arXiv subjects

D. M. Smith

Publications and source records attributed to D. M. Smith.

At least 37 records · Page 2Linked to original sources

arXiv2007

Long term monitoring of the BHC 1E 1740.7-2942

The microquasar 1E 1740.7-2942 is one of the most appealing source of the Galactic Centre region. The high energy feature detected once by SIGMA has been searched in the last years by INTEGRAL, but never confirmed. Classified as a persistent source, on 2004 it showed a quiescent-like state. In fact for few month 1E 1740.7-2942 was below the detector sensitivity level. We present the long term temporal behaviour of 1E 1740.7-2942 observed by INTEGRAL and RXTE in 2004 and 2005, as well as preliminary results on possible spectral transitions.

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arXiv2007

Spectral Analysis of GRBs Measured by RHESSI

The Ge spectrometer of the RHESSI satellite is sensitive to Gamma Ray Bursts (GRBs) from about 40 keV up to 17 MeV, thus ideally complementing the Swift/BAT instrument whose sensitivity decreases above 150 keV. We present preliminary results of spectral fits of RHESSI GRB data. After describing our method, the RHESSI results are discussed and compared with Swift and Konus.

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arXiv2006

The Giant Flare of December 27, 2004 from SGR 1806-20

The giant flare of December 27, 2004 from SGR 1806-20 represents one of the most extraordinary events captured in over three decades of monitoring the gamma-ray sky. One measure of the intensity of the main peak is its effect on X- and gamma-ray instruments. RHESSI, an instrument designed to study the brightest solar flares, was completely saturated for ~0.5 s following the start of the main peak. A fortuitous alignment of SGR 1806-20 near the Sun at the time of the giant flare, however, allowed RHESSI a unique view of the giant flare event, including the precursor, the main peak decay, and the pulsed tail. Since RHESSI was saturated during the main peak, we augment these observations with Wind and RHESSI particle detector data in order to reconstruct the main peak as well. Here we present detailed spectral analysis and evolution of the giant flare. We report the novel detection of a relatively soft fast peak just milliseconds before the main peak, whose timescale and sizescale indicate a magnetospheric origin. We present the novel detection of emission extending up to 17 MeV immediately following the main peak, perhaps revealing a highly-extended corona driven by the hyper-Eddington luminosities. The spectral evolution and pulse evolution during the tail are presented, demonstrating significant magnetospheric twist and evolution during this phase. Blackbody radii are derived for every stage of the flare, which show remarkable agreement despite the range of luminosities and temperatures covered. Finally, we place significant upper limits on afterglow emission in the hundreds of seconds following the giant flare.

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arXiv2006

INTEGRAL and RXTE monitoring of GRS 1758-258 in 2003 and 2004. A transition from the dim soft state to the hard state

The Galactic Center black hole candidate (BHC) GRS 1758-258 has been observed extensively within INTEGRAL's Galactic Center Deep Exposure (GCDE) program in 2003 and 2004, while also being monitored with RXTE. We present quasi-simultaneous PCA, ISGRI, and SPI spectra from four GCDE observation epochs, as well as the evolution of energy-resolved PCA and ISGRI light curves on time scales of days to months. We find that during the first epoch GRS 1758-258 displayed another of its peculiar dim soft states like the one observed in 2001, increasing the number of observed occurrences of this state to three. During the other epochs the source was in the hard state. The hard X-ray emission component in the epoch-summed spectra can be well described either by phenomenological models, namely a cutoff power law in the hard state and a pure power law in the dim soft state, or by thermal Comptonization models. A soft thermal component is clearly present in the dim soft state and might also contribute to the softer hard state spectra. We argue that in the recently emerging picture of the hardness-intensity evolution of black hole transient outbursts in which hard and soft states are observed to occur in a large overlapping range of luminosities (hysteresis), the dim soft state is not peculiar. As noted before for the 2001 dim soft state, these episodes seem to be triggered by a sudden decrease (within days) of the hard emission, with the soft spectral component decaying on a longer time scale (weeks). We discuss this behavior in terms of the existence of two independent accretion flows, the model previously suggested for the 2001 episode. (Abridged)

