Rossi X-ray Timing Explorer Satellite Research Articles

A QPO in Mkn 421 from Archival RXTE Data

We report a 325(−7, +8) day quasi-periodic oscillation (QPO) in the X-ray emission of the blazar Mkn 421 based on data obtained with the Rossi X-ray Timing Explorer satellite. The QPO is seen prominently in the ASM data (at least 15 cycles) due to the fact that it has had near-continuous sampling for more than a decade. The PCA data, where the sampling is not uniform and shows many large gaps, provide supporting evidence at lower significance. The QPOs are an important observable in accretion disks, can be modulated by various orbital timescales, and may be generated by a number of mechanisms. They have been studied extensively in X-ray binaries and should be present in active galactic nuclei (AGN) if they are governed by a common set of physical principles. In jetted sources, QPOs can probe jet–disk interactions or helical oscillations. This QPO has previously been claimed intermittently in X-ray, radio, and gamma-ray data, but the continuous, 15 yr extent (1996–2011) of the ASM observations (in which Mkn 421 is the brightest AGN observed) provides a unique window. The QPO appears to be present for nearly the entire extent of the ASM observations. We explore various physical origins and modulating mechanisms, particularly interpretations of the QPO as a result of disk–jet interactions, due to either an accretion disk limit cycle, jet instabilities, or helical motions. Limit cycle oscillations would not interact with either Keplerian or Lense–Thirring modulated oscillations; however, those associated with jet instabilities or helical motions in the jet would likely be modulated by Lense–Thirring precession.

Measurement of the vertical atmospheric density profile from the X-ray Earth occultation of the Crab Nebula with Insight-HXMT

Abstract. X-ray Earth occultation sounding (XEOS) is an emerging method for measuring the neutral density in the lower thermosphere. In this paper, the X-ray Earth occultation (XEO) of the Crab Nebula is investigated using the Hard X-ray Modulation Telescope (Insight-HXMT). The pointing observation data on the 30 September 2018 recorded by the low-energy X-ray telescope (LE) of Insight-HXMT are selected and analysed. The extinction light curves and spectra during the X-ray Earth occultation process are extracted. A forward model for the XEO light curve is established, and the theoretical observational signal for light curve is predicted. The atmospheric density model is built with a scale factor to the commonly used Mass Spectrometer Incoherent Scatter Radar Extended model (MSIS) density profile within a certain altitude range. A Bayesian data analysis method is developed for the XEO light curve modelling and the atmospheric density retrieval. The posterior probability distribution of the model parameters is derived through the Markov chain–Monte Carlo (MCMC) algorithm with the NRLMSISE-00 model and the NRLMSIS 2.0 model as basis functions, and the respective best-fit density profiles are retrieved. It is found that in the altitude range of 105–200 km, the retrieved density profile is 88.8 % of the density of NRLMSISE-00 and 109.7 % of the density of NRLMSIS 2.0 by fitting the light curve in the energy range of 1.0–2.5 keV based on the XEOS method. In the altitude range of 95–125 km, the retrieved density profile is 81.0 % of the density of NRLMSISE-00 and 92.3 % of the density of NRLMSIS 2.0 by fitting the light curve in the energy range of 2.5–6.0 keV based on the XEOS method. In the altitude range of 85–110 km, the retrieved density profile is 87.7 % of the density of NRLMSISE-00 and 101.4 % of the density of NRLMSIS 2.0 by fitting the light curve in the energy range of 6.0–10.0 keV based on the XEOS method. Goodness-of-fit testing is carried out for the validation of the results. The measurements of density profiles are compared to the NRLMSISE-00 and NRLMSIS 2.0 model simulations and the previous retrieval results with NASA's Rossi X-ray Timing Explorer (RXTE) satellite. For further confirmation, we also compare the measured density profile to the ones by a standard spectrum retrieval method with an iterative inversion technique. Finally, we find that the retrieved density profile from Insight-HXMT based on the NRLMSISE-00 and NRLMSIS 2.0 models is qualitatively consistent with the previous retrieved results from RXTE. The results of light curve fitting and standard energy spectrum fitting are in good agreement. This research provides a method for the evaluation of the density profiles from MSIS model predictions. This study demonstrates that the XEOS from the X-ray astronomical satellite Insight-HXMT can provide an approach for the study of the upper atmosphere. The Insight-HXMT satellite can join the family of the XEOS. The Insight-HXMT satellite with other X-ray astronomical satellites in orbit can form a space observation network for XEOS in the future.

