Microlensing variability in the gravitationally lensed quasar QSO 2237+0305 ≡ the Einstein Cross I. Spectrophotometric monitoring with the VLT

We present the continuation of our long-term spectroscopic monitoring of the gravitationally lensed quasar QSO 2237 + 0305. We investigate the chromatic variations observed in the UV/optical continuum of both quasar images A and B, and compare them with numerical simulations to infer the energy profile of the quasar accretion disk. Our procedure combines the microlensing ray-shooting technique with Bayesian analysis, and derives probability distributions for the source sizes as a function of wavelength. We find that the effective caustic crossing timescale is 4.0 +/- 1.0 months. Using a robust prior on the effective transverse velocity, we find that the source responsible for the UV/optical continuum has an energy profile well reproduced by a power-law R proportional to lambda(zeta) with zeta = 1.2 +/- 0.3, where R is the source size responsible for the emission at wavelength.. This is the first accurate, model-independent determination of the energy profile of a quasar accretion disk on such small scales.

[1]  D. Lynden-Bell,et al.  Galactic Nuclei as Collapsed Old Quasars , 1969, Nature.

[2]  B. Dewitt,et al.  Black holes (Les astres occlus) , 1973 .

[3]  G. Shields.  Thermal continuum from accretion disks in quasars , 1978, Nature.

[4]  John P. Huchra,et al.  2237 + 0305 - a new and unusual gravitational lens , 1985 .

[5]  Edwin L. Turner,et al.  High-resolution CCD imaging and derived gravitational lens models of 2237+0305 , 1988 .

[6]  J. Lasota,et al.  Slim Accretion Disks , 1988 .

[7]  William H. Press,et al.  Numerical recipes , 1990 .

[8]  J. Wambsganss,et al.  A Microlensing Model for QSO 2237+0305 , 1990 .

[9]  P. Schneider,et al.  Interpretation of the microlensing event in QSO 2237+0305 , 1990 .

[10]  J. Wambsganss,et al.  Expected color variations of the gravitationally microlensed QSO 2237 + 0305 , 1991 .

[11]  J. Huchra,et al.  The velocity field of clusters of galaxies within 100 megaparsecs. II: Northern clusters , 1991 .

[12]  D. Schneider,et al.  Initial light curve of Q2237 + 0305 , 1991 .

[13]  R. Blandford,et al.  Microlensing and the structure of active galactic nucleus accretion disks , 1991 .

[14]  D. Schneider,et al.  Hubble Space Telescope Wide Field Camera Imaging of the Gravitational Lens 2237+0305 , 1992 .

[15]  J. Wambsganss.  Probability distributions for the magnification of quasars due to microlensing , 1992 .

[16]  T. Kundić,et al.  Gravitational microlensing - The effect of random motion of individual stars in the lensing galaxy , 1993 .

[17]  S. Mao,et al.  Interpretation of microlensing events in Q2237 + 0305 , 1994 .

[18]  J. Wambsganss,et al.  Parameter degeneracy in models of the quadruple lens system Q2237+0305 , 1994, astro-ph/9408084.

[19]  M. Irwin,et al.  The statistics of microlensing light curves - I. Amplification probability distributions , 1995, astro-ph/9504018.

[20]  T. Kundić,et al.  Gravitational Microlensing by Random Motion of Stars: Analysis of Light Curves , 1995, astro-ph/9503035.

[21]  M. Irwin,et al.  The statistics of microlensing light curves — II. Temporal analysis , 1996 .

[22]  M. J. Lehner,et al.  The Macho Project: 45 Candidate Microlensing Events from the First Year Galactic Bulge Data , 1997 .

[23]  J. Krolik.  A New Equilibrium for Accretion Disks around Black Holes , 1998, astro-ph/9802276.

[24]  T. D. Matteo,et al.  Active Galactic Nuclei: From the Central Black Hole to the Galactic Environment , 2000 .

[25]  E. Turner,et al.  Limits on the microlens mass function of Q2237+0305 , 1999, astro-ph/9904359.

