoft X-ray spectra.

astro-ph↗

arXiv1995

Multifrequency Observations of Gamma-Ray Burst

Neither a flaring nor a quiescent counterpart to a gamma-ray burst has yet been convincingly identified at any wavelength region. The present status of the search for counterparts of classical gamma-ray bursts is given. Particular emphasis is put on the search for flaring counterparts, i.e. emission during or shortly after the gamma-ray emission.

astro-ph↗

arXiv1995

The gamma-ray burst GB 920622

We have analyzed the Ulysses, BATSE, and COMPTEL spectral data from the $γ$-ray burst of June 22, 1992 (GB 920622). COMPTEL data reveal a hard to soft evolution within the first pulse of the burst, while the mean hardness ratios of the three pulses are the same. Unlike the single instrument spectra, the composite spectrum of GB 920622 averaged over the total burst duration ranging from 20 keV up to 10 MeV cannot be fit by a single power law. Instead, the spectrum shows continuous curvature across the full energy range. COMPTEL imaging and BATSE/Ulysses triangulation constrain the source location of GB 920622 to a ring sector 1.1 arcmin wide and 2 degrees long. This area has been searched for quiescent X-ray sources using \ros survey data collected about two years before the burst. After the optical identification of the X-ray sources in and near the GRB location we conclude that no quiescent X-ray counterpart candidate for GB 920622 has been found.

astro-ph↗

arXiv1995

Search for GRB counterparts

A state-of-the-art review is given on the search for counterparts of $γ$-ray bursts. Particular emphasis is put on the search for flaring counterparts, i.e. emission during or shortly after the gamma-ray emission. With BACODINE working now for nearly one year, some of the highlights of the herewith triggered follow-up studies will be summarized.

astro-ph↗

arXiv1995

X-ray emission from the PSR B1259--63 system near apastron

The PSR B1259--63 system contains a 47 ms radio pulsar in a highly eccentric binary with a Be-star companion. Strongly time variable X-ray emission was reported from this system as the pulsar was near apastron in 1992-early 1993. The variability was primarily deduced from an apparent non-detection of the \psr system during a first pre-apastron \ros observation in February~1992. We have re-analyzed the \ros observations of the \psr system. Contrary to the results of a previous analysis, we find that the \psr system was detected by \ros during the first off-axis February~1992 observation. The intensity of the soft X-ray emission of the \psr system before and after the 1992 apastron appears to vary at most by a factor $\sim 2$. Our results sensibly constrain theoretical models of X-ray emission from the \psr system.

astro-ph↗

arXiv1994

Optical variability of the supersoft source RX~J0019.8+2156

We present a 100 yr optical lightcurve of the recently discovered supersoft X-ray source RX~J0019.8+2156 as deduced from photographic plates of Harvard and Sonneberg Observatory. Apart from the periodic orbital variations two different timescales of optical variability are discovered. The timescales and amplitudes of this variability are discussed in the framework of the steady nuclear burning model.

astro-ph↗