Search

We present mid-infrared (MIR) spectral-timing measurements of the prototypical Galactic microquasar GRS 1915+105. The source was observed with the Mid-Infrared Instrument (MIRI) onboard JWST in June 2023 at a MIR luminosity L(MIR)~10^{36} erg/s exceeding past IR levels by about a factor of 10. By contrast, the X-ray flux is much fainter than the historical average, in the source's now-persistent 'obscured' state. The MIRI low-resolution spectrum shows a plethora of emission lines, the strongest of which are consistent with recombination in the hydrogen Pfund (Pf) series and higher. Low amplitude (~1%) but highly significant peak-to-peak photometric variability is found on timescales of ~1,000 s. The brightest Pf(6-5) emission line lags the continuum. Though difficult to constrain accurately, this lag is commensurate with light-travel timescales across the outer accretion disc or with expected recombination timescales inferred from emission line diagnostics. Using the emission line as a bolometric indicator suggests a moderate (~5-30% Eddington) intrinsic accretion rate. Multiwavelength monitoring shows that JWST caught the source close in-time to unprecedentedly bright MIR and radio long-term flaring. Assuming a thermal bremsstrahlung origin for the MIRI continuum suggests an unsustainably high mass-loss rate during this time unless the wind remains bound, though other possible origins cannot be ruled out. PAH features previously detected with Spitzer are now less clear in the MIRI data, arguing for possible destruction of dust in the interim. These results provide a preview of new parameter space for exploring MIR spectral-timing in XRBs and other variable cosmic sources on rapid timescales., Comment: Dedicated to the memory of our colleague, Tomaso Belloni. Submitted 2024 June 21; Comments welcome

X-ray binaries exhibit a wide range of properties but there are few accepted methods to determine the nature of the compact object. Color-Color-Intensity diagrams have been suggested as a means of distinguishing between systems containing black holes from those containing neutron stars. However, this technique has been verified with data from only one instrument (RXTE/ASM) with a single set of X-ray colors defined using data available only in pre-determined energy bands. We test a selection of X-ray colors with a more sensitive instrument to determine the reliability of this method. We use data from the MAXI Gas Slit Camera, which allows users to specify energy-bands. We test X-ray colors that have been previously defined in the literature as well as ones that we define specifically in this paper. A representative set of systems are used to construct Color-Color-Intensity diagrams in each set of colors to determine which are best for separating different classes. For studying individual sources certain bands are more effective than others. For a specified energy range, the separation of soft states in black hole binaries was possible only where both soft and hard colors included information from the lowest energy band. We confirm that Color-Color-Intensity diagrams can distinguish between systems containing black holes or neutron stars in all X-ray colors tested; this suggests an universality in the accretion processes governing these different classes. We suggest possible physical processes driving different classes of X-ray binaries to different locations in Color-Color-Intensity diagrams., Comment: 23 pages, 19 figures. Accepted in New Astronomy

We present model fits to the X-ray line spectrum of the well known High Mass X-ray binary Cyg X-3. The primary observational dataset is a spectrum taken with the $Chandra$ X-ray Observatory High Energy Transmission Grating (HETG) in 2006, though we compare it to all the other observations of this source taken so far by this instrument. We show that the density must be $\geq 10^{12}$ cm$^{-3}$ in the region responsible for most of the emission. We discuss the influence of the dust scattering halo on the broad band spectrum and we argue that dust scattering and extinction is not the most likely origin for the narrow featureseen near the Si K edge. We identify the features of a wind in the profiles of the strong resonance lines and we show that the wind is more apparent in the lines from the lighter elements. We argue that this wind is most likely associated with the companion star. We show that the intensities of most lines can be fitted, crudely, by a single component photoionized model. However, the iron K lines do not fit with this model. We show that the iron K line variability as a function of orbital phase is different from the lower energy lines, which indicates that the lines arise in physically distinct regions. We discuss the interpretation of these results in the context of what is known about the system and similar sys, Comment: ApJ Submitted

We present a large-scale study of diffuse X-ray emission in the nearby massive stellar association Cygnus OB2 as part of the Chandra Cygnus OB2 Legacy Program. We used 40 Chandra X-ray ACIS-I observations covering $\sim$1.0 deg$^2$. After removing 7924 point-like sources detected in our survey, background-corrected X-ray emission, the adaptive smoothing reveals large-scale diffuse X-ray emission. Diffuse emission was detected in the sub-bands Soft [0.5 : 1.2] and Medium [1.2 : 2.5], and marginally in the Hard [2.5 : 7.0] keV band. From X-ray spectral analysis of stacked spectra we compute a total [0.5 : 7.0 keV] diffuse X-ray luminosity of L$_{\rm x}^{\rm diff}\approx$4.2$\times$10$^{\rm 34}$ erg s$^{-1}$, characterized with plasma temperature components at kT$\approx$ 0.11, 0.40 and 1.18 keV, respectively. The HI absorption column density corresponding to these temperatures has a distribution consistent with N$_{\rm H}$ = 0.43, 0.80 and 1.39 $\times$10$^{22}$ cm$^{-2}$. The extended medium band energy emission likely arises from O-type stellar winds thermalized by wind-wind collisions in the most populated regions of the association, while the soft band emission probably arises from less energetic termination shocks against the surrounding Interstellar-Medium. Super-soft and Soft diffuse emission appears more widely dispersed and intense than the medium band emission. The diffuse X-ray emission is generally spatially coincident with low-extinction regions that we attribute to the ubiquitous influence of powerful stellar winds from massive stars and their interaction with the local Interstellar-Medium. Diffuse X-ray emission is volume-filling, rather than edge-brightened, oppositely to other star-forming regions. We reveal the first observational evidence of X-ray haloes around some evolved massive stars., Comment: Accepted for publication on ApJSS, Cygnus OB2 Special Issue (2018). 20 pages, 19 figures

