Astrophysics Papers by Christopher Russell
We present recent work using three-dimensional (3D) Smoothed Particle Hydrodynamics (SPH) simulat... more We present recent work using three-dimensional (3D) Smoothed Particle Hydrodynamics (SPH) simulations to model the high ([Fe III], [Ar III], [Ne III] and [S III]) and low ([Fe II], [Ni II]) ionization forbidden emission lines observed in Eta Carinae using the HST/STIS. These structures are interpreted as the time-averaged, outer extensions of the primary wind and the wind-wind interaction region directly excited by the FUV of the hot companion star of this massive binary system. We discuss how analyzing the results of the 3D SPH simulations and synthetic slit spectra and comparing them to the spectra obtained with the HST/STIS helps us determine the absolute orientation of the binary orbit and helps remove the degeneracy inherent to models based solely on the observed RXTE X-ray light curve. A key point of this work is that spatially resolved observations like those with HST/STIS and comparison to 3D models are necessary to determine the alignment or misalignment of the orbital angular momentum axis with the Homunculus, or correspondingly, the alignment of the orbital plane with the Homunculus skirt.
Proceedings of the International Astronomical Union
ABSTRACT
Medium-dispersion long slit spectra, recorded by HST/STIS (R=8000, Theta=0.1"), resolve the ... more Medium-dispersion long slit spectra, recorded by HST/STIS (R=8000, Theta=0.1"), resolve the extended wind-wind interaction region of the massive binary, Eta Carinae. During the high state, extending for about five years of the 5.54-year binary period, lines of [N II], [Fe III], [S III], [Ar III] and [Ne III] extend outwards to 0.4" with a velocity range of -500 to +200
Cyg X-3 is a high mass X-ray binary consisting of a Wolf-Rayet star and a compact object in a ver... more Cyg X-3 is a high mass X-ray binary consisting of a Wolf-Rayet star and a compact object in a very short orbital period of 4.8h. The only confirmed microquasar with high energy gamma-ray emission, Cyg X-3 provides a unique opportunity to study the relationship between the accretion power and the power in high energy emission. Because of a compact orbit and a slow Wolf-Rayet wind, the flow structure around the compact object is thought to be strongly affected by the orbital motion, details of which can be obtained only by numerical simulations. In this paper, we report on the results from 3D hydrodynamic simulations of the wind accretion in Cyg X-3. For simplicity we adopt an anti-gravity-like force that emulates the radiative acceleration consistent with the beta-velocity wind. Due to the rapid orbital motion, the flow around the compact object has large density gradients. As a result, the accretion rate onto the compact object is significantly lower than that of the Bondi-Hoyle-Lyt...
The colliding wind binary (CWB) systems \eta\ Carinae and WR140 provide unique laboratories for X... more The colliding wind binary (CWB) systems \eta\ Carinae and WR140 provide unique laboratories for X-ray astrophysics. Their wind-wind collisions produce hard X-rays that have been monitored extensively by several X-ray telescopes, including RXTE. To interpret these RXTE X-ray light curves, we model the wind-wind collision using 3D smoothed particle hydrodynamics (SPH) simulations. Adiabatic simulations that account for the absorption of X-rays from an assumed point source at the apex of the wind-collision shock cone by the distorted winds can closely match the observed 2-10keV RXTE light curves of both \eta\ Car and WR140. This point-source model can also explain the early recovery of \eta\ Car's X-ray light curve from the 2009.0 minimum by a factor of 2-4 reduction in the mass loss rate of \eta\ Car. Our more recent models relax the point-source approximation and account for the spatially extended emission along the wind-wind interaction shock front. For WR140, the computed X-ray...
Proceedings of the International Astronomical Union, 2007
We study colliding winds in the superluminous binary η Carinae by performing threedimensional, Sm... more We study colliding winds in the superluminous binary η Carinae by performing threedimensional, Smoothed Particle Hydrodynamics (SPH) simulations. For simplicity, we assume both winds to be isothermal. We also assume that wind particles coast without any net external forces. We find that the lower density, faster wind from the secondary carves out a spiral cavity in the higher density, slower wind from the primary. Because of the phase-dependent orbital motion, the cavity is very thin on the periastron side, whereas it occupies a large volume on the apastron side. The model X-ray light curve using the simulated density structure fits very well with the observed light curve for a viewing angle of i = 54 • and φ = 36 • , where i is the inclination angle and φ is the azimuth from apastron.