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arXiv2005

XTE J1739-302 as a Supergiant Fast X-ray Transient

XTE J1739-302 is a transient X-ray source with unusually short outbursts, lasting on the order of hours. Here we give a summary of X-ray observations we have made of this object in outburst with the Rossi X-ray Timing Explorer (RXTE) and at a low level of activity with the Chandra X-ray Observatory, as well as observations made by other groups. Visible and infrared spectroscopy of the mass donor of XTE J1739-302 are presented in a companion paper. The X-ray spectrum is hard both at low levels and in outburst, but somewhat variable, and there is strong variability in the absorption column from one outburst to another. Although no pulsation has been observed, the outburst data from multiple observatories show a characteristic timescale for variability on the order of 1500-2000 s. The Chandra localization (right ascension 17h 39m 11.58s, declination -30o 20' 37.6'', J2000) shows that despite being located less than 2 degrees from the Galactic Center and highly absorbed, XTE J1739-302 is actually a foreground object with a bright optical counterpart. The combination of a very short outburst timescale and a supergiant companion is shared with several other recently-discovered systems, forming a class we designate as Supergiant Fast X-ray Transients (SFXTs). Three persistently bright X-ray binaries with similar supergiant companions have also produced extremely short, bright outbursts: Cyg X-1, Vela X-1, and 1E 1145.1-6141.

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arXiv2005

An exceptionally bright flare from SGR1806-20 and the origins of short-duration gamma-ray bursts

Soft-gamma-ray repeaters (SGRs) are galactic X-ray stars that emit numerous short-duration (about 0.1 s) bursts of hard X-rays during sporadic active periods. They are thought to be magnetars: strongly magnetized neutron stars with emissions powered by the dissipation of magnetic energy. Here we report the detection of a long (380 s) giant flare from SGR 1806-20, which was much more luminous than any previous transient event observed in our Galaxy. (In the first 0.2 s, the flare released as much energy as the Sun radiates in a quarter of a million years.) Its power can be explained by a catastrophic instability involving global crust failure and magnetic reconnection on a magnetar, with possible large-scale untwisting of magnetic field lines outside the star. From a great distance this event would appear to be a short-duration, hard-spectrum cosmic gamma-ray burst. At least a significant fraction of the mysterious short-duration gamma-ray bursts therefore may come from extragalactic magnetars.

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arXiv2004

1E 1740.7-2942: temporal and spectral evolution from INTEGRAL and RXTE observations

We present results of the monitoring of the black hole candidate 1E 1740.7-2942 with INTEGRAL, in combination with simultaneous observations by RXTE. We concentrate on broad-band spectra from INTEGRAL/IBIS and RXTE/PCA instruments. During our observations, the source spent most of its time in the canonical low/hard state with the measured flux variation within a factor of two. In 2003 September the flux started to decline and in 2004 February it was below the sensitivity level of the INTEGRAL and RXTE instruments. Notably, during the decline phase the spectrum changed, becoming soft and typical of black hole binaries in the intermediate/soft state.

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arXiv2004

INTEGRAL monitoring of the Black Hole candidate 1E 1740.7-2942

The brightest persistent Galactic black hole candidate close to the Galactic Centre, 1E 1740.7-2942, has long been observed with INTEGRAL. In this paper, we report on the long-term hard X-ray monitoring obtained during the first year of observations as part of the Galactic Centre Deep Exposure. We discuss the temporal and spectral behaviours in different energy bands up to 250 keV, as well as the hardness-flux correlations.

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arXiv2003

Chandra/HETGS Spectroscopy of the Galactic Black Hole GX 339-4: A Relativistic Iron Line and Evidence for a Seyfert-like Warm Absorber

We observed the Galactic black hole GX 339-4 with the Chandra High Energy Transmission Grating Spectrometer (HETGS) for 75 ksec during the decline of its 2002-2003 outburst. The sensitivity of this observation provides an unprecedented glimpse of a Galactic black hole at about a tenth of the luminosity of the outburst peak. The continuum spectrum is well described by a model consisting of multicolor disk blackbody (kT = 0.6 keV) and power-law (Gamma = 2.5) components. X-ray reflection models yield improved fits. A strong, relativistic Fe K-alpha emission line is revealed, indicating that the inner disk extends to the innermost stable circular orbit. The breadth of the line is sufficient to suggest that GX 339-4 may harbor a black hole with significant angular momentum. Absorption lines from H-like and He-like O, and He-like Ne and Mg are detected, as well as lines which are likely due to Ne II and Ne III. The measured line properties make it difficult to associate the absorption with the coronal phase of the interstellar medium. A scenario wherein the absorption lines are due to an intrinsic AGN-like warm-absorber geometry -- perhaps produced by a disk wind in an extended disk-dominated state -- may be more viable. We compare our results to Chandra observations of the Galactic black hole candidate XTE J1650-500, and discuss our findings in terms of prominent models for Galactic black hole accretion flows and connections to supermassive black holes.

