Spectroscopic reverberation mapping for SARM: The case of Mrk 1048 and Mrk 618

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.