Cygnus X-3 is a unique microquasar in the Galaxy hosting a Wolf-Rayet companion orbiting a compact object that most likely is a low-mass black hole. The unique source properties are likely due to the interaction of the compact object with the heavy stellar wind of the companion. In this paper, we concentrate on a very specific period of time prior to the massive outbursts observed from the source. During this period, Cygnus X-3 is in a so-called hypersoft state, where the radio and hard X-ray fluxes are found to be at their lowest values (or non-detected), the soft X-ray flux is at its highest values, and sporadic gamma-ray emission is observed. We will utilize multiwavelength observations in order to study the nature of the hypersoft state. We observed Cygnus X-3 during the hypersoft state with Swift and NuSTAR in the X-rays and SMA, AMI-LA, and RATAN-600 in the radio. We also considered X-ray monitoring data from MAXI and $\gamma$-ray monitoring data from AGILE and Fermi. We found that the spectra and timing properties of the multiwavelength observations can be explained by a scenario where the jet production is turned off or highly diminished in the hypersoft state and the missing jet pressure allows the wind to refill the region close to the black hole. The results provide proof of actual jet quenching in soft states of X-ray binaries., Comment: 15 pages, 13 figures. Accepted for publication in A&A

We report results from TeV gamma-ray observations of the microquasar Cygnus X-3. The observations were made with the Very Energetic Radiation Imaging Telescope Array System (VERITAS) over a time period from 2007 June 11 to 2011 November 28. VERITAS is most sensitive to gamma rays at energies between 85 GeV to 30 TeV. The effective exposure time amounts to a total of about 44 hours, with the observations covering six distinct radio/X-ray states of the object. No significant TeV gamma-ray emission was detected in any of the states, nor with all observations combined. The lack of a positive signal, especially in the states where GeV gamma rays were detected, places constraints on TeV gamma-ray production in Cygnus X-3. We discuss the implications of the results., Comment: 23 pages, 4 figures, 3 tables, accepted for publication in ApJ

We have performed a principal component analysis on the X-ray spectra of the microquasar Cygnus X-3 from RXTE, INTEGRAL and Swift during a major flare ejection event in 2006 May-July. The analysis showed that there are two main variability components in play, i.e. two principal components explained almost all the variability in the X-ray lightcurves. According to the spectral shape of these components and spectral fits to the original data, the most probable emission components corresponding to the principal components are inverse-Compton scattering and bremsstrahlung. We find that these components form a double-peaked profile when phase-folded with the peaks occurring in opposite phases. This could be due to an asymmetrical wind around the companion star with which the compact object is interacting., Comment: Proceedings of "An INTEGRAL view of the high-energy sky (the first 10 years)" the 9th INTEGRAL Workshop, October 15-19, 2012, Paris, France, in Proceedings of Science (INTEGRAL 2012), Eds. A. Goldwurm, F. Lebrun and C. Winkler, (http://pos.sissa.it/cgi-bin/reader/conf.cgi?confid=176), id PoS(INTEGRAL 2012)043

We analyse in detail the X-ray data of the microquasar Cygnus X-3 obtained during major radio flaring episodes in 2006 with multiple observatories. The analysis consists of two parts: probing the fast (~ 1 minute) X-ray spectral evolution with Principal Component Analysis followed by subsequent spectral fits to the time-averaged spectra (~ 3 ks). Based on the analysis we find that the overall X-ray variability during major flaring episodes can be attributed to two principal components whose evolution based on spectral fits is best reproduced by a hybrid Comptonization component and a bremsstrahlung or saturated thermal Comptonization component. The variability of the thermal component is found to be linked to the change in the X-ray/radio spectral state. In addition, we find that the seed photons for the Comptonization originate in two seed photon populations that include the additional thermal emission and emission from the accretion disc. The Comptonization of the photons from the thermal component dominates, at least during the major radio flare episode in question, and the Comptonization of disc photons is intermittent and can be attributed to the phase interval 0.2-0.4. The most likely location for Comptonization is in the shocks in the jet., Comment: 20 pages, 14 figures, accepted for publication in MNRAS

We present the AGILE-GRID monitoring of Cygnus X-3, during the period between November 2007 and July 2009. We report here the whole AGILE-GRID monitoring of Cygnus X-3 in the AGILE "pointing" mode data-taking, to confirm that the gamma-ray activity coincides with the same repetitive pattern of multiwavelength emission and to analyze in depth the overall gamma-ray spectrum by assuming both leptonic and hadronic scenarios. Seven intense gamma-ray events were detected in this period, with a typical event lasting one or two days. These durations are longer than the likely cooling times of the gamma-ray emitting particles, implying we see continuous acceleration rather than the result of an impulsive event such as the ejection of a single plasmoid which then cools as it propagates outwards. Cross-correlating the AGILE-GRID light curve with X-ray and radio monitoring data, we find that the main events of gamma-ray activity have been detected while the system was in soft spectral X-ray states (RXTE/ASM count rate > 3 counts/s), that coincide with local and often sharp minima of the hard X-ray flux (Swift/BAT count rate < 0.02 counts/cm^2/s), a few days before intense radio outbursts. [...] These gamma-ray events may thus reflect a sharp transition in the structure of the accretion disk and its corona, which leads to a rebirth of the microquasar jet and subsequent enhanced radio activity. [...] Finally, we examine leptonic and hadronic emission models for the gamma-ray events and find that both scenarios are valid. In the leptonic model - based on inverse Compton scatterings of mildly relativistic electrons on soft photons from the Wolf-Rayet companion star and from the accretion disk - the emitting particles may also contribute to the overall hard X-ray spectrum, possibly explaining the hard non-thermal power-law tail sometimes seen during special soft X-ray states in Cygnus X-3., Comment: Accepted for publication in A&A on July 26, 2012