Grouping bi-chi-squared method for pulsar navigation experiment using observations of Rossi X-ray timing explorer

X-ray pulsar-based navigation is a revolutionary technology which is capable of providing the required navigation information in the solar system. Performing as an important pulsar-based navigation technique, the Significance Enhancement of Pulse-profile with Orbit-dynamics (SEPO) can estimate orbital elements by using the distortion of pulse profile. Based on the SEPO technique, we propose a grouping bi-chi-squared technique and adopt the observations of Rossi X-ray Timing Explorer (RXTE) to carry out experimental verification. The pulsar timing is refined to determine spin parameters of the Crab pulsar (PSR B0531+21) during the observation periods, and a short-term dynamic model for RXTE satellite is established by considering the effects of diverse perturbations. Experimental results suggest that the position and velocity errors are 16.3 km (3σ) and 13.3 m/s (3σ) with two adjacent observations split by one day (exposure time of 1.5 ks), outperforming those of the POLAR experiment whose results are 19.2 km (3σ) and 432 m/s (3σ). The approach provided is particularly applicable to the estimation of orbital elements via using high-flux observations.

Low-frequency quasi-periodic oscillations and shocks in accretion on to black hole

ABSTRACT Low-frequency quasi-periodic oscillations (LFQPOs) have been routinely observed in black hole X-ray binaries (BHXRBs). These LFQPOs can be explained by axisymmetric shock oscillation in accretion flow around a rotating black hole. We address the physical origin of Type-C LFQPOs in BHXRBs observed by the Rossi X-ray Timing Explorer satellite considering a minimum number of free parameters, namely, specific energy and specific angular momentum of the infalling matter for a given set of BH mass and spin parameter. We apply the solution for a large number of BH candidates to further strengthen the scenario of an anticorrelation between the QPO frequency and the location of the shock. Our study also confirms that Compton cooling can be sufficient to explain the observed QPOs.

A Study on the Hysteresis Effect and Spectral Evolution in the Mini-Outbursts of Black Hole X-Ray Binary XTE J1550-564

One normal outburst and three mini-outbursts have been detected by Rossi X-ray Timing Explorer satellite after 2000 in the well-known black hole X-ray binary XTE J1550-564. In this work, we explore the hysteresis effect of the four outbursts, which is a phenomenon that a similar spectral state transition occurs at different luminosity in an outburst of black hole X-ray binary. A q-like track was found in the hardness-intensity diagram of the normal outburst in 2000 but not in the three mini-outbursts that only occur in the Low/Hard state. The results demonstrate that the hysteresis effect is not apparent in the three mini-outbursts and the X-ray spectra are harder than that of the normal outburst at the same photon count rate. Furthermore, the results of the correlation analysis show that the Γ − F2−10keV correlation of mini-outburst maintain negative in the Low/Hard state with the harder spectra than that of the normal outburst at the same X-ray flux. The X-ray spectral evolution can be well-explained by the state-transition model from the Shakura–Sunyaev disk to the advection-dominated accretion flow, which implies that the three mini-outbursts of XTE J1550-564 might originate from a smaller discrete accretion event.

