H$\alpha$ Reverberation Mapping of the Intermediate-Mass Active Galactic Nucleus in NGC 4395

TL;DR

This study uses high-cadence spectroscopic and imaging data to perform Hα reverberation mapping of the low-mass AGN in NGC 4395, constraining the black hole mass and broad line region properties.

Contribution

It demonstrates the effective use of spatially-resolved IFS data for flux calibration and provides updated measurements of the broad line width and black hole mass in NGC 4395.

Findings

Black hole mass estimated at (1.7±0.3)×10^4 solar masses.

Broad line lag constrained to less than 3 hours.

Second moment of Hα broad component measured as 586±19 km/s.

Abstract

We present the results of a high-cadence spectroscopic and imaging monitoring campaign of the active galactic nucleus (AGN) of NGC 4395. High signal-to-noise-ratio spectra were obtained at the Gemini-N 8 m telescope using the GMOS integral field spectrograph (IFS) on 2019 March 7, and at the Keck-I 10 m telescope using the Low-Resolution Imaging Spectrometer (LRIS) with slitmasks on 2019 March 3 and April 2. Photometric data were obtained with a number of 1 m-class telescopes during the same nights. The narrow-line region (NLR) is spatially resolved; therefore, its variable contributions to the slit spectra make the standard procedure of relative flux calibration impractical. We demonstrate that spatially-resolved data from the IFS can be effectively used to correct the slit-mask spectral light curves. While we obtained no reliable lag owing to the lack of strong variability pattern in the light curves, we constrain the broad line time lag to be less than 3 hr, consistent with the photometric lag of $\sim80$ min reported by Woo et al. (2019). By exploiting the high-quality spectra, we measure the second moment of the broad component of the H$\alpha$ emission line to be $586\pm19$ km s$^{-1}$, superseding the lower value reported by Woo et al. (2019). Combining the revised line dispersion and the photometric time lag, we update the black hole mass as $(1.7\pm 0.3)\times10^4$ M$_{\odot}$.