Cygnus X-3 (Cyg X-3) is a well-known microquasar producing variable emission at all wavelengths. Cyg X-3 is a prominent X-ray binary producing relativistic jets, and studying its high energy emission is crucial for the understanding of the fundamental acceleration processes in accreting compact objects. Aims. Our goal is to study extreme particle acceleration and {\gamma}-ray production above 100 MeV during special spectral states of Cyg X- 3 usually characterized by a low hard X-ray flux and enhanced soft X-ray states. We observed Cyg X-3 with the AGILE satellite in extended time intervals from 2009 Jun.-Jul., and 2009 Nov.-2010 Jul. We report here the results of the AGILE {\gamma}-ray monitoring of Cyg X-3 as well as the results from extensive multiwavelength campaigns involving radio (RATAN-600, AMI-LA and Mets\"{a}hovi Radio Observatories) and X-ray monitoring data (XTE and Swift). We detect a series of repeated {\gamma}-ray flaring activity from Cyg X-3 that correlate with the soft X-ray states and episodes of decreasing or non-detectable hard X-ray emission. Furthermore, we detect {\gamma}-ray enhanced emission that tends to be associated with radio flares greater than 1 Jy at 15 GHz, confirming a trend already detected in previous observations. The source remained active above 100 MeV for an extended period of time (almost 1.5 months in 2009 Jun.-Jul. and 1 month in 2010 May). We study in detail the short timescale {\gamma}-ray flares that occurred before or near the radio peaks. Our results confirm the transient nature of the extreme particle acceleration from the microquasar Cyg X-3. A series of repeated {\gamma}-ray flares shows correlations with radio and X-ray emission confirming a well established trend of emission. We compare our results with Fermi-LAT and MAGIC TeV observations of Cyg X-3., Comment: 16 pages, 8 figures, 7 tables; A&A (2011)

AGILE data on Cygnus X-3 are reviewed focussing on the correlation between the production of gamma-ray transient emission and spectral state changes of the source. AGILE clearly establishes a relation between enhanced gamma-ray emission and the "quenched" radio/hard X-ray states that precede in general major radio flares. We briefly discuss the theoretical implications of our findings., Comment: 2011 Fermi Symposium proceedings - eConf C110509

We have re-analyzed archival RXTE data of the X-ray binary Cygnus X-3 with a view to investigate the timing properties of the source. As compared to previous studies, we use an extensive sample of observations that include all the radio/X-ray spectral states that have been categorized in the source recently. In this study we identify two additional instances of Quasi-Periodic Oscillations that have centroid frequencies in the mHz regime. These events are all associated to a certain extent with major radio flaring, that in turn is associated with relativistic jet ejection events. We review briefly scenarios whereby the Quasi-Periodic Oscillations may arise., Comment: 5 pages, 3 figures, accepted for publication in MNRAS

Cygnus X-3 is one of the brightest X-ray and radio sources in the Galaxy, and is well known for its erratic behaviour in X-rays as well as in the radio, occasionally producing major radio flares associated with relativistic ejections. However, even after many years of observations in various wavelength bands Cyg X-3 still eludes clear physical understanding. Studying different emission bands simultaneously in microquasars has proved to be a fruitful approach towards understanding these systems, especially by shedding light on the accretion disc/jet connection. We continue this legacy by constructing a hardness-intensity diagram (HID) from archival Rossi X-ray Timing Explorer data and linking simultaneous radio observations to it. We find that surprisingly Cyg X-3 sketches a similar shape in the HID to that seen in other transient black hole X-ray binaries during outburst but with distinct differences. Together with the results of this analysis and previous studies of Cyg X-3 we conclude that the X-ray states can be assigned to six distinct states. This categorization relies heavily on the simultaneous radio observations and we identify one new X-ray state, the hypersoft state, similar to the ultrasoft state, which is associated to the quenched radio state during which there is no or very faint radio emission. Recent observations of GeV flux observed from Cyg X-3 (Tavani et al. 2009; Fermi LAT Collaboration et al. 2009) during a soft X-ray and/or radio quenched state at the onset of a major radio flare hint that a very energetic process is at work during this time, which is also when the hypersoft X-ray state is observed. In addition, Cyg X-3 shows flaring with a wide range of hardness., Comment: 17 pages, 9 figures, accepted for publication in MNRAS

Cygnus X-3 is a unique microquasar. Its X-ray emission shows a very strong 4.8-hour orbital modulation. But its mass-donating companion is a Wolf-Rayet star. Also unlike most other X-ray binaries Cygnus X-3 is relatively bright in the radio virtually all of the time (the exceptions being the quenched states). Cygnus X-3 also undergoes giant radio outbursts (up to 20 Jy). In this presentation we discuss and review the flaring behavior of Cygnus X-3 and its various radio/X-ray states. We present a revised set of radio/X-ray states based on Cygnus X-3's hardness-intensity diagram (HID). We also examine the connection of a certain type of activity to the reported AGILE/Fermi gamma-ray detections of Cygnus X-3., Comment: 8 pages, 7 figures. Invited review at the "7th Agile Meeting and The Bright Gamma-Ray Sky", held in Frascati, Italy, 29 September to 1 October 2009

The study of relativistic particle acceleration is a major topic of high-energy astrophysics. It is well known that massive black holes in active galaxies can release a substantial fraction of their accretion power into energetic particles, producing gamma-rays and relativistic jets. Galactic microquasars (hosting a compact star of 1-10 solar masses which accretes matter from a binary companion) also produce relativistic jets. However, no direct evidence of particle acceleration above GeV energies has ever been obtained in microquasar ejections, leaving open the issue of the occurrence and timing of extreme matter energization during jet formation. Here we report the detection of transient gamma-ray emission above 100 MeV from the microquasar Cygnus X-3, an exceptional X-ray binary which sporadically produces powerful radio jets. Four gamma-ray flares (each lasting 1-2 days) were detected by the AGILE satellite simultaneously with special spectral states of Cygnus X-3 during the period mid-2007/mid-2009. Our observations show that very efficient particle acceleration and gamma-ray propagation out of the inner disk of a microquasar usually occur a few days before major relativistic jet ejections. Flaring particle energies can be thousands of times larger than previously detected maximum values (with Lorentz factors of 105 and 102 for electrons and protons, respectively). We show that the transitional nature of gamma-ray flares and particle acceleration above GeV energies in Cygnus X-3 is clearly linked to special radio/X-ray states preceding strong radio flares. Thus gamma-rays provide unique insight into the nature of physical processes in microquasars., Comment: 29 pages (including Supplementary Information), 8 figures, 2 tables version submitted to Nature on August 7, 2009 (accepted version available at http://www.nature.com/nature/journal/vaop/ncurrent/pdf/nature08578.pdf)

