2025ApJ...995...31P

Robust extragalactic distance measurements are crucial for resolving the persistent discrepancy in the value of the Hubble constant (H₀). Active galactic nuclei (AGNs), through their compact and variable broad-line regions, enable the determination of geometric distances when reverberation mapping (RM) is combined with spectroastrometry. We report results from a spectroscopic RM campaign (2022 October-2023 March) targeting two narrow-line Seyfert 1 galaxies, Mrk 1048 and Mrk 618, using the 3.6 m Devasthal Optical Telescope and 2 m Himalayan Chandra Telescope. High-cadence spectrophotometric monitoring was carried out using onboard instruments such as ADFOSC, HFOSC, and TANSPEC, resulting in well-sampled continuum and emission-line light curves. The observed fractional variability (F_var) ranged from 4% to 14% across the g-band, Hβ, and Hα light curves. The time lags were measured using the interpolated cross-correlation function (ICCF), PyI²CCF, and JAVELIN methods. In the rest frame, the ICCF analysis yields Hβ lags of $10.{5}_{-4.2}^{+2.6}$ days for Mrk 1048 and $10.{2}_{-2.9}^{+3.4}$ days for Mrk 618, while the corresponding Hα lags are $18.{7}_{-5.4}^{+5.3}$ and $14.{4}_{-10.5}^{+4.6}$ days, respectively. The emission-line widths, measured from the rms spectra using σ_line, give virial black hole mass estimates of $6.{3}_{-2.1}^{+2.0}\times 1{0}^{7}\,{M}_{\odot }$ for Mrk 1048 and $1.{2}_{-0.6}^{+0.4}\times 1{0}^{7}\,{M}_{\odot }$ for Mrk 618. These results will serve as a basis for absolute geometric distance calibration when combined with VLTI/GRAVITY spectroastrometric measurements, thereby contributing to the development of AGNs as standardizable cosmological probes.