Abstract:
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 (Fvar) 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 σline, give virial black hole mass estimates of 6.3−2.1+2.0×107M⊙ for Mrk 1048 and 1.2−0.6+0.4×107M⊙ 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.