We revisit the discovery outburst of the X-ray transient XTE J1550-564 during which relativistic jets were observed in 1998 September, and review the radio images obtained with the Australian Long Baseline Array, and lightcurves obtained with the Molonglo Observatory Synthesis Telescope and the Australia Telescope Compact Array. Based on HI spectra, we constrain the source distance to between 3.3 and 4.9 kpc. The radio images, taken some two days apart, show the evolution of an ejection event. The apparent separation velocity of the two outermost ejecta is at least 1.3c and may be as large as 1.9c; when relativistic effects are taken into account, the inferred true velocity is >0.8c. The flux densities appear to peak simultaneously during the outburst, with a rather flat (although still optically thin) spectral index of -0.2., Comment: accepted for publication in MNRAS, 9 pages, 8 figures

We address the problem where the X-ray emission lines are formed and investigate orbital dynamics using Chandra HETG observations, photoionizing calculations and numerical wind-particle simulations.The observed Si XIV (6.185 A) and S XVI (4.733 A) line profiles at four orbital phases were fitted with P Cygni-type profiles consisting of an emission and a blue-shifted absorption component. In the models, the emission originates in the photoionized wind of the WR companion illuminated by a hybrid source: the X-ray radiation of the compact star and the photospheric EUV-radiation from the WR star. The emission component exhibits maximum blue-shift at phase 0.5 (when the compact star is in front), while the velocity of the absorption component is constant (around -900 km/s). The simulated FeXXVI Ly alpha line (1.78 A) from the wind is weak compared to the observed one. We suggest that it originates in the vicinity of the compact star, with a maximum blue shift at phase 0.25 (compact star approaching). By combining the mass function derived with that from the infrared HeI absorption (arising from the WR companion), we constrain the masses and inclination of the system. Both a neutron star at large inclination (over 60 degrees) and a black hole at small inclination are possible solutions., Comment: 8 pages, accepted 19/04/2009 for publication in Astronomy and Astrophysics

We present a detailed classification of the X-ray states of Cyg X-3 based on the spectral shape and a new classification of the radio states based on the long-term correlated behaviour of the radio and soft X-ray light curves. We find a sequence of correlations, starting with a positive correlation between the radio and soft X-ray fluxes in the hard spectral state, changing to a negative one at the transition to soft spectral states. The temporal evolution can be in either direction on that sequence, unless the source goes into a very weak radio state, from which it can return only following a major radio flare. The flare decline is via relatively bright radio states, which results in a hysteresis loop on the flux-flux diagram. We also study the hard X-ray light curve, and find its overall anticorrelation with the soft X-rays. During major radio flares, the radio flux responds exponentially to the level of a hard X-ray high-energy tail. We also specify the detailed correspondence between the radio states and the X-ray spectral states. We compare our results to those of black-hole and neutron-star binaries. Except for the effect of strong absorption and the energy of the high-energy break in the hard state, the X-ray spectral states of Cyg X-3 closely correspond to the canonical X-ray states of black-hole binaries. Also, the radio/X-ray correlation closely corresponds to that found in black-hole binaries, but it significantly differs from that in neutron-star binaries. Overall, our results strongly support the presence of a black hole in Cyg X-3., Comment: MNRAS, in press

The X-ray binary Cygnus X-3 is a highly variable X-ray source that displays a wide range of observed spectral states. One of the main states is significantly harder than the others, peaking at ~ 20 keV, with only a weak low-energy component. Due to the enigmatic nature of this object, hidden inside the strong stellar wind of its Wolf-Rayet companion, it has remained unclear whether this state represents an intrinsic hard state, with truncation of the inner disc, or whether it is just a result of increased local absorption. We study the X-ray light curves from RXTE/ASM and CGRO/BATSE in terms of distributions and correlations of flux and hardness and find several signs of a bimodal behaviour of the accretion flow that are not likely to be the result of increased absorption in a surrounding medium. Using INTEGRAL observations, we model the broad-band spectrum of Cyg X-3 in its apparent hard state. We find that it can be well described by a model of a hard state with a truncated disc, despite the low cut-off energy, if the accreted power is supplied to the electrons in the inner flow in the form of acceleration rather than thermal heating, resulting in a hybrid electron distribution and a spectrum with a significant contribution from non-thermal Comptonization, usually observed only in soft states. The high luminosity of this non-thermal hard state implies that either the transition takes place at significantly higher L/Ledd than in the usual advection models, or the mass of the compact object is > 20 Msun, possibly making it the most massive black hole observed in an X-ray binary in our Galaxy so far. We find that an absorption model as well as a model of almost pure Compton reflection also fit the data well, but both have difficulties explaining other results, in particular the radio/X-ray correlation., Comment: 14 pages, 13 figures, submitted to MNRAS

Aims: We investigate the X-ray spectral and timing properties of the high mass X-ray binary EXO 2030+375 observed during its June-September 2006 giant (type II) outburst. Methods: The data analyzed in this work are from partly simultaneous observations with INTEGRAL Swift. The pulse period P and its temporal derivative P_dot are measured. X-ray pulse profiles in different energy ranges and time intervals are constructed. Pulse averaged X-ray spectra for different time intervals are studied. Results: We report a strong spin-up of the source during the outburst, comparable to that observed in 1985 during the previous giant outburst when the source was discovered. The value of P_dot is found to be linearly related to the X-ray luminosity of the source during the outburst. For the first time the hard X-ray (>25 keV) characteristics of the source during a type II outburst are studied. The X-ray pulse profiles apparently change with luminosity. The X-ray spectral continuum in the 3--120 keV energy range is modeled with an absorbed power law with an exponential cutoff around E ~26 keV. An iron emission line at ~6-7 keV is observed. The spectrum reveals some features between 10 and 20 keV which can be modeled either by a broad emission line at ~13-15 keV (a ``bump'') or by two absorption lines at ~10 and ~20 keV., Comment: 5 pages, 5 figures, accepted for publication in A&A Letters

