X-ray Timing and Spectral studies of bare AGN Mrk 110

The origin of the soft X-ray excess below 2 keV in active galactic nuclei (AGNs) remains debated, with relativistic reflection from the inner accretion disk and warm Comptonization in an optically thick corona being the leading explanations. We investigate the timing and spectral properties of the Seyfert galaxy Mrk 110 using six XMM-Newton observations. A frequency-dependent lag analysis in the 7-9 $\times 10^{-5}$ Hz range reveals a soft X-ray lag of 889-3000s in the combined 2019 data, detected with a significance of 80%. The cross-correlation function analysis, supported by simulations, also detects lags of similar nature. Spectral modeling performed by adopting both proposed black hole masses in the literature for Mrk 110 confirms the presence of a warm corona in all observations, along with a weak relativistic reflection component and the reflection fraction remains low (Rf < 1). Interpreting the measured soft lag in terms of light travel time implies an emission radius 4.5 Rg for a supermassive black hole mass of $M = 1.4 \times 10^8$ solar mass , favoring a reflection scenario. However, if a lower mass of $M = 2 \times 10^7$ solar mass is adopted, the inferred radius increases, and both relativistic reflection and warm Comptonization can plausibly contribute to the observed soft lag. The warm corona radius appears larger in the high accretion state and smaller in a lower accretion state, although no trend can be established. The persistently low reflection fraction suggests an outflowing inner corona in Mrk 110, consistent with the recent detection of jet activity in this source.

💡 Research Summary

The authors present a comprehensive timing and spectral investigation of the Seyfert 1 galaxy Mrk 110 using six XMM‑Newton observations spanning from 2004 to 2020. The data set includes one early observation (Obs 1, 2004), four closely spaced observations in November 2019 (Obs 2a–d, collectively referred to as Obs 2), and a later observation in April 2020 (Obs 3). EPIC‑pn light curves were extracted with a 500 s binning, and simultaneous UV photometry from the Optical Monitor was also employed.

A hardness‑intensity analysis shows the source follows the typical “softer‑when‑brighter” trend seen in many radio‑quiet AGN, with a linear relation between hardness ratio and total count rate. Power‑spectral density (PSD) analysis of the 0.3–10 keV band, using 100 s time resolution, reveals a red‑noise power‑law shape with slopes α ranging from ~1.8 to ~2.1, consistent with AGN variability. These PSD slopes were later used to generate realistic simulated light curves for significance testing.

The core timing result is the detection of a soft X‑ray lag in the frequency range 7–9 × 10⁻⁵ Hz. In the combined 2019 data (Obs 2) the lag amplitude varies from –889 s at 7 × 10⁻⁵ Hz to –3067 s at 9 × 10⁻⁵ Hz. The 2004 observation shows a marginal soft lag (–401 ± 468 s) at the same frequency, while the 2020 observation does not reveal a significant lag. Monte‑Carlo simulations based on the Timmer & König method, incorporating the measured PSD slopes and observed count‑rate statistics, indicate that the observed lag in Obs 2 is significant at the ~80 % confidence level. Coherence between the soft (0.3–1 keV) and hard (1–5 keV) bands remains moderate (~0.5) at low frequencies but declines at higher frequencies where Poisson noise dominates.

Spectral modeling was performed with two competing black‑hole mass estimates for Mrk 110: a higher mass of 1.4 × 10⁸ M⊙ (derived from gravitational redshift and spectropolarimetry) and a lower mass of 2 × 10⁷ M⊙ (from reverberation mapping). In both cases the broadband X‑ray spectra (0.3–10 keV) require a warm, optically thick corona (kT≈0.2–0.4 keV, τ≈10–15) to account for the soft excess. A relativistic reflection component (modeled with relxill) is also present but weak, with a reflection fraction Rf < 1 in all observations.

Interpreting the measured soft lag as a light‑travel time delay yields an emission radius of ≈4.5 Rg for the higher black‑hole mass, supporting a scenario where the lag originates from reverberation of the inner accretion disc. For the lower mass, the same lag corresponds to a larger physical radius, allowing both warm‑corona Comptonization delays and disc‑reflection reverberation to contribute. The warm‑corona radius appears larger during higher accretion states (e.g., the 2019 observations) and smaller in lower states (2020), although the limited sample prevents a firm correlation.

The persistently low reflection fraction, together with recent VLBI detections of a relativistic jet in Mrk 110 (apparent speeds 2–3.6 c), suggests an outflowing inner corona. Such an outflow could diminish the reflected flux while simultaneously feeding the jet, providing a coherent picture that links the X‑ray corona, disc reverberation, and jet activity.

In summary, the paper demonstrates that Mrk 110 exhibits a measurable soft X‑ray lag consistent with disc reverberation for a high black‑hole mass, while also requiring a warm corona to explain the soft excess. The results imply that both relativistic reflection and warm‑corona Comptonization can operate simultaneously, with their relative importance possibly governed by the black‑hole mass and accretion rate. Future long‑duration, high‑signal‑to‑noise observations, combined with multi‑wavelength monitoring, will be essential to disentangle these components and to clarify the physical connection between the corona and the newly discovered jet in this source.