Variable X-ray Source Research Articles

RXTE observations of GRB afterglows

Variable X-ray sources that appear to be the afterglows of the strong gamma-ray bursts GRB 970616 and GRB 970828 have been discovered with the RXTE PCA. First seen less than 4 hours after the burst, the flux from the sources decreased with time. Although near the sensitivity limit of the PCA, the sources are the brightest afterglows yet seen in X-rays. Similar observations of two other bursts did not detect any afterglows. These results are part of a continuing collaboration between RXTE, BATSE, and IPN scientists to rapidly detect X-ray afterglows of bright gamma-ray bursts.

VARIABILITY OF ACTIVE GALACTIC NUCLEI

▪ Abstract A large collective effort to study the variability of active galactic nuclei (AGN) over the past decade has led to a number of fundamental results on radio-quiet AGN and blazars. In radio-quiet AGN, the ultraviolet (UV) bump in low-luminosity objects is thermal emission from a dense medium, very probably an accretion disk, irradiated by the variable X-ray source. The validity of this model for high-luminosity radio-quiet AGN is unclear because the relevant UV and X-ray observations are lacking. The broad-line gas kinematics appears to be dominated by virialized motions in the gravity field of a black hole, whose mass can be derived from the observed motions. The “accretion disk plus wind” model explains most of the variability (and other) data and appears to be the most appropriate model at present. Future investigations are outlined. In blazars, rapid variability at the highest energies (gamma-rays) implies that the whole continuum is relativistically boosted along the line of sight. The general correlation found between variations in TeV gamma rays and in X rays for Mrk 421, and between variations in GeV gamma rays and in the IR–optical–UV bands for 3C 279, two prototype objects, supports models in which the same population of relativistic electrons radiates the low-frequency continuum via synchrotron and the high frequency continuum via inverse Compton scattering of soft photons. Identifying the dominant source of soft photons, which is at present unclear, will strongly constrain the jet physics.

ROSAT HRI observations of the Local Group galaxies IC 10, NGC 147 and NGC 185

We report on pointed X-ray observations of IC 10, NGC 147 and NGC 185 made with the ROSAT High Resolution Imager (HRI). These are three Local Group galaxies that have never been previously studied in detail in the X-ray regime. IC 10 is the closest starburst galaxy to our own Galaxy, and NGC 147 and NGC 185 are companions to M31. We have discovered a variable X-ray source coincident with IC 10. The source is located near the centre of a large, non-thermal bubble of radio emission, and it is positionally coincident with an emission line star in IC 10 which has been classified as a WN type Wolf-Rayet star. We demonstrate that a confusing foreground or background source is improbable. The X-ray source is probably an X-ray binary in IC 10, and it may be a Wolf-Rayet + black hole binary. The source has mean and maximum 0.1--2.5 keV isotropic luminosities of about 2 and 4 times 10^{38} erg/s. We do not detect any sources in the central regions of NGC 147 or NGC 185. We place upper limits on their central X-ray emission, and we list all X-ray sources coincident with their outer extents. We also present the first X-ray detections of the well-studied Algol-type binary TV Cas and the W UMa-type binary BH Cas, which were both serendipitously observed during our IC 10 pointing.

The Discovery of 8.9 Second Pulsations from the Variable X-Ray Source 2E 0050.1−7247 in the Small Magellanic Cloud

During a systematic search for periodic signals in a sample of ROSAT PSPC light curves, we discovered ~8.9 s X-ray pulsations in 2E 0050.1-7247, a variable X-ray source in the Small Magellanic Cloud. The source was detected several times between 1979 and 1993 at luminosity levels ranging from ~5 × 1034 ergs s-1 up to 1.4 × 1036 ergs s-1 with both the Einstein IPC and ROSAT PSPC. The X-ray energy spectrum is consistent with a power-law spectrum that steepens as the source luminosity decreases. We revealed a pronounced Hα activity from at least two B stars in the X-ray error circles. These results strongly suggest that the X-ray pulsar 2E 0050.1-7247 is in a Be-type massive binary.

