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Register a new userThe Complex X-ray Spectrum of 3C 273: ASCA Observations
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M. Cappi
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shortened) Results obtained from 9 X-ray observations of 3C 273 performed by ASCA are presented (for a total exposure time of about 160 000 s). The analysis and interpretation of the results is complicated by the fact that 4 of these observations were used for on-board calibration of the CCDs spectral response. The present analysis shows that, in agreement with official recommendations, a conservative systematic error (at low energies) of about 2-3 x 10**20cm-2 must be assumed when analyzing ASCA SIS data. A soft-excess, with variable flux and/or shape, has been clearly detected as well as flux and spectral variability. An anti-correlation is found between the spectral index and the flux in the 2-10 keV energy range. Fitting the data with the latest available calibration matrices, we also detect an emission line at ~5.4-5.7 keV (~6.3-6.6 keV in the quasar frame) in (only) the two observations with lowest fluxes where it is weak (EW ~ 20-30 eV), narrow and consistent with being produced by Fe K emission from neutral matter. Overall, the observations are qualitatively consistent with a variable, non-thermal X-ray continuum emission, i.e., a power law with Gamma~1.6 (possibly produced in the innermost regions of the radio-optical jet), plus underlying ``Seyfert-like features, i.e., a soft-excess and Fe K line emission due to a reflection component. When the continuum (jet) emission is in a low state, the spectral features produced by the Seyfert-like spectrum (soft-excess, iron line and possibly a steep power law plus reflection continuum) are more easily seen.
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We present results from four recent Chandra monitoring observations of the jet in 3C 273 using the ACIS detector, obtained between November 2003 and July 2004. We find that the X-ray emission comes in two components: unresolved knots that are smaller than the corresponding optically emitting knots and a broad channel that is about the same width as the optical interknot region. We compute the jet speed under the assumption that the X-ray emission is due to inverse Compton scattering of the cosmic microwave background, finding that the dimming of the jet X-ray emission to the jet termination relative to the radio emission may be due to bulk deceleration.
Aims. The high energy spectrum of 3C 273 is usually understood in terms of inverse-Compton emission in a relativistic leptonic jet. This model predicts variability patterns and delays that could be tested with simultaneous observations from the radio to the GeV range. Methods. The instruments IBIS, SPI, JEM-X on board INTEGRAL, PCA on board RXTE, and LAT on board Fermi have enough sensitivity to follow the spectral variability of 3C 273 from the keV to the GeV. We looked for correlations between the different energy bands, including radio data at 37 GHz collected at the Metsahovi Radio Observatory and built quasi-simultaneous multiwavelength spectra in the high energy domain when the source is flaring either in the X-rays or in the {gamma} rays. Results. Both temporal and spectral analysis suggest a two-component model to explain the complete high energy spectrum. X-ray emission is likely dominated by a Seyfert-like component while the {gamma}-ray emission is dominated by a blazar-like component produced by the relativistic jet. The variability of the blazar-like component is discussed, comparing the spectral parameters in the two different spectral states. Changes of the electron Lorentz factor are found to be the most likely source of the observed variability.
The Fe-K line, an important physical diagnostic in the X-ray spectra of AGN, has been notoriously difficult to measure in the high-luminosity, radio-loud quasar 3C 273 (z=0.158). On the few occasions that it has been detected its intrinsic width has been thought to be narrow (FWHM < 10,000 km/s) with an equivalent width (EW) of a few tens of eV.This was consistent with the general trend that as one goes from low to high luminosity AGNs the Fe-K line goes from being strong (EW ~200-300 eV) and broad (FWHM ~ 100,000 km/s) to being weak and narrow, or absent altogether. Here we present the results of new ASCA and RXTE observations, together with archival ASCA data, and show for the first time that the Fe-K line in 3C 273 is as broad as that seen in Seyfert 1. The line is resolved in two of the observations, with a mean Gaussian width of 0.8 +/- 0.3 keV, or FWHM of 0.3 +/- 0.1c. The smallest and largest EW measured is 43 +/- 34 eV, and 133 (+52,-53) eV respectively (quasar frame). The Compton-reflection continuum is less than 10% of that expected from a centrally illuminated semi-infinite, face-on, Compton-thick disk, confirming previous studies that Compton reflection is negligible in 3C 273. The largest values of the Fe-K line EW are under-predicted if the line originates in the disk, unless a time lag longer than several days between line and continuum and/or an over-abundance of Fe is invoked. We cannot unambiguously constrain the disk inclination angle. About 60 deg is preferred for a cold disk, while a face-on disk is allowed if the ionization state of Fe is H-like.
A series of nine XMM-Newton observations of the radio-loud quasar 3C 273 are presented, concentrating mainly on the soft excess. Although most of the individual observations do not show evidence for iron emission, co-adding them reveals a weak, broad line (EW ~ 56 eV). The soft excess component is found to vary, confirming previous work, and can be well fitted with multiple blackbody components, with temperatures ranging between ~40 and ~330 eV, together with a power-law. Alternatively, a Comptonisation model also provides a good fit, with a mean electron temperature of ~350 eV, although this value is higher when the soft excess is more luminous over the 0.5-10 keV energy band. In the RGS spectrum of 3C 273, a strong detection of the OVII He-alpha absorption line at zero redshift is made; this may originate in warm gas in the local intergalactic medium, consistent with the findings of both Fang et al. (2003) and Rasmussen et al. (2003).
We present an analysis of the rosat and asca spectra of 21 broad line AGN (QSOs) with $zsim 1$ detected in the 2-10 keV band with the asca gis. The summed spectrum in the asca band is well described by a power-law with $Gamma=1.56pm0.18$, flatter that the average spectral index of bright QSOs and consistent with the spectrum of the X-ray background in this band. The flat spectrum in the asca band could be explained by only a moderate absorption ($sim 10^{22} rm cm^{-2}$) assuming the typical AGN spectrum ie a power-law with $Gamma$=1.9. This could in principle suggest that some of the highly obscured AGN, required by most X-ray background synthesis models, may be associated with normal blue QSOs rather than narrow-line AGN. However, the combined 0.5-8 keV asca-rosat spectrum is well fit by a power-law of $Gamma=1.7pm0.2$ with a spectral upturn at soft energies. It has been pointed out that such an upturn may be an artefact of uncertainties in the calibration of the ROSAT or ASCA detectors. Nevertheless if real, it could imply that the above absorption model suggested by the asca data alone is ruled out. Then a large fraction of QSOs could have ``concave spectra ie they gradually steepen towards softer energies. This result is in agreement with the bepposax hardness ratio analysis of $sim$ 100 hard X-ray selected sources.
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