Reverberation Mapping with Intermediate-Band Photometry: Detection of Broad-Line H$α$ Time Lags for Quasars at $0.2<z<0.4$

Abstract

We present a reverberation mapping (RM) experiment that combines broad- and intermediate-band photometry; it is the first such attempt targeting a sample of 13 quasars at $0.2<z<0.9$. The quasars were selected to have strong H$α$ or H$β$ emission lines that are located in one of three intermediate bands (with FWHM around 200 à ) centered at 8045, 8505, and 9171 à . The imaging observations were carried out in the intermediate bands and the broad $i$ and $z$ bands using the prime-focus imager 90Prime on the 2.3m Bok telescope. Because of the large ($\sim$1 deg$^2$) field-of-view (FoV) of 90Prime, we were able to include the 13 quasars within only five telescope pointings or fields. The five fields were repeatedly observed over 20--30 epochs that were unevenly distributed over a duration of 5--6 months. The combination of the broad- and intermediate-band photometry allows us to derive accurate light curves for both optical continuum (from the accretion disk) and line (from the broad-line region, or BLR) emission. We detect H$α$ time lags between the continuum and line emission in 6 quasars. These quasars are at a relatively low redshift range $0.2<z<0.4$. The measured lags are consistent with the current BLR size-luminosity relation for H$β$ at $z<0.3$. While this experiment appears successful in detecting lags of the bright H$α$ line, further investigation is required to see if it can also be applied to the fainter H$β$ line for quasars at higher redshifts. Finally we demonstrate that by using a small telescope with a large FoV, intermediate-band photometric RM can be efficiently executed for a large sample of quasars at $z>0.2$.

12 pages, 9 figures, accepted for publication in ApJ