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

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 Å) centered at 8045, 8505, and 9171 Å. 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 $z>0.2$.