The Burst and Transient Source Experiment (BATSE), aboard the Compton Gamma Ray Observatory (CGRO), provided a record of the low-energy gamma-ray sky (20-1000 keV) between 1991 April and 2000 May (9.1y). Using the Earth Occultation Technique to extract flux information, a catalog of sources using data from the BATSE large area detectors has been prepared. The first part of the catalog consists of results from the monitoring of 58 sources, mostly Galactic. For these sources, we have included tables of flux and spectral data, and outburst times for transients. Light curves (or flux histories) have been placed on the world wide web. We then performed a deep-sampling of 179 objects (including the aforementioned 58 objects) combining data from the entire 9.1y BATSE dataset. Source types considered were primarily accreting binaries, but a small number of representative active galaxies, X-ray-emitting stars, and supernova remnants were also included. The deep sample results include definite detections of 83 objects and possible detections of 36 additional objects. The definite detections spanned three classes of sources: accreting black hole and neutron star binaries, active galaxies and supernova remnants. Flux data for the deep sample are presented in four energy bands: 20-40, 40-70, 70-160, and 160-430 keV. The limiting average flux level (9.1 y) for the sample varies from 3.5 to 20 mCrab (5 sigma) between 20 and 430 keV, depending on systematic error, which in turn is primarily dependent on the sky location. To strengthen the credibility of detection of weaker sources (5-25 mCrab), we generated Earth occultation images, searched for periodic behavior using FFT and epoch folding methods, and critically evaluated the energy-dependent emission in the four flux bands., Comment: 64 pages, 17 figures, abstract abridged, Accepted by ApJS

We present the first INTEGRAL results on Cyg X-3 from the PV phase observations of the Cygnus region. The source was clearly detected by the JEM-X, ISGRI and SPI detectors. The INTEGRAL observations were supported by simultaneous pointed RXTE observations. Their lightcurves folded over the 4.8 hour binary period are compatible with the mean RXTE/ASM and CGRO/BATSE light curves. We fit our broad band X-ray/Gamma-ray spectra with a physical model, which represents the first such published model for Cyg X-3. The main physical processes in the source are thermal Comptonization and Compton reflection with parameters similar to those found for black-hole binaries at high Eddington rates., Comment: 6 pages, 5 colour figures, accepted for Astronomy and Astrophysics Letters, a special INTEGRAL issue

We report on the discovery of a 590 days long term periodicity in the hard X-ray component of the microquasar \grs found from a comprehensive study of more than four years of {\it RXTE} observations. The periodicity is also observed in the hard X-ray flux observed by BATSE, and in the radio flux as seen with the {\it Green Bank Interferometer} and the {\it Ryle Telescope}. We discuss various possible explanations, including the precession of a radiation induced warped accretion disk., Comment: 6 pages, 4 figuers, accepted for publication as ApJ Letter

We present the results from a multiwavelength campaign of GRS 1915+105 performed from 2000 April 16 to 25. This is one of the largest coordinated set of observations ever performed for this source, covering the wide energy band in radio (13.3-0.3 cm), near-infrared (J-H-K), X-rays and Gamma-rays (from 1 keV to 10 MeV). During the campaign GRS 1915+105 was predominantly in the "plateau" (or low/hard) state but sometimes showed soft X-ray oscillations: before April 20.3, rapid, quasi-periodic (~= 45 min) flare-dip cycles were observed. The radio flares observed on April 17 shows frequency- dependent peak delay, consistent with an expansion of synchrotron-emitting region starting at the transition from the hard-dip to the soft-flare states in X-rays. On the other hand, infrared flares on April 20 appear to follow (or precede) the beginning of X-ray oscillations with an inconstant time delay of ~= 5-30 min. This implies that the infrared emitting region is located far from the black hole by >~ 10E13 cm, while its size is <~ 10E12 cm constrained from the time variability. We find a good correlation between the quasi-steady flux level in the near-infrared band and in the X-ray band. From this we estimate that the reprocessing of X-rays, probably occurring in the outer parts of the accretion disk, accounts for about 20-30% of the observed K magnitude in the plateau state. The OSSE spectrum in the 0.05-10 MeV band is represented by a single power law with a photon index of 3.1 extending to ~1 MeV with no cutoff. The power-law slope above ~30 keV is found to be very similar between different states in spite of large flux variations in soft X-rays, implying that the electron energy distribution is not affected by the change of the state in the accretion disk., Comment: 31 pages, 11 figures. Accepted for publication in ApJ, vol. 571, 2002. Minor corrections. Figure 2 is revised (numbers on the top axis are corrected). References are updated

We have studied the 1999 soft X-ray transient outburst of XTE J1859+226 at radio and X-ray wavelengths. The event was characterised by strong variability in the disc, corona and jet - in particular, a number of radio flares (ejections) took place and seemed well-correlated with hard X-ray events. Apparently unusual for the `canonical soft' X-ray transient, there was an initial period of low/hard state behaviour during the rise from quiescence but prior to the peak of the main outburst - we show that not only could this initial low/hard state be an ubiquitous feature of soft X-ray transient outbursts but that it could also be extremely important in our study of outburst mechanisms., Comment: 12 pages, Accepted for publication in MNRAS