ROSAT monitoring of persistent giant and rapid variability in the narrow-line Seyfert 1 galaxy IRAS 13224-3809

We report evidence for persistent giant and rapid X-ray variability in the radio-quiet, ultrasoft, strong Fe II, narrow-line Seyfert 1 galaxy IRAS 13224-3809. Within a 30 day ROSAT High Resolution Imager (HRI) monitoring observation at least five giant amplitude count rate variations are visible, with the maximum observed amplitude of variability being about a factor of 60. We detect a rise by a factor of about 57 in just two days. IRAS 13224-3809 appears to be the most X-ray variable Seyfert known, and its variability is probably nonlinear. We carefully check the identification of the highly variable X-ray source with the distant galaxy, and it appears to be secure. We examine possible explanations for the giant variability. Unusually strong relativistic effects and partial covering by occulting structures on an accretion disc can provide plausible explanations of the X-ray data, and we explore these two scenarios. Relativistic boosting effects may be relevant to understanding the strong X-ray variability of some steep spectrum Seyferts more generally.

Doppler Tomography of Relativistic Accretion Disks

Spectral lines from a source orbiting around a compact object are studied. Time variations of observed frequency and count rate due to motion of the source and gravitational lensing are considered. Gravitational field of the central object is described by the Kerr metric. It is shown that: (i) simultaneous temporal and frequency resolution enables us to restrict parameters of the model (inclination angle, position of the source, angular momentum of the black hole); (ii) techniques of image restoration, familiar from other fields of astronomy, can be applied to study inner regions of active galactic nuclei. This contribution is relevant for extremely variable X-ray sources with high parameter of efficiency, such as Seyfert 1 galaxy PHL 1092 observed by ROSAT.

Stellar Variability in the Central Populations of 47 Tucanae from WF/PC Observations with the Hubble Space Telescope. II. Binary Systems

Stars in the core of 47 Tuc have been intensively surveyed for variability, using differential time series photometry, with the Planetary Camera of the Hubble Space Telescope. Some 20,000 stars were surveyed in the U band (F336W) in a 66" x 66" field centered on the core of 47 Tuc, with almost continuous observing over 38.5 hr, and using 1000 s exposure times. Using aperture photometry, PSF fitting, and power spectrum techniques, two W UMa systems were discovered, as well as six semidetached or detached binaries with periods between 0.41 and 1.5 days. The two faintest variables found are within the error circles of two highly variable X-ray sources and are good candidates for neutron star binaries or magnetic cataclysic variables. Two other certain variables were found, one of which is a possible cataclysmic variable. The radial distribution of the binaries is significantly more centrally concentrated than the average star, but it is consistent with the radial distribution of the blue stragglers found in the core. We show that the contact binary frequency and the period distribution of the binaries imply that few, if any, of the binaries are primordial.

A powerful and highly variable off-nuclear X-ray source in the composite starburst/Seyfert 2 galaxy NGC 4945

We report on a powerful and variable off-nuclear X-ray source in the nearby spiral galaxy NGC 4945. Two ROSAT PSPC observations show the source to brighten in 0.5--2.0 keV flux by a factor of about 9 on a time-scale of 11 months or less. It is seen by ASCA about one month after the second PSPC pointing, and is seen to have dimmed by a factor of > 7 in a ROSAT HRI pointing about one year after the second PSPC pointing. Its maximum observed 0.8--2.5 keV luminosity is about 8E38 erg/s, making it brighter than any known persistent X-ray binary in the Milky Way. Its total X-ray luminosity is probably larger than 1.2E39 erg/s. The observed variability argues against a superbubble interpretation, and the off-nuclear position argues against a low-luminosity active galactic nucleus. The source is therefore probably either an ultra-powerful X-ray binary or an ultra-powerful supernova remnant. Optical monitoring has not identified any supernovae in NGC 4945 during the time of the X-ray observations, and any supernova would have had to have been either very highly absorbed or intrinsically optically faint.

Generalized spectra model for 1-100 keV X-ray emission from Cygnus X-3 based on EXOSAT data

