Discovery of a relation between black hole mass and soft X-ray time lags in active galactic nuclei

Abstract

We carried out a systematic analysis of time lags between X-ray energy bands in a large sample (32 sources) of unabsorbed, radio quiet active galactic nuclei (AGN), observed by XMM-Newton. The analysis of X-ray lags (up to the highest/shortest frequencies/time-scales), is performed in the Fourier-frequency domain, between energy bands where the soft excess (soft band) and the primary power law (hard band) dominate the emission. We report a total of 15 out of 32 sources displaying a high frequency soft lag in their light curves. All 15 are at a significance level exceeding 97 per cent and 11 are at a level exceeding 99 per cent. Of these soft lags, 7 have not been previously reported in the literature, thus this work significantly increases the number of known sources with a soft/negative lag. The characteristic time-scales of the soft/negative lag are relatively short (with typical frequencies and amplitudes of ν ∼ 0.07 − 4 × 10 −3 Hz and τ ∼ 10 − 600 s, respectively), and show a highly significant ( > ∼ 4σ) correlation with the black hole mass. The measured correlations indicate that soft lags are systematically shifted to lower frequencies and higher absolute amplitudes as the mass of the source increases. To first approximation, all the sources in the sample are consistent with having similar mass-scaled lag properties. These results strongly suggest the existence of a mass-scaling law for the soft/negative lag, that holds for AGN spanning a large range of masses (about 2.5 orders of magnitude), thus supporting the idea that soft lags originate in the innermost regions of AGN and are powerful tools for testing their physics and geometry.

Multiwavelength campaign on Mrk 509

“…The moderate Eddington ratio of 0.3 in Mrk 509 may therefore be too low for blurred ionized reflection to become the dominant source of soft-excess in this object. However, lags have also been detected in many Seyfert galaxies without high accretion rates (De Marco et al 2013).…”

“…Soft X-ray reverberation lags have been found in ESO 113-G010 by Cackett et al (2013) and PG 1211+143 by De Marco et al (2011). Overall, soft lags have been found in more than 15 sources (De Marco et al 2013). Blurred ionized reflection has also been tested, as well as double Comptonization and ionized absorption by a high-velocity material, in Mrk 509 and Mrk 841, using average broad-band Suzaku data (Cerruti et al 2011).…”

X-ray reverberation around accreting black holes

“…Gebhardt et al 2000). 12 The observed scaling is flatter than expected from a linear relationship, but this can be explained as a bias due to the fact that we only sample the higher frequency end of the soft lag range in the highest mass objects, which leads to systematically shorter lags than would be seen if we could sample the maximum amplitude of soft lags seen at lower frequencies (De Marco et al 2013). Marco et al (2013).…”

“…De Marco et al (2013) conducted a systematic search through the XMMNewton archive for variable Seyfert galaxies with sufficiently long observations, Fig. 9 The ratio spectrum of 1H0707-495 to a continuum model (Fabian et al 2009).…”

Multi-Wavelength Variability

“…Subsequently, following the first highly significant detection in 1H0707-495 (Fabian et al 2009), soft highfrequency lags have been confirmed in XMM-Newton data from many other AGN (e.g. De Marco et al 2013), and these are now considered to be associated with reverberation from relatively close to the black hole, which we will not discuss further here (but see Reynolds, this volume). The origin of the hard lags observed in AGN at lower Fourier frequencies is still debated, however.…”