A QPO in NGC 4945 from Archival RXTE Data

Abstract We report the discovery of an ∼6 week quasi-periodic oscillation (QPO) in archival NGC 4945 data observed by the Rossi X-ray Timing Explorer (RXTE) satellite. QPOs are important observables in accretion disks and have been studied extensively in both neutron star (NS) and black hole (BH) X-ray binaries (XRB). QPOs should be present in active galactic nuclei (AGN) if galactic BHs and supermassive black holes are governed by a common set of physical processes. The search for QPOs in AGN has proven difficult because the time scales would be much longer, due to their higher mass. RXTE AGN light curves spanning 1996 to 2011 provide an excellent and perhaps unique opportunity to search for low-frequency QPOs. We investigated the 533 RXTE observations made of the Seyfert-2 AGN, NGC 4945. During a large cluster of observations in 2006–2007 (194 observations, spanning 396 days), the Lomb–Scargle periodogram shows a candidate QPO at 0.274 μHz (period ≈42.2 days). We estimate the uncertainties using the false alarm probability. We discuss the possible identification of this feature with the Lense–Thirring precession period.

Anomalous outbursts of H 1743-322

Abstract Using soft (1.5–3 keV) and hard (3–12 keV) photon counts of All Sky Monitor (ASM) in Rossi X-ray Timing Explorer (RXTE) satellite, we have proposed recently that there is a significant time lag between the infall time-scales of two components in the Two-Component Advective Flow paradigm, where a standard slow moving Keplerian disc is surrounded by a fast moving halo. The time lag is clearly due to the difference in viscosity in the flow components and the size of the Keplerian disc may be considered to be proportional to this arrival time lag. In this paper, using RXTE/ASM (1.5–12 keV) data, we examine eight successive outbursts of the low-mass X-ray binary H 1743-322 since 2003 from a new perspective. The day-to-day temporal evolution of a dynamic photon index, Θ, as well as its cross-correlation with the soft and hard energy fluxes show that the aforesaid time lag was the longest during the brightest outburst of 2003 – thereby indicating its largest Keplerian disc. The disc size diminished thereafter during subsequent weaker outbursts. Moreover, Θ decides spectral transitions of any outburst. We show from the behaviour of Θ alone that the outburst of October 2008 was anomalous while the outburst of 2003 was twin (anomalous + normal). In fact, each normal outburst was either preceded or followed by an otherwise premature outburst showing different degrees of anomaly. This makes H 1743-322 an enigmatic source and a subject of further study.

Does Cyg X-1 have a small accretion disc?

We analyze several outbursts of a few transient sources using Proportional Counter Array (PCA) data (2.5-25 keV) as well as All Sky Monitor (ASM) data (1.5-12 keV) of Rossi X-ray Timing Explorer (RXTE) satellite. We find a time delay between the arrival times of the Keplerian disc component and the halo of the Two-Component Advective Flow (TCAF) when the spectral data is fitted with TCAF solution. We compare this time delay from the spectral fits with the TCAF solution of the transient low mass X-ray binaries (LMXBs) e.g., GX 339-4, H 1743-322 and MAXI J1836-194 with that of the high mass X-ray Binary (HMXB), Cyg X-1. We find that several days of time delays are observed in LMXBs while for Cyg X-1 the delay is negligible. We interpret the large delay to be due to the viscous delay of a large Keplerian component to reach the inner region as compared to nearly free-fall time taken by the low angular momentum halo component. The delay is of the order of a few days for the low mass X-ray binaries (LMXBs) where the feeding is primarily through the Roche-lobe. However, it is negligible in a wind-fed system like Cyg X-1 since a very small Keplerian disc is created here by slowly redistributing the low angular momentum of the wind. As a consequence, sporadic soft or intermediate spectral states are observed.

