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Using two Chandra observations we have derived estimates of the dust distribution and distance to the eclipsing high mass X-ray binary (HMXB) Cen X-3 using the energy-resolved dust-scattered X-ray halo. By comparing the observed X-ray halos in 200 eV bands from 2-5 keV to the halo profiles predicted by the Weingartner & Draine interstellar grain model, we find that the vast majority (about 70%) of the dust along the line of sight to the system is located within about 300 pc of the Sun, although the halo measurements are insensitive to dust very close to the source. One of the Chandra observations occurred during an egress from eclipse as the pulsar emerged from behind the mass-donating primary. By comparing model halo light curves during this transition to the halo measurements, a source distance of 5.7 +/- 1.5 kpc (68% confidence level) is estimated, although we find this result depends on the distribution of dust on very small scales. Nevertheless, this value is marginally inconsistent with the commonly accepted distance to Cen X-3 of 8 kpc. We also find that the energy scaling of the scattering optical depth predicted by the Weingartner & Draine interstellar grain model does not accurately represent the results determined by X-ray halo studies of Cen X-3. Relative to the model, there appears to be less scattering at low energies or more scattering at high energies in Cen X-3.
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We analyze the ASCA spectrum of the Cen X-3 X-ray binary system in eclipse using atomic models appropriate to recombination-dominated level population kinetics in an overionized plasma. In order to estimate the wind characteristics, we first fit the eclipse spectrum to a single-zone photoionized plasma model. We then fit spectra from a range of orbital phases using global models of photoionized winds from the companion star and the accretion disk that account for the continuous distribution of density and ionization state. We find that the spectrum can be reproduced by a density distribution of the form derived by Castor, Abbot, & Klein (1975) for radiation-driven winds with with the value of the mass-loss rate divided by the terminal velocity consistent with values for isolated stars of the same stellar type. This is surprising because the neutron star is very luminous (~10^38 erg/s) and the X-rays from the neutron star should ionize the wind and destroy the ions that provide the opacity for the radiation-driven wind. Using the same functional form for the density profile, we also fit the spectrum to a spherically symmetric wind centered on the neutron star, a configuration chosen to represent a disk wind. We argue that the relatively modest orbital variation of the discrete spectrum rules out a disk wind hypothesis.
We report here an investigation of the X-ray eclipse transitions of the high mass X-ray binary pulsar Cen X-3 in different intensity states. Long term light curve of Cen X-3 obtained with RXTE-ASM spanning for more than 5000 days shows strong aperiodic flux variations with low and high states. We have investigated the eclipse transitions of Cen X-3 in different intensity states with data obtained from pointed observations with the more sensitive instruments on board ASCA, BeppoSAX, XMM-Newton, Chandra and RXTE. We found a very clear trend of sharp eclipse transitions in the high state and longer transitions in the low state. This is a confirmation of this feature first observed with the RXTE-ASM but now with much better clarity. From the light curves obtained from several missions, it is seen that the eclipse egress in the low state starts earlier by an orbital phase of 0.02 indicating that the observed X-rays originate from a much larger region. We have also performed spectral analysis of the post-eclipse part of each observations. From BeppoSAX observations, the out-of-eclipse X-ray fluxes is found to differ by a factor of ~ 26 during the high and low intensity states while the eclipse count rates differ by a factor of only ~ 4.7. This indicates that in the low state, there is an additional scattering medium which scatters some of the source photons towards the observer even when the neutron star is completely eclipsed. We could also resolve the three iron line components using XMM-Newton observation in the low state. By comparing the iron line equivalent width during the high and low states, it is seen that the width of iron line is relatively large during the low state which supports the fact that significant reprocessing and scattering of X-rays takes place in the low state.
Cen X-3 is a well-studied high-mass accreting X-ray binary and a variable source of high energy gamma rays from 100 MeV to 1 TeV. The object has been extensively monitored with the University of Durham Mark 6 telescope. Results of observations, including those taken in 1998 and 1999, are reported. There is no evidence for time variability in all the VHE data. There is also no evidence for correlation of the VHE flux with the X-ray flux detected by BATSE and RXTE/ASM. A search for periodic emission, at or close to the X-ray spin period, in the VHE data yielded a 3 sigma upper limit to the pulsed flux of 2.0 x 10^-12 cm^-2 s^-1.
The massive X-ray binary Cen X-3 was observed over approximately one quarter of the systems 2.08 day orbit, beginning before eclipse and ending slightly after eclipse center with the Chandra X-ray Observatory using its High-Energy Transmission Grating Spectrometer. The spectra show K shell emission lines from hydrogen- and helium-like ions of magnesium, silicon, sulfur, and iron as well as a K-alpha fluorescence emission feature from near-neutral iron. The helium-like n=2->1 triplet of silicon is fully resolved and the analogous triplet of iron is partially resolved. The helium-like triplet component flux ratios outside of eclipse are consistent with emission from recombination and subsequent cascades (recombination radiation) from a photoionized plasma. In eclipse, however, the w (resonance) lines of silicon and iron are stronger than that expected for recombination radiation, and are consistent with emission from a collisionally ionized plasma. The triplet line flux ratios at both phases can be explained more naturally, however, as emission from a photoionized plasma if the effects of resonant line scattering are included in addition to recombination radiation. We show that the emissivity due to resonant scattering depends sensitively on the line optical depth and, in the case of winds in X-ray binaries, this allows constraints on the wind velocity even when Doppler shifts cannot be resolved.
The radio galaxy Cen A has been detected all the way up to the TeV energy range. This raises the question about the dominant emission mechanisms in the high-energy domain. Spectral analysis allows us to put constraints on the possible emission processes. Here we study the hard X-ray emission as measured by INTEGRAL in the 3-1000 keV energy range, in order to distinguish between a thermal and non-thermal inverse Compton process. The hard X-ray spectrum of Cen A shows a significant cut-off at energies Ec = 434 (+106 -73) keV with an underlying power law of photon index 1.73 +- 0.02. A more physical model of thermal Comptonisation (compPS) gives a plasma temperature of kT = 206+-62 keV within the optically thin corona with Compton parameter y = 0.42 (+0.09 -0.06). The reflection component is significant at the 1.9 sigma level with R = 0.12 (+0.09 -0.10), and a reflection strength R>0.3 can be excluded on a 3 sigma level. Time resolved spectral studies show that the flux, absorption, and spectral slope varied in the range f(3-30 keV) = (1.2 - 9.2)e-10 erg/cm**2/s, NH = (7 - 16)e22 1/cm**2, and photon index 1.75 - 1.87. Extending the cut-off power law or the Comptonisation model to the gamma-ray range shows that they cannot account for the high-energy emission. On the other hand, also a broken or curved power law model can represent the data, therefore a non-thermal origin of the X-ray to GeV emission cannot be ruled out. The analysis of the SPI data provides no sign of significant emission from the radio lobes and gives a 3 sigma upper limit of f(40-1000 keV) < 0.0011 ph/cm**2/s. While gamma-rays, as detected by CGRO and Fermi, are caused by non-thermal (jet) processes, the main process in the hard X-ray emission of Cen A is still not unambiguously determined, being either dominated by thermal inverse Compton emission, or by non-thermal emission from the base of the jet.
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