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arXiv2003

Long Term Variability of Cyg X-1 I. X-ray spectral-temporal correlations in the hard state

We present the long term evolution of the timing properties of the black hole candidate Cygnus X-1 in the 0.002-128 Hz frequency range as monitored from 1998 to 2001 with the RXTE. The hard state power spectral density (PSD) is well modeled as the sum of four Lorentzians, which describe distinct broad noise components. Before 1998 July, Cyg X-1 was in a "quiet" hard state characterized primarily by the first three of these broad Lorentzians. Around 1998 May, this behavior changed: the total fractional rms amplitude decreased, the peak frequencies of the Lorentzians increased, the average time lag slightly increased, and the X-ray spectrum softened. The change in the timing parameters is mainly due to a strong decrease in the amplitude of the third Lorentzian. Since then, an unusually large number of X-ray flares have been observed. During these "failed state transitions", the X-ray power spectrum changes to that of the intermediate state. Modeling this PSD with the four Lorentzians, we find that the first Lorentzian component is suppressed relative to the second and third Lorentzian. Also the frequency-dependent time lags increase significantly. We confirm the interpretation as failed state transitions with observations from the 2001 Jan. and 2001 Oct. soft states. Such behavior suggests that some or all of the Lorentzian components are associated with the accretion disk corona. We discuss the physical interpretation of our results.

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arXiv2003

Extended Emission from Cygnus X-3 Detected with Chandra

We have discovered extended X-ray emission from the microquasar Cyg X-3 in archival Chandra X-ray Observatory observations. A 5" wide structure lies approximately 16" to the NE from the core point source and may be extended in that direction. This angular scale corresponds to a physical extent of roughly 0.8 lyr, at a distance of 2.5 lyr from Cyg X-3 (assuming a 10 kpc distance). The flux varied by a factor of 2.5 during the four months separating two of the observations, indicating significant substructure. The peak 2-10 keV luminosity was about 5e34 ergs/s. There may also be weaker, extended emission of similar scale oppositely directed from the core, suggesting a bipolar outflow. This structure is not part of the dust scattering halo, nor is it caused by the Chandra point spread function. In this Letter we describe the observations and discuss possible origins of the extension.

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arXiv2002

RHESSI as a Hard X-Ray Polarimeter

Polarization measurements of hard X-rays from solar flares can provide a direct handle on the extent to which the accelerated electrons are beamed, which, in turn, has important implications for particle acceleration models. Past efforts to measured X-ray polarization have met with only limited success. Although designed primarily as a hard X-ray imager and spectrometer, the Ramaty High Energy Solar Spectroscopic Imager (RHESSI) is also capable of measuring the polarization of hard X-rays (20-100 keV) from solar flares. This capability arises from the inclusion of a small unobstructed Be scattering element that is strategically located within the cryostat that houses the array of nine germanium detectors. The Ge detectors are segmented, with both a front and rear active volume. Low energy photons (below about 100 keV) can reach a rear segment of a Ge detector only indirectly, by scattering. Low energy photons from the Sun have a direct path to the Be and have a high probability of Compton scattering into a rear segment of a Ge detector. The azimuthal distribution of these scattered photons carries with it a signature of the linear polarization of the incident flux. Sensitivity estimates, based on Monte Carlo simulations and in-flight background measurements, indicate that a 20-100 keV polarization sensitivity of less than a few percent can be achieved for X-class flares.

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arXiv2002

Orbital and Super-Orbital Periods of 1E 1740.7-2942 and GRS 1758-258

Five years of Rossi X-ray Timing Explorer (RXTE) observations of the Galactic black-hole candidates 1E 1740.7-2942 and GRS 1758-258 show a periodic modulation with amplitude 3-4% in each source at 12.73 +/- 0.05 dy and 18.45 +/- 0.10 dy, respectively. We interpret the modulations as orbital, suggesting that the objects have red-giant companions. Combining the RXTE data with earlier data (Zhang, Harmon & Liang 1997) from the Burst and Transient Source Experiment on the Compton Gamma-Ray Observatory, we find a long period or quasi-period of about 600 dy in 1E 1740.7-2942, and a suggestion of a similar 600-dy period in GRS 1758-258. These timescales are longer than any yet found for either precessing systems like Her X-1 and SS 433 or binaries like LMC X-3 and Cyg X-1 with more irregular long periods.

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arXiv2002

Chandra High Resolution Camera Imaging of GRS 1758-258

We observed the "micro-quasar" GRS 1758-258 four times with Chandra. Two HRC-I observations were made in 2000 September-October spanning an intermediate-to-hard spectral transition (identified with RXTE). Another HRC-I and an ACIS/HETG observation were made in 2001 March following a hard-to-soft transition to a very low flux state. Based on the three HRC images and the HETG zero order image, the accurate position (J2000) of the X-ray source is RA = 18h 01m 12.39s, Dec = -25d 44m 36.1s (90% confidence radius = 0".45), consistent with the purported variable radio counterpart. All three HRC images are consistent with GRS 1758-258 being a point source, indicating that any bright jet is less than ~1 light-month in projected length, assuming a distance of 8.5 kpc.

