ChandraObservations of QSO 2237+0305

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Abstract
We present the observations of the gravitationally lensed system QSO 2237+0305 (Einstein Cross) performed with the Advanced CCD Imaging Spectrometer on board the Chandra X-Ray Observatory on 2000 September 6 and on 2001 December 8 for 30.3 and 9.5 ks, respectively. Imaging analysis resolves the four X-ray images of the Einstein Cross. A possible fifth image is detected; however, the poor signal-to-noise ratio of this image combined with contamination produced by a nearby brighter image make this detection less certain. We investigate possible origins of the additional image. Fits to the combined spectrum of all images of the Einstein Cross assuming a simple power law with Galactic and intervening absorption at the lensing galaxy yields a photon index of 1.90 consistent with the range of Γ measured for large samples of radio-quiet quasars. For the first Chandra observation of the Einstein Cross this spectral model yields a 0.4-8.0 keV X-ray flux of 4.6 × 10-13 ergs cm-2 s-1 and a 0.4-8.0 keV lensed luminosity of 1.0 × 1046 ergs s-1. The source exhibits variability over both long and short timescales. The X-ray flux has dropped by 20% between the two observations, and the Kolmogorov-Smirnov test showed that image A is variable at the 97% confidence level within the first observation. Furthermore, a possible time delay of 2.7 hr between images A and B with image A leading is detected in the first Chandra observation. The X-ray flux ratios of the images are consistent with the optical flux ratios that are affected by microlensing, suggesting that the X-ray emission is also microlensed. A comparison between our measured column densities and those inferred from extinction measurements suggests a higher dust-to-gas ratio in the lensing galaxy than the average value of our Galaxy. Finally, we report the detection at the 99.99% confidence level of a broad emission feature near the redshifted energy of the Fe Kα line in only the spectrum of image A. The rest frame energy, width, and equivalent width of this feature are Eline = 5.7 keV, σline = 0.87 keV, and EW = 1200 eV, respectively.
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