[26]  M. Sambridge.  Geophysical inversion with a neighbourhood algorithm—II. Appraising the ensemble , 1999 .

[27]  E. Turner,et al.  A small source in Q2237+0305? , 1999, astro-ph/9911245.

[28]  E. Agol,et al.  Magnetic Stress at the Marginally Stable Orbit: Altered Disk Structure, Radiation, and Black Hole Spin Evolution , 1999, astro-ph/9908049.

[29]  P. Wozniak,et al.  The Optical Gravitational Lensing Experiment: A Hunt for Caustic Crossings in QSO 2237+0305 , 2000, astro-ph/0004287.

[30]  Determining the microlens mass function from quasar microlensing statistics , 2000, astro-ph/0008008.

[31]  C. Alard,et al.  The Optical Gravitational Lensing Experiment Monitoring of QSO 2237+0305* , 1999, astro-ph/9904329.

[32]  Evidence for a Source Size of Less than 2000 AU in Quasar 2237+0305 , 2000, astro-ph/0012216.

[33]  E. Agol,et al.  The size of the mid-IR emission region of a quasar inferred from microlensed images of Q2237 0305 , 2001, astro-ph/0112281.

[34]  B. McLeod,et al.  QSO 2237+0305 VR Light Curves from Gravitational LensES International Time Project Optical Monitoring , 2002, astro-ph/0204426.

[35]  Cambridge,et al.  Optical monitoring of the gravitationally lensed quasar Q2237+0305 from APO between June 1995 and January 1998 , 2002, astro-ph/0207117.

[36]  J. J. Bock,et al.  Peculiar Velocity Limits from Measurements of the Spectrum of the Sunyaev-Zeldovich Effect in Six Clusters of Galaxies , 2003, astro-ph/0303510.

[37]  C. Kochanek.  Quantitative Interpretation of Quasar Microlensing Light Curves , 2003, astro-ph/0307422.

[38]  P. Schechter,et al.  Size Is Everything: Universal Features of Quasar Microlensing with Extended Sources , 2004, astro-ph/0408195.

[39]  G. Lewis,et al.  Limits on the transverse velocity of the lensing galaxy in Q2237+0305 from the lack of strong microlensing variability , 2004, astro-ph/0411239.

[40]  K. Ulaczyk,et al.  The Optical Gravitational Lensing Experiment. OGLE-III Long Term Monitoring of the Gravitational Lens QSO 2237+0305 , 2006, astro-ph/0701300.

[41]  C. Keeton,et al.  Microlensing of an extended source by a power-law mass distribution , 2006, astro-ph/0612542.

[42]  P. Schechter,et al.  X-Ray and Optical Flux Ratio Anomalies in Quadruply Lensed Quasars. I. Zooming in on Quasar Emission Regions , 2006, astro-ph/0607655.

[43]  Z. Kuncic,et al.  Towards a new standard model for black hole accretion , 2007, 0705.0791.

[44]  D. Long,et al.  The multiple quasar Q2237+0305 under a microlensing caustic , 2007, 0711.4265.

[45]  Georges Meylan,et al.  Microlensing variability in the gravitationally lensed quasar QSO 2237+0305 ≡ the Einstein Cross I. Spectrophotometric monitoring with the VLT , 2008 .

[46]  Christopher S. Kochanek,et al.  The Spatial Structure of an Accretion Disk , 2007, 0707.0003.

[47]  A. Eigenbrod.  Microlensing variability in the gravitationally lensed quasar QSO $2237+0305 \equiv$ the Einstein Cross , 2008 .

[48]  Physikalisches Kolloquium,et al.  Fakultät für Physik , 2009 .

[49]  R. Rosenfeld.  Nature , 2009, Otolaryngology--head and neck surgery : official journal of American Academy of Otolaryngology-Head and Neck Surgery.

[50]  R. Davies,et al.  Astronomical Society of the Pacific Conference Series , 2010 .