The Astrophysical Journal
X-ray emission from the supermassive binary system Eta Carinae declines sharply around periastron... more X-ray emission from the supermassive binary system Eta Carinae declines sharply around periastron. This X-ray minimum has two distinct phases - the lowest flux phase in the first ~3 weeks and a brighter phase thereafter. In 2009, the Chandra X-ray Observatory monitored the first phase five times and found the lowest observed flux at ~1.9e-12 ergs cm-2 s-1 (3-8 keV). The spectral shape changed such that the hard band above ~4 keV dropped quickly at the beginning and the soft band flux gradually decreased to its lowest observed value in ~2 weeks. The hard band spectrum had begun to recover by that time. This spectral variation suggests that the shocked gas producing the hottest X-ray gas near the apex of the wind-wind collision (WWC) is blocked behind the dense inner wind of the primary star, which later occults slightly cooler gas downstream. Shocked gas previously produced by the system at earlier orbital phases is suggested to produce the faint residual X-ray emission seen when the...
X-ray emission from the supermassive binary system, Eta Carinae, declines sharply around periastr... more X-ray emission from the supermassive binary system, Eta Carinae, declines sharply around periastron. This X-ray minimum has two distinct phases --- the lowest flux phase in the first ~3 weeks and a slightly brighter phase thereafter. In 2009, the Chandra X-ray Observatory monitored the first phase five times and found the lowest observed flux at ~1.9e-12 ergs cm-2 s-1 (3-8 keV). The spectral shape changed such that the hard band above ~4 keV dropped quickly at the beginning and the soft band flux gradually decreased to its lowest observed value in ~2 weeks. The hard band spectrum had begun to recover by that time. This spectral variation suggests that the shocked gas producing the hottest X-ray gas near the apex of the wind-wind collision (WWC) is blocked behind the dense inner wind of the primary star, which later occults slightly cooler gas downstream. Shocked gas previously produced by the system at earlier orbital phases is suggested to produce the faint residual X-ray emission ...
Proceedings of the International Astronomical Union, 2010
The colliding wind binary (CWB) systems η Carinae and WR140 provide unique laboratories for X-ray... more The colliding wind binary (CWB) systems η Carinae and WR140 provide unique laboratories for X-ray astrophysics. Their wind-wind collisions produce hard X-rays that have been monitored extensively by several X-ray telescopes, including RXTE. To interpret these RXTE X-ray light curves, we model the wind-wind collision using 3D smoothed particle hydrodynamics (SPH) simulations. Adiabatic simulations that account for the absorption of X-rays from an assumed point source at the apex of the wind-collision shock cone by the distorted winds can closely match the observed 2-10keV RXTE light curves of both η Car and WR140. This point-source model can also explain the early recovery of η Car's X-ray light curve from the 2009.0 minimum by a factor of 2-4 reduction in the mass loss rate of η Car. Our more recent models relax the point-source approximation and account for the spatially extended emission along the wind-wind interaction shock front. For WR140, the computed X-ray light curve again matches the RXTE observations quite well. But for η Car, a hot, post-periastron bubble leads to an emission level that does not match the extended X-ray minimum observed by RXTE. Initial results from incorporating radiative cooling and radiatively-driven wind acceleration via a new anti-gravity approach into the SPH code are also discussed.
Colliding wind binaries (CWBs) are unique laboratories for X-ray astrophysics. The massive stars ... more Colliding wind binaries (CWBs) are unique laboratories for X-ray astrophysics. The massive stars in these systems possess powerful stellar winds with speeds up to $\sim$3000 km s$^{-1}$, and their collision leads to hot plasma (up to $\sim10^8$K) that emit thermal X-rays (up to $\sim$10 keV). Many X-ray telescopes have observed CWBs, including Suzaku, and our work aims to model these X-ray observations. We use 3D smoothed particle hydrodynamics (SPH) to model the wind-wind interaction, and then perform 3D radiative transfer to compute the emergent X-ray flux, which is folded through X-ray telescopes' response functions to compare directly with observations. In these proceedings, we present our models of Suzaku observations of the multi-year-period, highly eccentric systems $\eta$ Carinae and WR 140. The models reproduce the observations well away from periastron passage, but only $\eta$ Carinae's X-ray spectrum is reproduced at periastron; the WR 140 model produces too much ...
Astronomy & Astrophysics, 2014
We present results from a study of the eclipsing, colliding-wind binary V444 Cyg that uses a comb... more We present results from a study of the eclipsing, colliding-wind binary V444 Cyg that uses a combination of X-ray and optical spectropolarimetric methods to describe the 3-D nature of the shock and wind structure within the system. We have created the most complete X-ray light curve of V444 Cyg to date using 40 ksec of new data from Swift, and 200 ksec of new and archived XMM-Newton observations. In addition, we have characterized the intrinsic, polarimetric phase-dependent behavior of the strongest optical emission lines using data obtained with the University of Wisconsin's Half-Wave Spectropolarimeter. We have detected evidence of the Coriolis distortion of the wind-wind collision in the X-ray regime, which manifests itself through asymmetric behavior around the eclipses in the system's X-ray light curves. The large opening angle of the X-ray emitting region, as well as its location (i.e. the WN wind does not collide with the O star, but rather its wind) are evidence of radiative braking/inhibition occurring within the system. Additionally, the polarimetric results show evidence of the cavity the wind-wind collision region carves out of the Wolf-Rayet star's wind.