The 1998 September outburst of the black-hole X-ray binary XTE J1550-564 was monitored at X-ray, optical and radio wavelengths. We divide the outburst sequence into five phases and discuss their multiwavelength properties. The outburst starts with a hard X-ray spike, while the soft X-ray flux rises with a longer timescale. We suggest that the onset of the outburst is determined by an increased mass transfer rate from the companion star, but the outburst morphology is determined by the distribution of specific angular momentum in the accreting matter. The companion in XTE J1550-564 is likely to be an active magnetic star, with a surface field strong enough to influence the dynamics of mass transfer. We suggest that its magnetic field can create a magnetic bag capable of confining gas inside the Roche lobe of the primary. The impulsive rise in the hard X-rays is explained by the inflow of material with low angular momentum onto the black hole, on a free-fall timescale, when the magnetic confinement breaks down. At the same time, high angular momentum matter, transferred via ordinary Roche-lobe overflow, is responsible for the formation of a disk., Comment: to appear in ApJ, vol 564, January 10, 2002

An Earth orbiting detector sensitive to gamma ray photons will see step-like occultation features in its counting rate when a gamma ray point source crosses the Earth's limb. This is due to the change in atmospheric attenuation of the gamma rays along the line of sight. In an uncollimated detector, these occultation features can be used to locate and monitor astrophysical sources provided their signals can be individually separated from the detector background. We show that the Earth occultation technique applied to the Burst and Transient Source Experiment (BATSE) on the Compton Gamma Ray Observatory (CGRO) is a viable and flexible all-sky monitor in the low energy gamma ray and hard X-ray energy range (20 keV - 1 MeV). The method is an alternative to more sophisticated photon imaging devices for astronomy, and can serve well as a cost-effective science capability for monitoring the high energy sky. Here we describe the Earth occultation technique for locating new sources and for measuring source intensity and spectra without the use of complex background models. Examples of transform imaging, step searches, spectra, and light curves are presented. Systematic uncertainties due to source confusion, detector response, and contamination from rapid background fluctuations are discussed and analyzed for their effect on intensity measurements. A sky location-dependent average systematic error is derived as a function of galactic coordinates. The sensitivity of the technique is derived as a function of incident photon energy and also as a function of angle between the source and the normal to the detector entrance window. Occultations of the Crab Nebula by the Moon are used to calibrate Earth occultation flux measurements independent of possible atmospheric scattering effects., Comment: 39 pages, 24 figures. Accepted for publication in the Astrophysical Journal Supplements

We report on radio and X-ray observations of the black hole candidate (BHC) XTE J1550-564 performed during its 2000 X-ray outburst. Observations have been conducted with the Australia Telescope Compact Array (ATCA) and have allowed us to sample the radio behavior of XTE J1550-564 in the X-ray Low Hard and Intermediate/Very High states. We observed optically thin radio emission from XTE J1550-564 five days after a transition to an Intermediate/Very High state, but we observed no radio emission six days later, while XTE J1550-564 was still in the Intermediate/Very High state. In the Low Hard state, XTE J1550-564 is detected with an inverted radio spectrum. The radio emission in the Low Hard state most likely originates from a compact jet; optical observations suggest that the synchrotron emission from this jet may extend up to the optical range. The total power of the compact jet might therefore be a significant fraction of the total luminosity of the system. We suggest that the optically thin synchrotron radio emission detected five days after the transition to the Intermediate/Very High state is due to a discrete ejection of relativistic plasma during the state transition. Subsequent to the decay of the optically thin radio emission associated with the state transition, it seems that in the Intermediate/Very High state the radio emission is quenched by a factor greater than 50, implying a suppression of the outflow. We discuss the properties of radio emission in the X-ray states of BHCs., Comment: 15 pages, including 3 figures. Accepted for publication in ApJ, scheduled for the vol. 553 Jun 1, 2001 issue

We have discovered with the Wide Field Cameras on board BeppoSAX the weak transient X-ray source SAXJ2239.3+6116 whose position coincides with that of 4U2238+60/3A2237+608 and is close to that of the fast transient AT2238+60 and the unidentified EGRET source 3EG2227+6122. The data suggest that the source exhibits outbursts that last for a few weeks and peak to a flux of 4E-10 erg/s/cm2 (2-10 keV) at maximum. During the peak the X-ray spectrum is hard with a photon index of -1.1+/-0.1. Follow-up observations with the Narrow-Field Instruments on the same platform revealed a quiescent emission level that is 1E+3 times less. Searches through the data archive of the All-Sky Monitor on RXTE result in the recognition of five outbursts in total from this source during 1996-1999, with a regular interval time of 262 days. Optical observations with the KPNO 2.1 m telescope provide a likely optical counterpart. It is a B0 V to B2 III star with broadened emission lines at an approximate distance of 4.4 kpc. The distance implies a 2-10 keV luminosity in the range from 1E+33 to 1E+36 erg/s. The evidence suggests that SAXJ2239.3+6116 is a Be X-ray binary with an orbital period of 262 days., Comment: submitted to A&A Main Journal

BATSE/OSSE observations of the high redshift quasar 4C 71.07 indicate that this is the brightest and furthest AGN so far detected above 20 keV. BATSE Earth occultation data have been used to search for emission from 4C 71.07 from nearly 3 years of observation. The mean source flux over the whole period in the BATSE energy range 20-100 keV is (13.2 +/- 1.06) x 10^(-11) erg cm^(-2) s^(-1) corresponding to a luminosity of 2 x 10^(48) erg s^(-1). The BATSE light curve over the 3 years of observations shows several flare-like events, one of which (in January 1996) is associated with an optical flare (R=16.1) but with a delay of 55 days. The OSSE/BATSE spectral analysis indicates that the source is characterized by a flat power spectrum (Gamma about 1.1 - 1.3) when in a low state; this spectral form is consistent within errors with the ASCA and ROSAT spectra. This means that the power law observed from 0.1 to 10 keV extends up to at least 1 MeV but steepens soon after to meet EGRET high energy data. BATSE data taken around the January 1996 flare suggests that the spectrum could be steeper when the source is in a bright state. The nuF-nu representation of the source is typical of a low frequency peaked/gamma-ray dominated blazar, with the synchrotron peak in the mm-FIR band and the Compton peak in the MeV band. The BATSE and OSSE spectral data seem to favour a model in which the high energy flux is due to the sum of the synchrotron self-Compton and the external Compton contributions; this is also supported by the variability behaviour of the source., Comment: 19 pages, LaTeX, plus 4 .ps figures. accepted by Astrophysical Journal

