We monitored the z=0.158 quasar 3C273 between 2015 and 2019 in the optical (BVrz) and near-infrared (JHK) with the aim to perform dust reverberation mapping. Accounting for host galaxy and accretion disk contributions, we obtained pure dust light curves in JHK. Cross correlations between the V-band and the dust light curves yield an average rest-frame delay for the hot dust of tau_cent ~ 410 days. This is a factor 2 shorter than expected from the the dust ring radius R_x ~ 900 light days reported from interferometric studies. The dust covering factor (CF) is about 8%, much smaller than predicted from the half covering angle of 45 degree found for active galactic nuclei (AGN). We analyse the asymmetric shape of the correlation functions and explore whether an inclined bi-conical bowl-shaped dust torus geometry could bring these findings (tau_cent, R_x and CF) into a consistent picture. The hot varying dust emission originates from the edge of the bowl rim with a small covering angle 40 < theta < 45 degree, and we see only the near side of the bi-conus. Such a dust gloriole with R_x = 900 +/- 200 ld and an inclination i = 12 degree matches the data remarkably well. Comparing the results of 3C273 with literature for less luminous AGN, we find a lag-luminosity relation tau propto L^alpha with alpha between 0.33 and 0.40, flatter than the widely adopted relation with alpha = 0.5. We address several explanations for the new lag - luminosity relation.

中文翻译：

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3C 273 的尘埃混响：环面结构和滞后-光度关系

我们在 2015 年至 2019 年期间在光学 (BVrz) 和近红外 (JHK) 中监测了 z=0.158 类星体 3C273，目的是进行尘埃混响映射。考虑到宿主星系和吸积盘的贡献，我们在 JHK 中获得了纯尘埃光曲线。V 波段和尘埃光曲线之间的互相关产生了 tau_cent ~ 410 天的热尘埃的平均静止帧延迟。这比干涉测量研究报告的尘埃环半径 R_x ~ 900 光日的预期短 2 倍。尘埃覆盖因子 (CF) 约为 8%，远小于根据为活动星系核 (AGN) 发现的 45 度半覆盖角所预测的。我们分析了相关函数的不对称形状，并探讨了倾斜的双圆锥碗状尘埃环几何形状是否可以带来这些发现（tau_cent，R_x 和 CF) 变成一致的图片。热变化的粉尘排放起源于碗边缘的小覆盖角 40 < theta < 45 度，我们只看到双圆锥的近侧。这种具有 R_x = 900 +/- 200 ld 且倾角 i = 12 度的尘埃光环与数据非常吻合。将 3C273 的结果与亮度较低的 AGN 的文献进行比较，我们发现滞后光度关系 tau propto L^alpha 的 alpha 介于 0.33 和 0.40 之间，比广泛采用的 alpha = 0.5 的关系更平坦。我们对新的滞后 - 亮度关系提出了几种解释。这种具有 R_x = 900 +/- 200 ld 且倾角 i = 12 度的尘埃光环与数据非常吻合。将 3C273 的结果与亮度较低的 AGN 的文献进行比较，我们发现滞后光度关系 tau propto L^alpha 的 alpha 介于 0.33 和 0.40 之间，比广泛采用的 alpha = 0.5 的关系更平坦。我们对新的滞后 - 亮度关系提出了几种解释。这种具有 R_x = 900 +/- 200 ld 且倾角 i = 12 度的尘埃光环与数据非常吻合。将 3C273 的结果与亮度较低的 AGN 的文献进行比较，我们发现滞后光度关系 tau propto L^alpha 与 alpha 在 0.33 和 0.40 之间，比广泛采用的 alpha = 0.5 的关系更平坦。我们对新的滞后 - 亮度关系提出了几种解释。