WR 140 is a canonical massive "colliding wind" binary system in which periodically-varying X-ray ... more WR 140 is a canonical massive "colliding wind" binary system in which periodically-varying X-ray emission is produced by the collision between the wind of the WC7 and O4-5 star components in the space between the two stars. We have obtained X-ray observations using the RXTE satellite observatory through almost one complete orbital cycle including two consecutive periastron passages. We discuss the results of this observing campaign, and the implications of the X-ray data for our understanding of the orbital dynamics and the stellar mass loss.
ABSTRACT The Rossi X-ray Timing Explorer has, for the first time, provided detailed measures of t... more ABSTRACT The Rossi X-ray Timing Explorer has, for the first time, provided detailed measures of the X-ray spectral variations in the two most important, high mass, evolved, highly eccentric colliding wind binaries, Eta Carinae and WR 140 though multiple orbital cycles. We report on the breakthroughs RXTE has achieved for these two binaries in observations spanning 15 years.
We discuss X-ray and Gamma-ray observations of the superluminous extremely massive binary system ... more We discuss X-ray and Gamma-ray observations of the superluminous extremely massive binary system Eta Carinae through the most recent X-ray minimum/periastron passage in January 2009. X-ray monitoring with RXTE shows that the minimum began on 16 January 2009, consistent with a period of 2024 days. Prior to the X-ray minimum, the flux of the system in the 2-10 keV band
The extremely massive (> 90 M_&sun;) and luminous ( = 5 × 10^{6} L_&sun;) star Eta Carinae, wi... more The extremely massive (> 90 M_&sun;) and luminous ( = 5 × 10^{6} L_&sun;) star Eta Carinae, with its spectacular bipolar ``Homunculus'' nebula, comprises one of the most remarkable and intensely observed stellar systems in the Galaxy. However, many of its underlying physical parameters remain unknown. Multiwavelength variations observed to occur every 5.54 years are interpreted as being due to the collision of a massive wind from the primary star with the fast, less dense wind of a hot companion star in a highly elliptical (e ˜ 0.9) orbit. Using three-dimensional (3-D) Smoothed Particle Hydrodynamics (SPH) simulations of the binary wind-wind collision, together with radiative transfer codes, we compute synthetic spectral images of [Fe III] emission line structures and compare them to existing Hubble Space Telescope/Space Telescope Imaging Spectrograph (HST/STIS) observations. We are thus able, for the first time, to tightly constrain the absolute orientation of the bina...
The Astrophysical Journal, 2010
We report continued monitoring of the superluminous binary system η Car by the Proportional Count... more We report continued monitoring of the superluminous binary system η Car by the Proportional Counter Array on the Rossi X-ray Timing Observatory (RXTE) through the 2009 X-ray minimum. The RXTE campaign shows that the minimum began on 2009 January 16, consistent with the phasings of the two previous minima, and overall, the temporal behavior of the X-ray emission was similar to that observed by RXTE in the previous two cycles. However, important differences did occur. The 2-10 keV X-ray flux and X-ray hardness decreased in the 2.5 year interval leading up to the 2009 minimum compared to the previous cycle. Most intriguingly, the 2009 X-ray minimum was about 1 month shorter than either of the previous two minima. During the egress from the 2009 minimum the X-ray hardness increased markedly as it had during egress from the previous two minima, although the maximum X-ray hardness achieved was less than the maximum observed after the two previous recoveries. We suggest that the cycle-to-cycle variations, especially the unexpectedly early recovery from the 2009 X-ray minimum, might have been the result of a decline in η Car's wind momentum flux produced by a drop in η Car's mass loss rate or wind terminal velocity (or some combination), though if so the change in wind momentum flux required to match the X-ray variation is surprisingly large.