The Burst and Transient Source Experiment (BATSE) on the Compton Gamma Ray Observatory (CGRO) has triggered on 1637 cosmic gamma-ray bursts between 1991 April 19 and 1996 August 29. These events constitute the Fourth BATSE burst catalog. The current version (4Br) has been revised from the version first circulated on CD-ROM in September 1997 (4B) to include improved locations for a subset of bursts that have been reprocssed using additional data. A significant difference from previous BATSE catalogs is the inclusion of bursts from periods when the trigger energy range differed from the nominal 50-300 keV. We present tables of the burst occurrence times, locations, peak fluxes, fluences, and durations. In general, results from previous BATSE catalogs are confirmed here with greater statistical significance., Comment: 45 pages, 12 Postscript figures, accepted for publication in Ap. J. Suppl

The Burst and Transient Source Experiment (BATSE) on the Compton Gamma-Ray Observatory has been shown to be sensitive to non-transient hard X-ray sources in our galaxy, down to flux levels of 100 mCrab for daily measurements, 3 mCrab for integrations over several years. We use the continuous BATSE database and the Earth Occultation technique to extract average flux values between 20 and 200 keV from complete radio- and X-ray- selected BL Lac samples over a 2 year period., Comment: 4 Pages, to appear in the proceedings of the conference "BL Lac Phenomenon", Turku, Finland, 22-26 June 1998, ed. L. Takalo

Cyg X-3 is an unusual X-ray binary which shows remarkable correlative behavior between the hard X-ray, soft X-ray, and the radio. We present an analysis of these long term light curves in the context of spectral changes of the system. This analysis will also incorporate a set of pointed RXTE observations made during a period when Cyg X-3 made a transition from a quiescent radio state to a flaring state (including a major flare) and then returned to a quiescent radio state., Comment: 4 pages, 4 color figures, epsfig.sty included. To appear in the 3rd INTEGRAL Workshop Proceedings (The Extreme Universe)

Using CGRO/BATSE hard X-ray (HXR) data and GHz radio monitoring data from the Green Bank Interferometer (GBI), we have performed a long term study ($\sim$ 1800 days) of the unusual X-ray binary Cyg X-3 resulting in the discovery of a remarkable relationship between these two wavelength bands. We find that, during quiescent radio states, the radio flux is strongly anticorrelated with the intensity of the HXR emission. The relationship switches to a correlation with the onset of major radio flaring activity. During major radio flaring activity the HXR drops to a very low intensity during quenching in the radio and recovers during the radio flare. Injection of plasma into the radio jets of Cyg X-3 occurs during changes in the HXR emission and suggests that disk-related and jet-related components are responsible for the high energy emission., Comment: 19 pages, Latex, 4 figures, 2 tables, submitted to Ap.J

In the period between May 1997 and August 1997 a series of pointed RXTE observations were made of Cyg X-3. During this period Cyg X-3 made a transition from a quiescent radio state to a flare state (including a major flare) and then returned to a quiescent radio state. Analyses of the observations are made in the context of concurrent observations in the hard X-ray (CGRO/BATSE), soft X-ray (RXTE/ASM) and the radio (Green Bank Interferometer, Ryle Telescope, and RATAN-600). Preliminary analyses of the observations are presented., Comment: 4 pages, 4 figures. newarcrc.sty included. To appear in 2nd Workshop of Relativistic Jets from Galactic Sources, R.N. Ogley and S.J. Bell Burnell eds, NewAR 42, in press

The hard X-ray emission from Cygnus X-1 has been monitored continually by BATSE since the launch of CGRO in April 1991. We present the hard X-ray intensity and spectral history of the source covering a period of more than five years. Power spectral analysis shows a significant peak at the binary orbital period. The 20-100 keV orbital light curve is roughly sinusoidal with a minimum near superior conjunction of the X-ray source and an rms modulation fraction of approximately 1.7%. No longer-term periodicities are evident in the power spectrum. We compare our results with other observations and discuss the implications for models of the source geometry., Comment: 5 pages (aipproc LaTeX), 4 PostScript figures, to appear in Proceedings of the Fourth Compton Symposium, ed. C. D. Dermer, M. S. Strickman, and J. D. Kurfess, AIP Conf. Proc. 410 (New York: AIP), in press

The operation of CGRO/BATSE continues to produce, after more than 5 years, a valuable database for the study of long-term variability in bright hard X-ray sources. The all-sky capability of BATSE provides, using the Earth occultation technique, up to approximately 30 flux measurements per day for each source. The long BATSE baseline and the numerous rising and setting occultation flux measurements allow searches for periodic and quasi-periodic signals from hours to hundreds of days. We present initial results from our study of the hard X-ray variability in 24 of the brightest BATSE sources. Power density spectra are computed for each source. In addition, we present profiles of the hard X-ray orbital modulations in 8 X-ray binaries (Cen X-3, Cyg X-1, Cyg X-3, GX 301-2, Her X-1, OAO1657-415, Vela X-1 and 4U1700-37), several-hundred-day modulations in the amplitude and width of the main high state in the 35-day cycle in Her X-1, and variations in outburst durations and intensities in the recurrent X-ray transients., Comment: 4 pages (LaTeX) including 3 PostScript figures; to appear in Proceedings of the 2nd INTEGRAL Workshop "The Transparent Universe" (St. Malo, France), eds. C. Winkler et al., ESA SP-382, in press

Using the unique X-ray imaging capabilities of the Chandra X-ray observatory, a 2006 observation of Cygnus X-3 has provided insight into a singular feature associated with this well-known microquasar. This extended emission, located approx.16" from Cygnus X-3, varies in flux and orbital phase (shifted by 0.56 in phase) with Cygnus X-3, acting like a celestial X-ray "mirror." The feature s spectrum, flux, and time variations allow us to determine the location, size, density, and mass of the scatterer. We find that the scatterer is a Bok Globule located along our line of sight, and we discuss its relationship to Cygnus X-3. This is the first time such a feature has been identified with Chandra.