A search for very high-energy flares from the microquasars GRS 1915+105, Circinus X-1, and V4641 Sgr using contemporaneous H.E.S.S. and RXTE observations

Context. Microquasars are potential γ-ray emitters. Indications of transient episodes of γ-ray emission were recently reported in at least two systems: Cyg X-1 and Cyg X-3. The identification of additional γ-ray-emitting microquasars is required to better understand how γ-ray emission can be produced in these systems.Aim. Theoretical models have predicted very high-energy (VHE) γ-ray emission from microquasars during periods of transient outburst. Observations reported herein were undertaken with the objective of observing a broadband flaring event in the γ-ray and X-ray bands.Methods. Contemporaneous observations of three microquasars, GRS 1915+105, Circinus X-1, and V4641 Sgr, were obtained using the High Energy Spectroscopic System (H.E.S.S.) telescope array and the Rossi X-ray Timing Explorer (RXTE) satellite. X-ray analyses for each microquasar were performed and VHE γ-ray upper limits from contemporaneous H.E.S.S. observations were derived.Results. No significant γ-ray signal has been detected in any of the three systems. The integral γ-ray photon flux at the observational epochs is constrained to be I(>560 GeV) < 7.3 × 10−13 cm−2 s−1, I(>560 GeV ) < 1.2 × 10−12 cm−2 s−1, and I(>240 GeV) < 4.5 × 10−12 cm−2 s−1 for GRS 1915+105, Circinus X-1, and V4641 Sgr, respectively.Conclusions. The γ-ray upper limits obtained using H.E.S.S. are examined in the context of previous Cherenkov telescope observations of microquasars. The effect of intrinsic absorption is modelled for each target and found to have negligible impact on the flux of escaping γ-rays. When combined with the X-ray behaviour observed using RXTE, the derived results indicate that if detectable VHE γ-ray emission from microquasars is commonplace, then it is likely to be highly transient.

Time delay estimation method of X-ray pulsar observed profile based on the optimal frequency band

In order to improve the time delay estimation accuracy of the observed profile in the X-ray pulsar based navigation, the spectral characteristics of the observed profile of X-ray pulsar and the drawback of the classical Taylor fast Fourier transform (FFT) time delay estimation method are analyzed. It is found that when estimating the time delay, we can abandon the higher frequency components that are always affected by noise seriously, but only utilize the information about the low frequency part. Based on this idea, by modifying the weigh function of the classical Taylor FFT time delay estimation method, a new time delay estimation algorithm based on the optimal frequency band is proposed, in which the optimal frequency band is determined by establishing the relationship between the selected frequency band and the time delay estimation accuracy under different signal-to-noise ratios (SNRs). Then by using the real data obtained with the proportional counter array, the low-energy (2-60 keV) detection instrument boarded on the Rossi X-ray Timing Explorer satellite, the optimal frequency as a function the SNR of observed profile is given for the PSR B0531+21 (namely the Crab pulsar) through the Monte-Carlo technique. Since the parameters of different pulsars are known, in practical navigation, the optimal frequency in an observation time for a certain pulsar can be estimated in advance by using the simulation data or the obtained real data of the pulsar, which can remarkably alleviate the onboard computational burden. Finally, a series of numerical simulations and experiments using real data of Crab pulsar are designed to evaluate the performance of the proposed time delay estimation algorithm. The main results can be summarized as follows: the proposed estimator outperforms the normally used fast approximate maximum-likelihood (FAML), cross correlation (CC), nonlinear least square (NLS) and weighted nonlinear least-square (WNLS) estimators when the observation time is short or the source flux is small; when the observation time is long or the source flux is large, its estimation accuracy is almost the same as those of CC and NLS estimators and lower than those of the FAML and WNLS estimators, but its computational complexity is smaller than those of NLS, FAML and WNLS estimators. The above results indicate the high estimation accuracy and high computational efficiency of the proposed time delay estimation method, which can be used in the case that the observation time is restricted to be short or the source flux of the usable pulsar is small in X-ray pulsar based navigation.

X-Ray Pulsar Profile Recovery Based on Tracking-Differentiator

The profile recovery is an important work in X-ray pulsar-based navigation. It is a key step for the analysis on the pulsar signal’s characteristic and the computing of time of arrival (TOA). This paper makes an argument for an algorithm based on the tracking-differentiator (TD) to recover the profile from the low Signal-to-Noise Ratio (SNR) signals. In the method, a TD filter with cascade structure is designed which has very low phase delay and amplitude distortion. In the simulation experiment, two typical pulsars (PSR B0531+21 and PSR B1937+21) are used to verify the algorithm’s performance. The simulation results show that the method satisfies the application requirements in the aspects of SNR and profile fidelity. By processing the data collected by the Rossi X-Ray Timing Explorer (RXTE) satellite in space, similar results can also be achieved.

