A Soft X-Ray Lag Detected in Centaurus A

Abstract

We performed time lag analysis on the X-ray light curves of Centaurus A (Cen A) obtained by the Gas Slit Camera (GSC) aboard the Monitor of All-sky X-ray Image ({\it MAXI}) in three energy bands (2--4 keV, 4--10 keV, and 10--20 keV). We discovered a soft X-ray lag relative to higher energies (soft lag) on a timescale of days in a flaring episode by employing the discrete correlation function (DCF) and the z-transformed discrete correlation function (ZDCF) method. In the episode, a peak and a centroid in the DCF and the ZDCF was observed at a soft lag of $\sim 5$ days in 2--4 keV versus 4--10 keV and in 4--10 keV versus 10--20 keV, and $\sim 10$ days in 2--4 keV versus 10--20 keV. We found it difficult to explain the observed X-ray variation by a single energy injection with the one-zone synchrotron self-Compton (SSC) model, in which the soft lags in these three energy bands reflect the different cooling times of the relativistic electrons, by assuming the magnetic field and minimum Lorentz factor estimated from a broad-band SED. Alternatively, if the phenomenon is interpreted as cooling of Comptonizing electrons in a corona covering the accretion disk, the temperature of the corona producing the variable X-rays should be $\sim$10 keV for reconciliation with the soft lag in the energy range of 2--20 keV.

23 pages, 7 figures, accepted version

References in corpus (10)

• Swift BAT Survey of AGN
• An absorption origin for the X-ray spectral variability of MCG-6-30-15
• A soft X-ray reverberation lag in the AGN ESO 113-G010
• Observations of MCG-5-23-16 with Suzaku, XMM-Newton and NuSTAR: Disk tomography and Compton hump reverberation
• One-zone SSC model for the core emission of Centaurus A revisited
• VHE Gamma-Ray Induced Pair Cascades in the Radiation Fields of Dust Tori of AGN: Application to Cen A
• The 37-month MAXI/GSC source catalog in the high Galactic-latitude sky
• Energy Dependent time lags in the Seyfert 1 galaxy NGC 4593
• The hard X-ray continuum of Cen A observed with INTEGRAL SPI
• MAXI: all-sky observation from the International Space Station

Cited by in corpus (4)

• High-Energy Neutrinos from Magnetized Coronae of Active Galactic Nuclei and Prospects for Identification of Seyfert Galaxies and Quasars in Neutrino Telescopes
• The 7-year MAXI/GSC X-ray Source Catalog in the High Galactic-Latitude Sky (3MAXI)
• X-ray and infrared diagnostics of nearby active galactic nuclei with MAXI and AKARI
• Distributed accelerators in the jet of Centaurus A: the origin of the spectral hardening of very high energy gamma-rays