arXiv subjects

R. A. Remillard

Publications and source records attributed to R. A. Remillard.

At least 55 records · Page 3Linked to original sources

arXiv2001

Multiwavelength Observations of the Black Hole Candidate XTE J1550-564 during the 2000 Outburst

We report optical, infrared, and X-ray light curves for the outburst, in 2000, of the black hole candidate XTE J1550-564. We find that the start of the outburst in the H and V bands precedes that seen in the RXTE All Sky Monitor by 11.5 +/- 0.9 and 8.8 +/- 0.6 days, respectively; a similar delay has been observed in two other systems. About 50 days after the primary maxima in the VIH light curves, we find secondary maxima, most prominently in H. This secondary peak is absent in the X-ray light curve, but coincides with a transition to the low/hard state. We suggest that this secondary peak may be due to non-thermal emission associated with the formation of a jet.

astro-ph↗

arXiv2001

Radio Emission and the Timing Properties of the Hard X-ray State of GRS 1915+105

We combine a complete sample of 113 pointed observations taken with the Rossi X-ray Timing Explorer between 1996-1999, monitoring observations taken with the Ryle telescope and the Green Bank Interferometer, and selected observations with the Very Large Array to study the radio and X-ray properties of GRS 1915+105 when its X-ray emission is hard and steady. We establish that radio emission always accompanies the hard-steady state of GRS 1915+105, but that the radio flux density at 15.2 GHz and the X-ray flux between 2-200 keV are not correlated. Therefore we study the X-ray spectral and timing properties of GRS 1915+105 using three approaches: first, by describing in detail the properties of three characteristic observations, then by displaying the time evolution of the timing properties during periods of both faint and bright radio emission, and lastly by plotting the timing properties as a function of the the radio flux density. We find that as the radio emission becomes brighter and more optically thick, 1) the frequency of a ubiquitous 0.5-10 Hz QPO decreases, 2) the Fourier phase lags between hard (11.5-60 keV) and soft (2-4.3 keV) in the frequency range of 0.01-10 Hz change sign from negative to positive, 3) the coherence between hard and soft photons at low frequencies decreases, and 4) the relative amount of low frequency power in hard photons compared to soft photons decreases. We discuss how these results reflect upon basic models from the literature describing the accretion flow around black holes and the possible connection between Comptonizing electrons and compact radio jets.

astro-ph↗

arXiv2001

Properties of Black Holes in Stellar Binary Systems

Recent X-ray timing and spectral observations of black hole binaries in outburst have redefined methods for investigating the properties of Galactic black holes and the physics of accretion flows. High-frequency X-ray QPOs in 5 systems (60--300 Hz) continue to be investigated as a possible means to constrain black hole mass and spin via interpretation as oscillations due to strong-field effects in General Relativity. In principle, black hole mass and spin may also be constrained via determination of the inner radius of the accretion disk, using spectroscopic parallax of the X-ray thermal component. However, an accurate application of this technique is fraught with difficulties. Monitoring programs that track the evolution of the thermal and power-law components in the X-ray spectrum provide new insights into modes of energy flow and the formation of relativistic jets.

astro-ph↗

arXiv2001

Use of Gas Electron Multiplier (GEM) Detectors for an Advanced X-ray Monitor

We describe a concept for a NASA SMEX Mission in which Gas Electron Multiplier (GEM) detectors, developed at CERN, are adapted for use in X-ray astronomy. These detectors can be used to obtain moderately large detector area and two-dimensional photon positions with sub mm accuracy in the range of 1.5 to 15 keV. We describe an application of GEMs with xenon gas, coded mask cameras, and simple circuits for measuring event positions and for anticoincidence rejection of particle events. The cameras are arranged to cover most of the celestial sphere, providing high sensitivity and throughput for a wide variety of cosmic explosions. At longer timescales, persistent X-ray sources would be monitored with unprecedented levels of coverage. The sensitivity to faint X-ray sources on a one-day timescale would be improved by a factor of 6 over the capability of the RXTE All Sky Monitor.

astro-ph↗

arXiv2000

Hard X-rays from Type II bursts of the Rapid Burster and its transition toward quiescence

We report on 4 BeppoSAX Target Of Opportunity observations of MXB 1730-335, the Rapid Burster (RB), made during the 1998 February-March outburst. In the first observation, approximately 20 days after the outburst peak, the X-ray light curve showed Type II bursts at a rate of 43 per hour. Nine days later, during the second BeppoSAX pointing, only 5 Type II bursts were detected at the beginning of the observation. During the third pointing no X-ray bursts were detected and in the fourth and final observation the RB was not detected at all. Persistent emission from the RB was detected up to 10 keV during the first three pointings. The spectra of the persistent and bursting emissions below 10 keV were best fit with a model consisting of two blackbodies. An additional component (a power law) was needed to describe the 1-100 keV bursting spectrum when the persistent emission was subtracted. To our knowledge, this is the first detection of the RB beyond 20 keV. We discuss the evolution of the spectral parameters for the bursting and persistent emission during the outburst decay. The light curve, after the second BeppoSAX pointing, showed a steepening of the previous decay trend, and a sharper decay rate leading to quiescence was observed with BeppoSAX in the two subsequent observations. We interpret this behaviour as caused by the onset of the propeller effect. Finally, we infer a neutron star magnetic field B ~ 4 10^8 Gauss.