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arXiv2001

Two Different Long-term Behaviors in Black-Hole Candidates: Evidence for Two Accretion Flows?

We discuss the results of long-term hard x-ray monitoring of Galactic black-hole candidates 1E 1740.7-2942, GRS 1758-258, Cyg X-1, GX 339-4, and Cyg X-3 with the Rossi X-Ray Timing Explorer (RXTE). The objects divide into two classes. In the first class, exemplified by Cyg X-1, luminosity and spectral hardness evolve simultaneously. In the second class, the relation is more complicated: the softest spectra occur while the count rate is dropping. Most models of accretion, tailored to Cyg X-1, do not predict the second sort of behavior. One interpretation is a simple model with two simultaneous, independent accretion flows: a thin disk and a hot halo. A drop in the accretion rate affecting both flows would propagate through the halo immediately but might take up to several weeks to propagate through the disk. While the inner halo is thus temporarily depleted compared to the disk, a temporary soft state is expected. This picture is supported by the observation that those sources which show delays (1E 1740.7-2942, GRS 1758-258, and GX 339-4) are expected to have low-mass companions, and those which do not (Cyg X-1, Cyg X-3) are known or thought to have high-mass companions. Low-mass companions imply accretion by Roche-lobe overflow, with a high specific angular momentum in the accreting material, and therefore a large disk with a long viscous timescale. Wind accretion from massive companions is expected to result in a much smaller disk, and thus little viscous delay.

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arXiv2001

The Spectrum and Accurate Location of GRS 1758-258

We observed the "micro-quasar" GRS 1758-258 four times with Chandra. Two HRC-I observations were made in 2000 September-October spanning an intermediate-to-hard spectral transition (identified with RXTE). Another HRC-I and an ACIS/HETGS observation were made in 2001 March following a hard-to-soft transition to a very low flux state. The accurate position (J2000) of the X-ray source is RA= 18 01 12.40, Dec= -25 44 36.0 (90% confidence radius = 0".6), consistent with the purported variable radio counterpart. All images are consistent with GRS 1758-258 being a point source, indicating that any bright jet is less than ~1 light-month in projected length, assuming a distance of 8.5 kpc. The March spectrum is well-fit with a multi-color disk-blackbody with an inner temperature of 0.50+/-0.01 keV, interstellar absorption of nH = (1.59 +/- 0.05)x10^22 cm^-2, and (un-absorbed) 1-10 keV luminosity of (4.5x10^36)(D/8.5kpc)^2 ergs/sec. No narrow emission lines are apparent in the spectrum and upper limits to line equivalent widths are given.

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arXiv2001

A Transition to the Soft State in GRS 1758-258

Near the end of 2001 February, the black-hole candidate (BHC) GRS 1758-258 made an abrupt transition from a standard hard (low) state to a soft state. Unlike Cyg X-1 and other BHCs, whose luminosity increases during this transition, GRS 1758-258 was dimmer after the transition. We present observations with the Proportional Counter Array on the Rossi X-ray Timing Explorer and interpret the phenomenon in the context of a ``dynamical'' soft state model. Using this model we predicted that mass transfer from the companion had ceased, and that the luminosity should decay on a timescale of a few weeks. The most recent data support this prediction, being consistent with a decay time of 34 dy. The current state is consistent with the ``off'' state of GRS 1758-258 reported by GRANAT/Sigma in 1991-1992.

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arXiv2001

Cygnus X-1 from RXTE: Monitoring the short term variability

We present temporal and spectral results from monitoring Cygnus X-1 with the Rossi X-ray Timing Explorer (RXTE) in 1998 and 1999. We concentrate on the long term evolution of the hard state timing properties, comparing it to the 1996 soft state evolution. This leads to the following results: 1. the hard and soft state time lag spectra are very similar, 2. during state transitions, the lags in the 1-10 Hz range increase by more than an order of magnitude, 3. in the hard state itself, flaring events can be seen --- the temporal and spectral evolution during the flare of 1998 July identifies it as a ``failed state transition''. During (failed) state transitions, the time lag spectra and the power spectra change predominantly in the 1-10 Hz range. We suggest that this additional variability is produced in ejected coronal material disrupting the synchrotron radiation emitting outflows present in the hard state.

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