The Burst and Transient Source Experiment (BATSE),aboard the COmptOn Gamma Ray Observatory (CGRO), provided a record of the hard X-ray/low energy gamma ray sky between April 1991 and June 2000. During that time, a catalog of known sources was derived from existing catalogs such as HEAO A-4 (Levine et al. 19841, as well as new transient sources discovered with RATSE and other X-ray monitors operating in the CGRO era. The Earth Occultation Technique (Harmon et al. 2001, astro-ph/0109069) was used to monitor a combination of these sources, mostly galactic, totaling about 175 objects. The catalog will present the global properties of these sources and their probability of detection (>lO mCrab, 20-100 keV) with BATSE. Systematic errors due to unknown sources or background components are included. Cursory analyses to search for new transients (35-80 mCrab in the 20-100 keV band) and super-orbital periods in known binary sources are also presented. Whole mission light curves and associated data production/analysis tools are being delivered to the HEASARC for public use.

The Burst and Transient Source Experiment (BATSE), aboard the Compton Gamma Ray Observatory (CGRO), provided a record of the low-energy gamma-ray sky (approx. 20-1000 keV) between 1991 April and 2000 May (9.1y). BATSE monitored the high energy sky using the Earth occultation technique (EOT) for point sources whose emission extended for times on the order of the CGRO orbital period (approx. 92m) or greater. Using the EOT to extract flux - 2 - information, a catalog of sources using data from the BATSE large area detectors has been prepared. The first part of the catalog consists of results from the all-sky monitoring of 58 sources, mostly Galactic, with intrinsic variability on timescales of hours to years. For these sources, we have included tables of flux and spectral data, and outburst times for transients. Light curves (or flux histories) covering the entire nine mission are being placed on the world wide web. We then performed a deep-sampling of these 58 objects, plus a selection of 121 more objects, combining data from the entire 9.ly BATSE dataset. Source types considered were primarily accreting binaries, but a small number of representative active galaxies, X-ray-emitting stars, and supernova remnants were also included. The sample represents a compilation of sources monitored and/or discovered with BATSE and other high energy instruments between 1991 and 2000, known sources taken from the HEAO 1 A-4 (Levine et al. 1984) and Macomb and Gehrels (1999) catalogs. The deep sample results include definite detections of 82 objects and possible detections of 36 additional objects. The definite detections spanned three classes of sources: accreting black hole and neutron star binaries, active galaxies and supernova remnants. The average fluxes measured for the fourth class, the X-ray emitting stars, were below the confidence limit for definite detection. Flux data for the deep sample are presented in four energy bands: 20-40, 40-70, 70-160, and 160-430 keV. The limiting average flux level (9.1 y) for the sample varies from 3.5 to 20 mCrab (5delta) between 20 and 430 keV, depending on systematic error, which in turn is primarily dependent on the sky location. To strengthen the credibility of detection of weaker sources (approx.5-25 mCrab), we generated Earth occultation images, searched for periodic behavior using FFT and epoch folding methods, and critically evaluated the energy-dependent emission in the four flux bands. The deep sample results are intended for guidance in performing future all-sky surveys or pointed observations in the hard X-ray and low-energy gamma-ray band, as well as more detailed studies with the BATSE EOT.

The Burst and Transient Source Experiment (BATSE), aboard the Compton Gamma Ray Observatory (CGRO), provided a record of the hard X-ray/low energy gamma ray sky between April 1991 and June 2000. During that time, a catalog of known sources was derived from existing catalogs such as HEAO A-4 (Levine et al. 1984), as well as new transient sources discovered with BATSE and other X-ray monitors operating in the CGRO era. The Earth Occultation Technique (Harmon et al. 2001, astro-ph/0109069) was used to monitor a combination of these sources, mostly galactic, totaling to about 175 objects. The catalog will present the global properties of these sources and their probability of detection (greater than 10 mCrab, 20-100 keV) with BATSE. Systematic errors due to unknown sources or background components are included. Cursory analyses to search for new transients (35-80 mCrab in the 20-100 keV band) and super-orbital periods in known binary sources are also presented. Whole mission light curves and associated data production/analysis tools are being delivered to the High Energy Astrophysics Science Archive Research Center (HEASARC) for public use.

The 1998 September outburst of the X-ray binary XTE J1550-564 was monitored at X-ray, optical and radio wavelengths. We divide the outburst sequence into five phases and discuss their multi-wavelength properties. The outburst starts with a hard X-ray spike, while the soft X-ray flux rises with a longer timescale. We suggest that the onset of the outburst is determined by an increased mass transfer rate from the companion star, but the outburst morphology is determined by the distribution of specific angular momentum in the accreting matter. The companion in XTE J1550-564 is likely an active magnetic star, with a surface field strong enough to influence the dynamics of mass transfer. We suggest that its magnetic field can create a magnetic bag capable of confining gas inside the Roche lobe of the primary. The impulsive rise in the hard X-rays is explained by the inflow of material with low angular momentum onto the black hole, on a free-fall timescale, when the magnetic confinement breaks down. At the same time, high angular momentum matter, transferred via ordinary Roche-lobe overflow, is responsible for the formation of a disk.

We have studied the 1999 soft X-ray transient outburst of XTE J1859+226 at radio and X-ray wavelengths. The event was characterized by strong variability in the disc, corona and jet-in particular, a number of radio flares (ejections) took place and seemed well-correlated with hard X-ray events. Apparently unusual for the canonical soft X-ray transient, there was an initial period of low/hard state behavior during the rise from quiescence but prior to the peak of the main outburst - we show that not only could this initial low/hard state be an ubiquitous feature of soft X-ray transient outbursts but that it could also be extremely important in our study of outburst mechanisms.