Denoising of X-ray pulsar observed profile in the undecimated wavelet domain

The low intensity of the X-ray pulsar signal and the strong X-ray background radiation lead to low signal-to-noise ratio (SNR) of the X-ray pulsar observed profile obtained through epoch folding, especially when the observation time is not long enough. This signifies the necessity of denoising of the observed profile. In this paper, the statistical characteristics of the X-ray pulsar signal are studied, and a signal-dependent noise model is established for the observed profile. Based on this, a profile noise reduction method by performing a local linear minimum mean square error filtering in the un-decimated wavelet domain is developed. The detail wavelet coefficients are rescaled by multiplying their amplitudes by a locally adaptive factor, which is the local variance ratio of the noiseless coefficients to the noisy ones. All the nonstationary statistics needed in the algorithm are calculated from the observed profile, without a priori information. The results of experiments, carried out on simulated data obtained by the ground-based simulation system and real data obtained by Rossi X-Ray Timing Explorer satellite, indicate that the proposed method is excellent in both noise suppression and preservation of peak sharpness, and it also clearly outperforms four widely accepted and used wavelet denoising methods, in terms of SNR, Pearson correlation coefficient and root mean square error.

Timing and spectral studies of the transient X-ray pulsar GX 304−1 during an outburst

We present the timing and spectral properties of the transient X-ray pulsar GX 304-1 during its recent outburst in 2010 August, using observations carried out with the Proportional Counter Array (PCA) instrument on-board the Rossi X-ray Timing Explorer (RXTE) satellite. We detected strong intensity and energy dependent variations in the pulse profiles during the outburst. The pulse profile showed significant evolution over the outburst. It showed complex structures consisting of a main peak with steps on both sides during the start of the outburst. On some days, a sharp dip like feature was seen which disappeared at the end of the outburst; when the profile evolved into a sinusoidal shape. At low energies, the pulse profiles appeared complex, consisting of multiple peaks and a narrow minimum. The amplitude of the second brightest peak in low energies decreased with energy, and above 12 keV, the shape of the pulse profile changed to a single broad peak with a dip like feature. The dip had energy dependence, both in phase and in width. We detected Quasi-periodic Oscillations (QPO) at 0.125 Hz with a harmonic. The QPO feature had a low rms value of 2.9% and it showed a positive energy dependence up to 40 keV with the rms value increasing to 9% at 40 keV. The QPO frequency decreased from 0.128 Hz to 0.108 Hz in 12 days. During most of the outburst, the 3-30 keV spectrum of GX 304-1 can be well fitted with a partial covering power-law model with a high energy cut-off and iron fluorescent line emission. For a few of the observations carried out during the decay of the outburst, the partial covering absorption component is found to change to single component absorption. We also found that the partial covering and high energy cut-off parameters vary significantly with the pulse phase.

Broad-band spectral analysis of Aql X-1

We present the results of a broadband spectral study of the transient Low Mass X-ray Binary Aql X-1 observed by Suzaku and Rossi X-ray Timing Explorer satellites. The source was observed during its 2007 outburst in the High/Soft (Banana) state and in the Low/Hard (Extreme Island) state. Both the Banana state and the Extreme Island state spectra are best described by a two component model consisting of a soft multi-colour blackbody emission likely originating from the accretion disk and a harder Comptonized emission from the boundary layer. Evidence for a hard tail (extending to ~50 keV) is found during the Banana state; this further (transient) component, accounting for atleast ~1.5% of the source luminosity, is modeled by a power-law. Aql X-1 is the second Atoll source after GX 13+1 to show a high energy tail. The presence of a weak but broad Fe line provides further support for a standard accretion disk extending nearly to the neutron star surface. The input photons for the Comptonizing boundary layer could either be the disk photons or the hidden surface of the star or both. The luminosity of the boundary layer is similar to the disk luminosity in the banana state and is about six times larger in the extreme island state. The temperature of the Comptonizing boundary layer changes from ~2 keV in the banana state to ~20 keV in the extreme island state.

