Broad Band Photometric Reverberation Mapping of NGC 4395

TL;DR

This study demonstrates that broad band photometric reverberation mapping can effectively measure the size of the broad line region and estimate the black hole mass in low luminosity AGN like NGC 4395 using simple, accessible observational techniques.

Contribution

The paper introduces a photometric reverberation mapping method using broad band filters to estimate black hole masses in AGN, providing a simpler alternative to spectroscopic RM.

Findings

Measured Hα lag of 3.6 ± 0.8 hours in NGC 4395.

Estimated black hole mass of (4.9 ± 2.6) × 10^4 solar masses.

Validated the photometric RM approach against previous spectroscopic results.

Abstract

We present results of broad band photometric reverberation mapping (RM) to measure the radius of the broad line region, and subsequently the black hole mass (M$_{\rm BH}$), in the nearby, low luminosity active galactic nuclei (AGN) NGC 4395. Using the Wise Observatory's 1m telescope equipped with the SDSS g$'$, r$'$ and i$'$ broad band filters, we monitored NGC 4395 for 9 consecutive nights and obtained 3 light curves each with over 250 data points. The g$'$ and r$'$ bands include time variable contributions from H$\beta$ and H$\alpha$ (respectively) plus continuum. The i$'$ band is free of broad lines and covers exclusively continuum. We show that by looking for a peak in the difference between the cross-correlation and the auto-correlation functions for all combinations of filters, we can get a reliable estimate of the time lag necessary to compute M$_{\rm BH}$. We measure the time lag for H$\alpha$ to be $3.6 \pm 0.8 $ hours, comparable to previous studies using the line resolved spectroscopic RM method. We argue that this lag implies a black hole mass of M$_{\rm BH} = (4.9 \pm 2.6) \times 10^{4}$ \Msun .