astro-ph↗

arXiv2000

The transient X-ray source SAX J2239.3+6116 and its optical counterpart

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.

astro-ph↗

arXiv1999

X-ray Nova XTE J1550-564: Discovery of a QPO Near 185 Hz

We have investigated the X-ray timing properties of XTE J1550-564 during 60 RXTE PCA observations made between 1998 September 18 and November 28. We detect quasi-periodic oscillations (QPOs) near 185 Hz during four time intervals. The QPO widths (FWHM) are near 50 Hz, and the rms amplitudes are about 1% of the mean flux at 2-30 keV. This is the third Galactic black hole candidate known to exhibit a transient X-ray timing signature above 50 Hz, following the 67 Hz QPO in GRS1915+105 and the 300 Hz QPO in GRO J1655-40. However, unlike the previous cases, which appear to show stationary frequencies, the QPO frequency in XTE J1550-564 must vary by at least 10% to be consistent with observations. The occurrences and properties of the QPO were insensitive to large changes in the X-ray intensity (1.5 to 6.8 Crab). However, the QPO appearance was accompanied by changes in the energy spectrum, namely, an increase in the temperature and a decrease in the normalization of the thermal component. The QPO is also closely related to the hard X-ray power-law component of the energy spectrum since the fractional amplitude of the QPO increases with photon energy. The fast QPOs in accreting black hole binaries are thought to be effects of general relativity; however, the relevance of the specific physical models that have been proposed remains largely uncertain. Low frequency QPOs in the range 3-13 Hz were often observed. Occasionally at high luminosity the rms QPO amplitude was 15% of the flux, a level previously reached only by GRS1915+105.

astro-ph↗

arXiv1999

X-ray Nova XTE J1550-564: RXTE Spectral Observations

Excellent coverage of the 1998 outburst of the X-ray Nova XTE J1550-564 was provided by the Rossi X-ray Timing Explorer. XTE J1550-564 exhibited an intense (6.8 Crab) flare on 1998 September 19 (UT), making it the brightest new X-ray source observed with RXTE. We present a spectral analysis utilizing 60 Proportional Counter Array spectra from 2.5-20 keV spanning 71 days, and a nearly continuous All Sky Monitor light curve. The spectra were fit to a model including multicolor blackbody disk and power-law components. XTE J1550-564 is observed in the very high, high/soft, and intermediate canonical outburst states of Black Hole X-ray Novae.

astro-ph↗

arXiv1999

X-ray Nova XTE J1550-564: Optical Observations

We report the identification of the optical counterpart of the X-ray transient XTE J1550-564 described in two companion papers by Sobczak et al (1999) and Remillard et al (1999). We find that the optical source brightened by approximately 4 magnitudes over the quiescent counterpart seen at B~22 on a SERC survey plate, and then decayed by approximately 1.5 magnitudes over the 7 week long observation period. There was an optical response to the large X-ray flare described by Sobczak et al (1999), but it was much smaller and delayed by roughly 1 day.

astro-ph↗

arXiv1998

Multifrequency observations of XTE J0421+560/CI Cam in outburst

We report on two X-ray observations of the transient source XTE J0421+560 performed by BeppoSAX, and on a series of observations performed by the 0.7m Teramo-Normale Telescope. Outburst peak occurrence time and duration depend on photon energy: the outburst peak is achieved first in the X-ray band, then in the optical and finally in the radio. An exponential decay law fits well the X-ray data except in the TOO2 0.5-1.0 keV band, where erratic time variability is detected. During TOO1 the e-folding time scale decreases with energy up to ~ 20 keV, when it achieves a saturation; during TOO2 it decreases up to ~ 2 keV and then increases. This change is correlated with a spectral change, characterized by the onset of a soft (< 2 keV) component in TOO2 (Orr et al. 1998). This component might originate from the relativistic jets, while the hard component is more likely associated to processes occurring in the circumstellar matter and/or near the compact object. Optical observations show that the object appears intrinsically red even during the outburst. The nature of the compact object is discussed.