The X-ray spectrum of the highly variable X-ray source, Cyg X-3, has so far defied a consistent explanation based on simple emission models. We have extracted two of the best data sets from the EXOSAT archives and performed a detailed spectral analysis for its 'high' and 'low' states. The analysis of the less frequently occurring 'low' state is presented for the first time for the EXOSAT data. Combining data from the medium-energy argon and xenon detectors and the gas scintillation proportional counter, with a better energy resolution, and carrying out a simultaneous fit, we find that the X-ray continuum in both the 'high' and 'low' state can be explained as a sum of a blackbody emission and emission from a Comptonized plasma cloud with a common absorption. The Comptonization model is sufficient as well as preferable to many other models, in explaining the observed X-ray emission up to 100 keV. In addition, we find an emission-line feature due to ionized iron (Fe XX-Fe XXVI) and absorption features due to cold iron (Fe I) as well as highly ionized iron (Fe XXV-Fe XXXVI). The presence of absorption due to Fe I has been shown for the first time here. This is the simplest and the most generalized spectral model for the 1-100 keV X-ray emission from Cyg X-3, to date. We find that the blackbody temperature derived in the 'high ' state (1.47 keV) is much lower than that derived for the 'low' state (2.40 keV) and is associated with an increase in the blackbodly radius in the 'high' state. The ratio of blackbody flux to the total flux is approximately 0.61 in the 'high' state and ~0.44 in the 'low' state. The Fe line energy is significantly higher in the 'high' state (~6.95 keV) compared to the 'low' state (~6.56 keV). The Comptonization parameter changes from 2 to ~15 in going from the 'high' to the 'low' state implying a highly saturated Comptonization in the 'low' state. The Comptonized region has high electron temperature and low opacity in the 'high' state and vice versa in the 'low' state. The orbital light curve is mostly explained by variations in the intensities of the continuum components. We discuss the likely origin of different emission regions, continuum and line, and interpret them in terms of an accretion disk corona.

Two very luminous variable X-ray sources in M82

We report on a variability study of X-ray sources in the starburst galaxy M82, based on ROSAT and Einstein High-Resolution Imager observations. In particular, we concentrate our analysis on two bright sources which exhibit significant variability. The brightest source in M82 is located in the central core of the galaxy, is variable within individual ROSAT observations, and indication are that it might have varied between the Einstein and the ROSAT observations. It is the most luminous X-ray binary candidate yet known. The other source is located outside the crowded central region and was very bright in the Einstein observation, but was not detected by ROSAT, despite the larger effective area of the instrument and the much longer exposure. The detection of variability poses strong constraints on the X-ray luminosity of individual components of the X-ray sources. Even if both sources are radiating at the Eddington limit, their mass would be at least several solar masses, making them candidates for extragalactic black holes.

Detection of an Unresolved Nuclear Radio Source in M31: Erratum

view Abstract Citations (2) References (1) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Detection of an Unresolved Nuclear Radio Source in M31: Erratum Crane, Patrick C. ; Dickel, John R. ; Cowan, John J. Abstract There were two typographical errors in the Letter "Detection of an Unresolved Nuclear Radio Source in M31" by Patrick C. Crane, John R. Dickel, & John J. Cowan (ApJ, 390, L9 [1992]). A comparison of the radio source's brightness included in the Abstract and the radio position for (J2000) listed in Table 1 were incorrect. 1. The corrected Abstract is shown below: An unresolved radio source with a flux density of 28 +/- 5 microJy has been detected at the center of the galaxy M 31. This source coincides with the optical stellar nucleus and with a variable Einstein X-ray source but is only 1/300 as luminous as the Galactic center source Sgr A and one fifth as luminous as Sgr A^*^. Consequently, this object is the weakest known nuclear radio source. No evidence is found for radio emission from the remnant of supernova 1885A in M31. An upper limit of 15 microJy (3 σ has been determined for the flux density of this source, which is 1/15 the value for Tycho's SNR if it were at the same distance. 2. The right ascension (J2000) given in Table 1 for the radio nucleus is incorrect. The correct right ascension (J2000) is 00^h^42^m^44.329^s^ +/- 0.15". Publication: The Astrophysical Journal Pub Date: July 1993 DOI: 10.1086/186924 Bibcode: 1993ApJ...411L.107C Keywords: ERRATA; ADDENDA full text sources ADS | data products NED (1) Related Materials (1) Original Paper: 1992ApJ...390L...9C

A supersoft variable low-luminosity X-ray source in the globular cluster M3

view Abstract Citations (40) References (37) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A Supersoft Variable Low-Luminosity X-Ray Source in the Globular Cluster M3 Hertz, P. ; Grindlay, J. E. ; Bailyn, C. D. Abstract The globular cluster M3 (NGC 5272) was observed twice with the ROSAT high-resolution imager in order to study the low-luminosity X-ray source 1E 1339.8+2837. In 1992 January 1E 1339.8+2837 had an X-ray luminosity of 2 x 10^35^ ergs s^-1^ (0.1-2.4 keV), over an order of magnitude brighter than it was when observed with the Einstein Observatory. The source was unresolved and very soft (kT ~ 20 eV); such supersoft outbursts would be difficult to detect in the vast majority of globular clusters which are more heavily absorbed than M3. In 1992 June the source was too faint to be detected (<2.8 x 10^33^ ergs s^-1^). The soft outburst luminosity (L_bol_ ~ 1.2 x 10^36^ ergs s^-1^) and the blackbody radius (R~7.7 x 10^8^ cm) suggest that 1E 1339.8+2837 is a cataclysmic variable in which much of the luminosity is generated by steady nuclear burning of accreted material on the surface of the white dwarf primary. Subject headings globular clusters: individual (M3) - novae, cataclysmic variables - X-raYs: stars Publication: The Astrophysical Journal Pub Date: June 1993 DOI: 10.1086/186886 Bibcode: 1993ApJ...410L..87H Keywords: Cataclysmic Variables; Globular Clusters; Rosat Mission; Stellar Luminosity; X Ray Sources; Neutron Stars; Satellite Imagery; X Ray Stars; Astrophysics full text sources ADS | data products SIMBAD (8) NED (2) HEASARC (1)