Monthly Notices of the Royal Astronomical Society: Letters, 2008
The very massive star system η Carinae exhibits regular 5.54-year (2024-day) period disruptive ev... more The very massive star system η Carinae exhibits regular 5.54-year (2024-day) period disruptive events in wavebands ranging from the radio to X-ray. There is a growing consensus that these events likely stem from periastron passage of an (as yet) unseen companion in a highly eccentric (ǫ ∼ 0.9) orbit. This paper presents three-dimensional (3-D) Smoothed Particle Hydrodynamics (SPH) simulations of the orbital variation of the binary wind-wind collision, and applies these to modeling the X-ray light curve observed by the Rossi X-ray Timing Explorer (RXTE). By providing a global 3-D model of the phase variation of the density of the interacting winds, the simulations allow computation of the associated variation in X-ray absorption, presumed here to originate from near the apex of the wind-wind interaction cone. We find that the observed RXTE light curve can be readily fit if the observer's line of sight is within this cone along the general direction of apastron. Specifically, the data are well fit by an assumed inclination i = 45 • for the orbit's polar axis, which is thus consistent with orbital angular momentum being along the inferred polar axis of the Homunculus nebula. The fits also constrain the position angle φ that an orbital-plane projection makes with the apastron side of the semi-major axis, strongly excluding positions φ < 9 • along or to the retrograde side of the axis, with the best fit position given by φ = 27 • . Overall the results demonstrate the utility of a fully 3-D dynamical model for constraining the geometric and physical properties of this complex colliding-wind binary system.
Monthly Notices of the Royal Astronomical Society, 2009
The highly eccentric binary system, η Car, provides clues to the transition of massive stars from... more The highly eccentric binary system, η Car, provides clues to the transition of massive stars from hydrogen-burning via the CNO cycle to a helium-burning evolutionary state. The fastmoving wind of η Car B creates a cavity in η Car A's slower, but more massive, stellar wind, providing an in situ probe. The Hubble Space Telescope/Space Telescope Imaging Spectrograph (HST/STIS), with its high spatial and spectral resolutions, is well matched to follow temporal spatial and velocity variations of multiple wind features. We use observations obtained across 1998-2004 to produce a rudimentary three-dimensional model of the wind interaction in the η Car system. Broad (±500 km s −1 ) [Fe II] emission line structures extend 0.7 arcsec (∼1600 au) from the stellar core. In contrast, [Fe III], [Ar III], [Ne III] and [S III] lines extend only 0.3 arcsec (700 au) from NE to SW and are blue shifted from −500 to +200 km s −1 . All observed spectral features vary with the 5.54-year orbital period. The highly ionized, forbidden emission disappears during the low state, associated with periastron passage. The high-ionization emission originates in the outer wind interaction region that is directly excited by the far-ultraviolet radiation from η Car B. The HST/STIS spectra reveal a time-varying, distorted paraboloidal structure, caused by the interaction of the massive stellar winds. The model and observations are consistent with the orbital plane aligned with the skirt of the Homunculus. However, the axis of the distorted paraboloid, relative to the major axis of the binary orbit, is shifted in a prograde rotation along the plane, which projected on the sky is from NE to NW.
Monthly Notices of the Royal Astronomical Society, 2013
Recent work suggests that the mass-loss rate of the primary star η A in the massive colliding win... more Recent work suggests that the mass-loss rate of the primary star η A in the massive colliding wind binary η Carinae dropped by a factor of 2−3 between 1999 and 2010. We present results from large-(±1545 au) and small-(±155 au) domain, 3D smoothed particle hydrodynamics (SPH) simulations of η Car's colliding winds for three η A mass-loss rates (Ṁ ηA = 2.4, 4.8, and 8.5 × 10 −4 M ⊙ yr −1 ), investigating the effects on the dynamics of the binary wind-wind collision (WWC). These simulations include orbital motion, optically thin radiative cooling, and radiative forces. We find thatṀ ηA greatly affects the time-dependent hydrodynamics at all spatial scales investigated. The simulations also show that the post-shock wind of the companion star η B switches from the adiabatic to the radiative-cooling regime during periastron passage (φ ≈ 0.985 − 1.02). This switchover starts later and ends earlier the lower the value ofṀ ηA and is caused by the encroachment of the wind of η A into the acceleration zone of η B 's wind, plus radiative inhibition of η B 's wind by η A . The SPH simulations together with 1D radiative transfer models of η A 's spectra reveal that a factor of two or more drop inṀ ηA should lead to substantial changes in numerous multiwavelength observables. Recent observations are not fully consistent with the model predictions, indicating that any drop inṀ ηA was likely by a factor 2 and occurred after 2004. We speculate that most of the recent observed changes in η Car are due to a small increase in the WWC opening angle that produces significant effects because our line-of-sight to the system lies close to the dense walls of the WWC zone. A modest decrease inṀ ηA may be responsible, but changes in the wind/stellar parameters of η B , while less likely, cannot yet be fully ruled out. We suggest observations during η Car's next periastron in 2014 to further test for decreases inṀ ηA . IfṀ ηA is declining and continues to do so, the 2014 X-ray minimum should be even shorter than that of 2009.
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Astrophysics Papers by Christopher Russell