CAUGHT IN THE ACT: CHANDRA OBSERVATIONS OF MICROLENSING OF THE RADIO-LOUD QUASAR MG J0414+0534

摘要

We present results from monitoring of the distant (z = 2.64), gravitationally lensed quasar MG J0414+0534 with the Chandra X-Ray Observatory. An Fe Kα line at 6.49 +- 0.09 keV (rest frame) with an equivalent width of ～ 190 eV, consistent with fluorescence from a cold medium, is detected at the 99% confidence level in the spectrum of the brightest image A. During the last two observations of our monitoring program, we detected a fivefold increase of the equivalent width of a narrow Fe Kα line in the spectrum of image B but not in the brighter image A, whereas image C is too faint to resolve the line. The continuum emission component of image B did not follow the sudden enhancement of the iron line in the last two observations. We propose that the sudden increase in the iron-line strength from ～190 to 900 eV only in image B can be explained with a caustic crossing due to microlensing that selectively enhances a strip of the line emission region of the accretion disk. The nonenhancement of the continuum emission in the spectrum of image B suggests that the continuum emission region is concentrated closer to the center of the accretion disk than the iron-line emission region and that the magnification caustic has not reached close enough to the former region to amplify it. A model of a caustic crossing event predicts discontinuities in the light curve of the magnification and provides an upper limit of ～5 x 10~(-4) pc on the outer radius of the Fe Kα emission region. The nondetection of any relativistic or Doppler shifts of the iron line in the spectrum of image B implies that the magnification caustic for the last two observations was located at a radius greater than ～100 gravitational radii. Each observation of the quadruply lensed quasar MG J0414+0534 provides a view of the quasar at four different epochs spaced by the time delays between the lensed images. We produced a light curve of the quasar X-ray flux by normalizing the flux of each image to the mean flux of that image over all observations. We find significant deviations of the normalized light curve from unity especially for the faintest image C. A plausible mechanism to explain the flux variability of image C is a microlensing event. Finally, spectral analysis of MG J0414+0534 indicates the presence of significant absorption in excess of the Galactic value. For absorption at the redshift of the lensed quasar, we find an intrinsic column density of N_H ～ 5 x 10~(22) cm~(-2), consistent with the reddening observed in the optical band.