Detection of an unresolved nuclear radio source in M31

view Abstract Citations (65) References (32) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Detection of an Unresolved Nuclear Radio Source in M31 Crane, Patrick C. ; Dickel, John R. ; Cowan, John J. Abstract An unresolved radio source with a flux density of 28 +/- 5 microJy has been detected at the center of the galaxy M 31. This source coincides with the optical stellar nucleus and with a variable Einstein X-ray source but is only 1/300 as luminous as the Galactic center source Sgr A and one fifth as luminous as Sgr A^*^. Consequently, this object is the weakest known nuclear radio source. No evidence is found for radio emission from the remnant of supernova 1885A in M31. An upper limit of 15 microJy (3 σ has been determined for the flux density of this source, which is 1/15 the value for Tycho's SNR if it were at the same distance. 2. The right ascension (J2000) given in Table 1 for the radio nucleus is incorrect. The correct right ascension (J2000) is 00^h^42^m^44.329^s^ +/- 0.15". Publication: The Astrophysical Journal Pub Date: May 1992 DOI: 10.1086/186359 Bibcode: 1992ApJ...390L...9C Keywords: Andromeda Galaxy; Galactic Nuclei; Radio Sources (Astronomy); Supernova Remnants; Supernovae; Synchrotron Radiation; X Ray Sources; Astrophysics; GALAXIES: INDIVIDUAL MESSIER NUMBER: M31; GALAXIES: NUCLEI; ISM: INDIVIDUAL ALPHANUMERIC: SN 1985A; ISM: SUPERNOVA REMNANTS; STARS: SUPERNOVAE: GENERAL full text sources ADS | data products SIMBAD (5) NED (4) Related Materials (1) Erratum: 1993ApJ...411L.107C

Is the Galactic Centre gamma-ray source 1E1740.7 – 2942 accreting from a molecular cloud?

FOR twenty years there have been detections of an intermittent source of 511-keV electron–positron annihilation radiation at the centre of our Galaxy1–3. Until recently, none of the balloon or satellite experiments lucky enough to catch the source in its 'on' state had spatial resolution better than several degrees, but on 13–14 October 1990 the French SIGMA experiment aboard the Soviet GRANAT spacecraft observed a day-long burst of narrow-band γ rays, probably annihilation radiation, from within 1.5 arcmin of the known X-ray source IE 1740.7–2942, about 50 arcmin from Sgr A West, which contains the dynamical centre of the Milky Way4–6. Here we report millimetre-wavelength observations showing that this variable X-ray source lies on a line of sight to the dense 105M⊙ molecular cloud G–0.86–0.08, near the Galactic Centre. We suggest that the γ-ray source, probably a compact object, possibly a black hole, is accreting dense gas directly from the molecular cloud.

Neutron stars and black holes in binary systems

Abstract Neutron stars and black holes can find themselves with close stellar companions through birth or later accident (section 1). The systems give rise to intense variable X-ray sources and born again pulsars (section 2) and perhaps to still odder things (section 6). They provide the best measurements of neutron star masses and the most persuasive evidence for the existence of black holes in the real world (section 3). Most of the basic evolutionary (section 1) and radiation mechanisms (section 4) required to model the observations have been in place for a decade or more and have survived discovery of new phenomena every couple of years. Many of these involve disks of gas accreting onto the stars (section 5). These disks are perhaps prototypes for those in the cores of quasars. Some problems remain with (a) relative numbers of objects in various evolutionary phases and (b) a few extreme individual systems such as SS 433 and 1957 + 20 (section 6).

