# The long term X-ray time-lags of NGC 4051

## TL;DR

This study measures the lowest frequency X-ray time-lags in NGC 4051, revealing a power-law behavior consistent with black hole binaries, thus supporting the analogy between active galactic nuclei and X-ray binaries.

## Contribution

First measurement of low-frequency X-ray time-lags in an AGN, extending the frequency range and confirming the power-law behavior across a broad spectrum.

## Key findings

- X-ray time-lags follow a power-law with slope ~ -1 from 10^{-7} to 10^{-3} Hz.
- Time-lags at higher energies are delayed relative to 2-4 keV band.
- Results support the analogy between AGN and Galactic X-ray binaries.

## Abstract

We present the long term, frequency-dependent, X-ray time lags of the Seyfert galaxy NGC 4051. We used Rossi X-ray Timing Explorer light curves in the 2-4, 4-7 and 7-10 keV bands and we measured the time-lags at $10^{-7}-10^{-6}$ Hz. This is the lowest frequency range that AGN time-lags have ever been measured. The variations in the higher energy bands are delayed with respect to the variations we observe at 2-4 keV, in agreement with the time-lags at high frequencies. When we combine our results with those from the model fitting of the time lags at higher frequencies we find that that the X-ray hard lags in NGC 4051 follow a power law of slope $\sim -1$ over a broad frequency range, from $\sim 10^{-7}$ to $\sim 10^{-3}$ Hz. This is consistent with the time-lags of Cyg X-1, a result which further supports the analogy between active galaxies and Galactic X-ray black hole binaries.