astro-ph↗

arXiv1998

A Multiwavelength Study of the Extreme AGN J2310-437

We present new X-ray, radio, and optical data for the unusual galaxy/cluster system J2310-437. Our results confirm the presence of an active nucleus, and suggest an interpretation as an anomalous BL Lac object of bulk relativistic Doppler factor < 2, with an optically deficient radio-to-X-ray spectrum. The radio, optical, and soft X-ray flux densities could lie along a single power-law function, lacking the curvature typical of BL Lac objects. Compared with other known sources that may have comparable multifrequency spectra, J2310-437 is the most extreme. Its low isotropic optical/UV radiation is consistent with the intensity of external photons governing the electron spectral break through Compton cooling; in this source the external photon density would be too low to produce a spectral break below the X-ray.

astro-ph↗

arXiv1998

X-ray Observations of the Seyfert galaxy LB 1727 (1H 0419-577)

We discuss the properties of the Seyfert 1.5 galaxy LB 1727, also known as 1H 0419-577, from X-ray observations obtained by ASCA and ROSAT along with optical observations from earlier epochs. ASCA shows only modest (< 20%) variations in X-ray flux within or between the observations. In contrast, a daily monitoring campaign over 1996 Jun - Sept by the ROSAT HRI instrument reveals the soft X-ray (0.1-2 keV) flux to have increased by a factor ~3. The 2 - 10 keV continuum can be parameterized as a power-law with a photon index Gamma ~ 1.45-1.68 across ~0.7 - 11 keV in the rest-frame. We also report the first detection of iron Kalpha line emission in this source. Simultaneous ASCA and ROSAT data show the X-ray spectrum to steepen sharply at a rest-energy \~0.75 keV, the spectrum below this energy can be parameterized as a power-law of slope Gamma ~3.6. We show that LB 1727 is one of the few Seyferts for which we can rule out the possibility that the presence of a warm absorber is solely responsible for the spectral steepening in the soft X-ray regime. Consideration of the overall spectral-energy-distribution for this source indicates the presence of a pronounced XUV-bump visible in optical, ultraviolet and soft X-ray data.

astro-ph↗

arXiv1998

RXTE Observations of 0.1-300 Hz QPOs in the Microquasar GRO J1655-40

We have investigated 52 RXTE pointed observations of GRO J1655-40 spanning the X-ray outburst that commenced on 1996 April 25 and lasted for 16 months. Our X-ray timing analyses reveal four types of QPOs: three with relatively stable central frequencies at 300 Hz, 9 Hz, and 0.1 Hz, and a fourth that varied over the range 14-28 Hz. The 300 Hz and 0.1 Hz QPOs appear only at the highest observed luminosities (Lx > 0.15 Ledd), where the power-law component dominates the X-ray spectrum. At lower luminosity, the disk flux exceeds the power-law flux and only two of the QPOs are observed: the spectrally "soft" 9 Hz QPO, and the narrow, "hard" QPO that varies from 14-28 Hz as the hard flux decreases. The 300 Hz QPO is likely to be analogous to the stationary QPO at 67 Hz seen in the microquasar GRS1915+105. We discuss models of these high-frequency QPOs which depend on effects due to general relativity. The 9 Hz QPO displays a spectrum consistent with a thermal origin, but this frequency does not appear to be consistent with any of the natural time scales associated with the disk, or with the inferred values of the mass and rapid spin of the black hole. The mechanism for the 14-28 Hz QPOs appears to be linked to the power-law component, as do the 1-10 Hz QPOs in GRS1915+105. Finally, we show data for GRO J1655-40 and GRS1915+105 as each source teeters between relative stability and a state of intense oscillations at 0.1 Hz. A comparison of the sources' spectral parameters allows us to speculate that the black hole mass in GRS1915+105 is very large, possibly in the range 39-70 Msun.

astro-ph↗

arXiv1998

X-ray Transients Observed with the RXTE All Sky Monitor 1996-1997

Highlights from the RXTE All Sky Monitor (ASM) during 1996 and 1997 are reviewed with particular attention to X-ray transients. The ASM has detected 117 sources. These include 12 recurrent transients and 10 new X-ray sources, some of which began their outbursts before the launch of RXTE. The majority of the outburst profiles are strikingly different from the classical form, with its fast rise and slow decay. Some of the light curves appear quasi-persistent, which suggests that they are associated with secular changes in the accretion rate from the donor star rather than with a cyclic instability in the accretion disk. The nature of the compact object is uncertain in many cases, but there is a likelihood that the majority of new sources are black hole systems, while the majority of recurrent transients contain neutron stars. There is a correlation between the gross shape of the energy spectrum, as seen in the ASM hardness ratio, HR2 (5-12 keV / 3-5 keV), and the accretion subclass. These data, with important contributions from SAX, CGRO, and Granat, provide a deeper probe of the X-ray sky than has been available previously. However, there are important systematic effects to be investigated before a meaningful evaluation of transient occurrence rates can be derived.