X-ray evidence of an obscured nucleus in the type 2 Seyfert galaxy Mkn3

SEYFERT galaxies comprise the majority of active galactic nuclei (AGNs), in which a bright compact nucleus is thought to be powered by matter accreting onto a massive black hole. Seyfert galaxies are classified as type 1 or 2 according to the presence or absence of broad emission lines in the optical spectrum. The high velocities indicated by the broad lines in Seyfert 1 galaxies are taken to be good evidence of a compact, massive object, as are the strong and variable hard X-ray sources that are also generally observed in these objects. In contrast, Seyfert 2 galaxies possess neither of these characteristics, so the theory that they too have an accreting massive black hole is less compelling. Since the discovery1 by spectropolarimetry of a 'hidden' Seyfert 1 nucleus in the prototypical Seyfert 2, NGC1068, the long-standing hope that the two classes may be unified has been revived. Here we present evidence from observations by the Ginga satellite that another Seyfert 2, Mkn3, has the X-ray spectral signature of a hidden type 1 nucleus.

Two variable X-ray sources in M31

The entire set Einstein observations of M31, consisting of five imaging proportional counter and nine high resolution imager observations, has been analyzed for variability of the X-ray sources. The time scales explored range up to about 10 to the 5th s, the maximum time span of individual observations. Variability above the adopted 99.73 percent significance level is detected in two sources, and constraints on their possible periodicity are derived. The observed X-ray luminosities, the pattern of variability, and the optical identifications, suggest the possibility that one of them is a low-mass X-ray binary and the other one a compact massive X-ray binary. 17 refs.

CAL 87 - an eclipsing black hole binary?

view Abstract Citations (86) References (20) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS CAL 87: an Eclipsing Black Hole Binary? Cowley, A. P. ; Schmidtke, P. C. ; Crampton, D. ; Hutchings, J. B. Abstract Simultaneous spectroscopic and photometric observations have been made of the optical counterpart of the variable X-ray source CAL 87, an apparent low-mass X-ray binary in the Large Magellanic Cloud. Photometric observations show a pronounced light variation (> 1 mag) with a period of 10.6 hr. The shape of the optical light curve, showing a deep minimum with very broad wings, suggests an eclipse of a very extended disk structure, which is the dominant light source. There appears to be little X-ray heating of the nondegenerate companion, possibly due to the opacity of the thick disk. The only strong feature in the optical spectrum is a variable He II (λ4686) emission line. Near minimum light, weak absorption lines (e.g., H and K of Ca II, G band) presumably from the nondegenerate companion are visible, indicating a late-F spectral type. Radial velocities of the He II emission lines show a small-amplitude variation (K ~ 40 km s^-1^) through the orbital period. The phasing of these velocities indicates that the emission arises on the degenerate star side of the center of mass, as is the case for many low-mass X-ray binaries. If this velocity reflects the motion of the compact star, then the mass ratio must be large (~ 10), and there is a strong probability that the system may be an eclipsing black hole binary. Publication: The Astrophysical Journal Pub Date: February 1990 DOI: 10.1086/168381 Bibcode: 1990ApJ...350..288C Keywords: Black Holes (Astronomy); Eclipsing Binary Stars; Light Curve; Stellar Spectrophotometry; X Ray Binaries; Emission Spectra; Line Spectra; Spectrum Analysis; Astrophysics; BLACK HOLES; STARS: ECLIPSING BINARIES; STARS: INDIVIDUAL ALPHANUMERIC: CAL 87; X-RAYS: BINARIES full text sources ADS | data products SIMBAD (9) NED (1)

High Energy Transient Ionospheric Monitoring Network

AbstractContinuous, wide sky coverage is essential for the detection and monitoring of infrequent, short-lived events of astrophysical interest such as supernova and nova explosions, variable X-ray sources, gamma ray bursts, gravity waves and stellar and solar flares. We propose to (1) examine past radio propagation records and (2) develop new computer based radio receivers to monitor and log ionospheric perturbations associated with these events.

High-Resolution Observations of the M81 Nucleus

The nucleus of the nearby Sb galaxy M81 has been shown to exhibit many of the features prominent in luminous active galactic nuclei: a variable, compact VLBI radio source (Kellermann et al. 1976, Jones, Sramek and Terzian 1981), a central variable X-ray source (Elvis and van Speybroeck 1982, Fabbiano 1988), and broad wings of Balmer emission similar in extent to those typical to Seyfert 1 nuclei (Peimbert and Torres-Peimbert 1981, Shuder and Osterbrock 1981). The nucleus also shows a narrow-line spectrum in many ways typical of LINERs (Heckman 1980), now known to be representative of early-type spiral nuclei in general.