MAXI investigation into the longterm X-ray variability from the very-high-energy gamma-ray blazar Mrk 421

TL;DR

This study used MAXI X-ray data to analyze the long-term variability of Mrk 421, revealing a power-law PSD with a break consistent with previous observations, and providing insights into its variability characteristics.

Contribution

First long-term X-ray variability analysis of Mrk 421 using MAXI data, estimating PSD index and break frequency, and comparing with previous high-frequency observations.

Findings

PSD index in low frequencies is approximately 1.6

PSD break frequency is constrained around 5 x 10^-6 Hz

Low-frequency PSD aligns with typical Seyfert galaxy variability

Abstract

The archetypical very-high-energy gamma-ray blazar Mrk 421 was monitored for more than 3 years with the Gas Slit Camera onboard Monitor of All Sky X-ray Image (MAXI), and its longterm X-ray variability was investigated. The MAXI lightcurve in the 3 -- 10 keV range was transformed to the periodogram in the frequency range $f = 1 \times 10^{-8}$ -- $2 \times 10^{-6}$ Hz. The artifacts on the periodogram, resulting from data gaps in the observed lightcurve, were extensively simulated for variations with a power-law like Power Spectrum Density (PSD). By comparing the observed and simulated periodograms, the PSD index was evaluated as $\alpha = 1.60 \pm 0.25$. This index is smaller than that obtained in the higher frequency range ($f > 1 \times 10^{-5}$ Hz), namely, $\alpha = 2.14 \pm 0.06$ in the 1998 ASCA observation of the object. The MAXI data impose a lower limit on the PSD break at $f_{\rm b} = 5 \times 10^{-6}$ Hz, consistent with the break of $f_{\rm b} = 9.5 \times 10^{-6}$ Hz, suggested from the ASCA data. The low frequency PSD index of Mrk 421 derived with MAXI falls well within the range of the typical value among nearby Seyfert galaxies ($\alpha = 1$ -- $2$). The physical implications from these results are briefly discussed.