ON THE NATURE OF THE FIRST TRANSIENT Z SOURCE XTE J1701–462: ITS ACCRETION DISK STRUCTURE, NEUTRON STAR MAGNETIC FIELD STRENGTH, AND HARD TAIL

Using the data from the Rossi X-Ray Timing Explorer satellite, we investigate the spectral evolution along a "Z" track and a "v" track on the hardness-intensity diagrams of the first transient Z source XTE J1701-462. The spectral analyses suggest that the inner disk radius depends on the mass accretion rate, in agreement with the model prediction, R_in \propto ((dM/dt)_disk)^{2/7}, for a radiation pressure dominated accretion disk interacting with the magnetosphere of a neutron star (NS). The changes in the disk mass accretion rate (dM/dt)_disk are responsible for the evolution of the "Z" or "v" track. The radiation pressure thickens the disk considerably, and also produces significant outflows. The NS surface magnetic field strength, derived from the interaction between the magnetosphere and the radiation pressure dominated accretion disk, is ~(1--3)X10^9 G, which is possibly between normal atoll and Z sources. A significant hard tail is detected in the horizontal branches and we discuss several possible origins of the hard tail.

Steady jets from radiatively efficient hard states in GRS 1915+105

Recent studies of different X-ray binaries (XRBs) have shown a clear correlation between the radio and X-ray emission. We present evidence of a close relationship found between the radio and X-ray emission at different epochs for GRS1915+105, using observations from the Ryle Telescope and Rossi X-ray Timing Explorer satellite. The strongest correlation was found during the hard state (also known as the `plateau' state), where a steady AU-scale jet is known to exist. Both the radio and X-ray emission were found to decay from the start of most plateau states, with the radio emission decaying faster. An empirical relationship of $S_{\rm{radio}}\propto S_{\rm{X-ray}}^{\xi}$ was then fitted to data taken only during the plateau state, resulting in a power-law index of $\xi\sim1.7\pm0.3$, which is significantly higher than in other black hole XRBs in a similar state. An advection-flow model was then fitted to this relationship and compared to the universal XRB relationship as described by Gallo et al. (2003). We conclude that either (I) the accretion disk in this source is radiatively efficient, even during the continuous outflow of a compact jet, which could also suggest a universal turn-over from radiatively inefficient to efficient for all stellar-mass black holes at a critical mass accretion rate ($\dot{m}_{\rm{c}}\approx10^{18.5}$ g/s); or (II) the X-rays in the plateau state are dominated by emission from the base of the jet and not the accretion disk (e.g. via inverse Compton scattering from the outflow).

Decade time-scale modulation of low-mass X-ray binaries

Regular observations by the All Sky Monitor aboard the Rossi X-ray Timing Explorer satellite have yielded well-sampled light-curves with a time baseline of over ten years. We find that up to eight of the sixteen brightest persistent low mass X-ray binaries show significant, possible sinusoidal, variations with periods of order ten years. We speculate on its possible origin and prevalence in the population of low mass X-ray binaries and we find the presence of a third object in the system, or long-period variability intrinsic to the donor star, as being attractive origins for the X-ray flux modulation we detect. For some of the objects in which we do not detect a signal, there is substantial short-term variation which may hide modest modulation on long time-scales. Decade time-scale modulations may thus be even more common.

