Abstract
The origin of the Soft X-ray excess in the spectra of several nearby active galactic nucleus is still highly debated. In this work we report the monotonic spectral hardening of a nearby bare Seyfert galaxy ESO 511-G030 over a time period of ∼110 ks. We found that both the soft (0.3–2 keV) and the hard X-ray (2–10 keV) bands have shown interesting variability during the 110 ks observation. The soft X-ray flux shows a sharp drop in the last ∼50 ks, by ∼30%, while the hard X-ray flux initially increased and then decreased. The overall effect of these variations is a monotonic increase in the hardness ratio. We conclude that: (A) The soft X-ray flux variations may be related to the changes in the primary hard X-ray. (B) The soft X-ray flux decreases by ∼36% in a timespan of ∼50 ks, indicating that its origin could be from a region as close as <0.5 light-day from the supermassive black hole. A detailed hardness ratio resolved spectral analysis with state-of-the-art reflection models will be reported in a future paper.
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1. Introduction
Active galactic nuclei (AGNs) emit radiation in panchromatic wavelength range. In particular, the UV and X-ray emission probes the dynamics and physics of matter in the sub-pc to a few 10 s of pc scale region around the super massive black hole (Fabian et al. 2009; García et al. 2014; Ghosh & Laha 2020a, 2020b; Laha et al. 2018a, 2019a; Civano et al. 2019; Lopez-Rodriguez et al. 2019; Nyland et al. 2019). Apart from the power law, the AGN X-ray spectra have several discrete and continuum features which arise when the primary hard X-ray photons interact with the surrounding ionized and neutral matter in the vicinity of the supermassive black hole (SMBH). One of them is the soft X-ray excess, which is the excess emission in the 0.3–2 keV over the primary power law component and is ubiquitously found in nearby AGN (Pal et al. 2021, 2020, 2016; Pal & Dewangan 2013). The origin of the soft-excess is a highly debated topic in astrophysics.
ESO 511-G030 (ESO511 from now) is a well studied bright nearby AGN which has exhibited the presence of strong soft excess. ESO511 has also shown X-ray flux and spectral variability the timescales of <day to years. It is a bare AGN with no signature of ionized absorption in the X-ray spectra which are detected in ∼50% of nearby AGN population (Tombesi et al. 2010; Reeves et al. 2020; Laha et al. 2011, 2013, 2016a, 2014, 2021, 2019b, 2018b, 2017, 2016b, 2020). Previous studies (Ghosh & Laha 2020b) on the source found that both the relativistic reflection (García et al. 2014) as well as two-corona models (Done et al. 2012) can describe the origin of its soft X-ray excess. In this work we show the interesting ks scale flux and spectral variability of the source, using an arxival XMM-Newton observation in 2007 (obsid: 0502090201).
2. Results and Conclusions
The XMM-Newton EPIC-pn data has been reprocessed following the methods enumerated in Ghosh & Laha (2020b). Figure 1 left panel shows the 0.3–2 keV, 2–10 keV and the hardness ratio of the source for a total of ∼110 ks. We find that there is a gradual monotonic spectral hardening throughout the observation. However, the soft and the hard X-ray flux variations are erratic. We have divided the total exposure time into five segments, with segments 1, 2, 3, 4 and 5 having 17 ks, 20 ks, 20 ks, 20 ks and 30 ks respectively. The segment durations have been chosen such that we have enough signal to noise ratio to carry out a hardness ratio resolved spectroscopy. Figure 1 right panel shows the 2–10 keV spectra of the source for the five time segments mentioned in the left panel. The spectra are fitted with a simple powerlaw in the 3–5 keV energy band, with a common slope of Γ = 2.1 and extrapolated to other bands.
Figure 1. Left: The 0.3–2 keV, 2–10 keV and hardness ratio (HR) light curves in the top, middle and lowest panels respectively. The vertical dotted lines indicate the five different time segments used to study the HR resolved spectra of this source. Right: The 3–5 keV energy range is fitted with a power law with a slope Γ = 2.1 and the rest of the energy bands extrapolated, for the five time segments discussed in the left panel. The gradual spectral hardening has contributions from both the decrease of soft X-ray excess flux in the time segments 4 and 5, as well as increase in the 2–10 keV flux in the segments 2, 3 and 4.
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From Figure 1 left panel we find that the soft X-ray spectra varies moderately in the first four time segments, but in the fifth time segment drops to ∼11 counts s−1 compared to the first segment ∼15 counts s−1. However, for the case of the 2–10 keV hard X-ray spectra, the count rate in segment 1 and segment 5 are nearly similar, with some high states in between (with an increase of ∼0.6 counts s−1). The monotonic increase in spectral hardening can therefore be attributed mostly to the decrease in the soft X-ray excess flux, although there is some contributions of increase in the 2–10 keV flux in the segments 3 and 4. The right panel of Figure 1 shows the spectral hardening for the five different time segments, which has both contributions from a decreasing soft X-ray excess flux as well as increase in the 2–10 keV flux in segments 3 and 4. We conclude that: (A) The soft X-ray flux variations may be related to the changes in the primary hard X-ray. (B) The soft X-ray flux decreases by ∼36% in a timespan of ∼50 ks, indicating that its origin could be from a region as close as <0.5 light-day from the SMBH. A detailed hardness ratio resolved spectral analysis with state-of-the-art reflection models will be reported in a future paper.