astro-ph↗

arXiv1998

Monitoring the Galactic Microquasars with the Rossi X-ray Timing Explorer

The Galactic microquasars GRS1915+105 and GRO J1655-40 were targets for dedicated monitoring programs with RXTE throughout 1996 and 1997. We review recent results with particular attention to the X-ray QPOs seen in these sources. We confirm the reappearance of 67 Hz oscillations in GRS1915+105 in several different spectral states. Six intervals, each of 1 to 6 months duration, yield QPO detections with a central frequency of 66.7 +- 1.4 Hz and a FWHM of 3.4 +- 1.0 Hz. Comparing these observing intervals, the average X-ray luminosity varies by a factor of 4 with no significant change in the central QPO frequency. GRS1915+105 has been active in X-rays since 1992, and it shows no signs of evolving toward quiescence. In contrast, GRO J1655-40, which began its most recent X-ray outburst on 1996 April 25, is seen to fade below 20 mCrab on 1997 Aug 17. There are no reappearances of the 300 Hz QPO during the entire second half of the X-ray outburst; during this time the X-ray spectrum is dominated by the soft, thermal component. There are still several models seeking to explain the high frequency QPOs in microquasars. While it is likely that the cause invokes some effect of General Relativity, the correct model and the details of the emission mechanism remain uncertain.

astro-ph↗

arXiv1998

ASCA Observation of the polar RX J1802.1+1804

We present X-ray data of RX J1802.1+1804 obtained by ASCA. Although it shows a clear orbital intensity modulation with an amplitude of nearly 100% below 0.5 keV in ROSAT data, the ASCA light curves are nearly flat except for a possible dip lasting about one-tenth of the orbital period. We discuss this within the model assumption of a stream-eclipsing geometry as derived from the ROSAT observations. The ASCA X-ray spectrum can be represented by a two temperature optically thin thermal plasma emission model with temperatures of ~1keV and >7 keV, suggesting postshock cooling as observed in EX Hya. A remarkable feature of the spectrum is the strong iron K_alpha emission line whose equivalent width is ~4 keV. To account for this, an iron abundance of greater than at least 1.3 times Solar is required. A combined spectral analysis of the ROSAT PSPC and ASCA data indicates that the N_H-corrected flux ratio of the soft blackbody (0.1-2.4 keV) to the hard optically thin thermal plasma emission (2-10 keV) is as large as ~10^4.

astro-ph↗

arXiv1998

Bipolar Jets and Orbital Dynamics of the Supersoft X-Ray Source RX J0019.8+2156

From 1994 to 1997 we monitored the optical spectrum of RXJ0019.8+2156. This supersoft X-ray source is one of only two accreting white dwarfs in the Galaxy thought to be burning hydrogen on their surface as a consequence of a high rate of mass transfer from a binary companion. Accurate orbital ephemerides are derived from radial velocity measure- ments of HeII emission lines, which exhibit a stable velocity semi- amplitude of 71.2 +- 3.6 km/s. We report the discovery of transient, low-velocity, bipolar jets. These jets are respresented by redshifted and blueshifted pairs of emission lines with an outflow velocity of v cos(i) = 815 km/s, where i is the binary inclination angle. When present, the jet lines also exhibit an orbital modulation of 71 km/s, which indicates that the jets are oriented nearly perpendicular to the orbital plane. On most occasions, the Hline profiles are further altered by P Cygni absorption. We show that the jets and the P Cygni features have very different temporal characteristics and binary phase dependence. The jet material and the absorbing wind must therefore occupy very different geometries. Finally, the measured mass function is combined with binary evolution models to suggest a limit, i < 40 deg. A particular model invoked to explain a high rate of mass transfer requires 16 < i < 25. However, at such small inclination it is difficult to explain the large amplitude of the orbital light curve. By contrast, a simple model fit to the jet outflow lines indicates an orbital inclination angle of 35 < i < 60.

astro-ph↗

arXiv1998

The X-ray stream-eclipsing polar RX J1802.1+1804

We present X-ray and optical data of the newly discovered AM Her variable RX J1802.1+1804 = V884 Her. This X-ray source was observed in the ROSAT All-Sky-Survey in September 1990 and subsequently discovered as a highly variable and soft X-ray source. Follow-up pointed observations confirmed the strong variability and revealed a periodic flux modulation including a sharp and nearly complete eclipse of the X-ray emission. Based on the shape and duration of this eclipse as well as the lack of optical eclipses we favour an interpretation in terms of a self-eclipse by the accretion stream. The X-ray spectrum averaged over the full period is dominated by soft emission below 0.5 keV. The ratio of soft (blackbody) to hard (bremsstrahlung) bolometric energy flux is 1670, distinguishing this object as another example of a polar with an extreme strong excess in soft X-rays.

astro-ph↗