ECLIPSE TIMINGS OF THE TRANSIENT LOW-MASS X-RAY BINARY EXO 0748–676. IV. THE ROSSI X-RAY TIMING EXPLORER ECLIPSES

We report our complete database of X-ray eclipse timings of the low-mass X-ray binary EXO 0748−676 observed by the Rossi X-Ray Timing Explorer (RXTE) satellite. As of this writing we have accumulated 443 full X-ray eclipses, 392 of which have been observed with the Proportional Counter Array on RXTE. These include both observations where an eclipse was specifically targeted and those eclipses found in the RXTE data archive. Eclipse cycle count has been maintained since the discovery of the EXO 0748−676 system in 1985 February. We describe our observing and analysis techniques for each eclipse and describe improvements we have made since the last compilation by Wolff et al. The principal result of this paper is the database containing the timing results from a seven-parameter fit to the X-ray light curve for each observed eclipse along with the associated errors in the fitted parameters. Based on the standard O − C analysis, EXO 0748−676 has undergone four distinct orbital period epochs since its discovery. In addition, EXO 0748−676 shows small-scale events in the O − C curve that are likely due to short-lived changes in the secondary star.

Linking jet emission and X-ray properties in the peculiar neutron star X-ray binary Circinus X-1

We present the results of simultaneous X-ray and radio observations of the peculiar Z-type neutron star X-ray binary Cir X-1, observed with the Rossi X-ray timing explorer satellite and the Australia Telescope Compact Array in 2000 October and 2002 December. We identify typical Z source behaviour in the power density spectra as well as characteristic Z patterns drawn in an X-ray hardness-intensity diagram. Power spectra typical of bright atoll sources have also been identified at orbital phases after the periastron passage, while orbital phases before the periastron passage are characterized by power spectra that are typical neither of Z nor of atoll sources. We investigate the coupling between the X-ray and the radio properties, focusing on three orbital phases when an enhancement of the radio flux density has been detected, to test the link between the inflow (X-ray) and the outflow (radio jet) to/from the compact object. In two out of three cases we associate the presence of the radio jet to a spectral transition in the X-rays, although the transition does not precede the radio flare, as detected in other Z sources. An analogous behaviour has recently been found in the black hole candidate GX 339-4. In the third case, the radio light curve shows a similar shape to the X-ray light curve. We discuss our results in the context of jet models, considering also black hole candidates.

The Hard X-ray Spectra of the Polars V2301 Oph, BL Hyi, and WW Hor

ABSTRACTThe hard X-ray spectra of the Polars V2301 Oph and BL Hyi acquired using the Proportional Counting Array (PCA) detector on board the Rossi X-ray Timing Explorer (RXTE) satellite were modeled using theoretical spectra from two-temperature radiative shock models for field-aligned accretion flows onto magnetic white dwarfs. The radiative shock models included the cooling processes e-i and e-e bremsstrahlung and Compton cooling and, importantly, self-consistently included the effects of cyclotron emission through an explicit solution of the radiative transfer equation rather than through the use of the approximate cyclotron cooling function derived by Chanmugam, Langer, and Shaviv. The spectra also included the effects of reflection from the surface of the white dwarf. We also discussed the eclipsing Polar WW Hor within the context of the theoretical shock models. WW Hor was observed and detected by XMM-Newton in the hard X-rays, and, unlike the above named Polars, WW Hor also exhibits an optical light curve with properties characteristic of a shock with cyclotron emission. We find that: (i) based on the X-ray continuum, the mass of the white dwarf in V2301 Oph is ) M⊙ for the known magnetic field strength B∗ = 7 MG. Using the observed optical flux of V2301 Oph, we set the further upper limit on the white dwarf mass of M∗ ∼ 1.2 M⊙. (ii) Based on the X-ray continuum and the shapes of the B and R optical light curves, we infer that the mass of the white dwarf in WW Hor to M∗ ≳ 0.4 M⊙ with magnetic field B∗ ∼ 15 MG. Using the observed optical flux of WW Hor, we suggest that the lower limit on the mass of the white dwarf mass in WW Hor is M∗ ∼ 0.8 M⊙. We were unable to model the hard X-ray spectrum of BL Hyi using a fitting function composed of an absorbed shock spectrum.