X-ray eclipse tomography: Resolving the extent of X-ray-emitting region(s) in the Seyfert galaxy ESO362-G18

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X-ray spectral variability in active galactic nuclei (AGNs) can be either intrinsic or caused by external processes, such as variable absorption. Several transient occultation events from individual clouds or X-ray eclipses have been identified in a number of AGNs in the past two decades. We report results from the analysis of two Swift monitoring campaigns of the Seyfert galaxy ESO 362-G18, a changing-look AGN that showed transitions between optical spectral types 1.5 and 1.9 in the past, most likely induced by variable absorption. We identified two X-ray eclipses during the first Swift campaign, one of which we were able to follow almost entirely from ingress to egress. We used time-resolved spectroscopy to follow the evolution of the X-ray spectrum with time in order to derive the properties of the absorbing cloud and the size of the main X-ray-emitting region. The cloud is located towards the innermost dust sublimation zone, intermediate between the dust-free broad-line region and the dusty torus. The X-ray-emitting region is confined within ∼ 35 \sim 35 gravitational radii ( R g R_{g} ) from the centre. A deeper analysis indicated that the eclipsing cloud likely comprises a denser core and a more tenuous atmosphere. We were also able to estimate the size of the soft excess and hot corona X-ray-emitting regions separately. These are two of the most relevant continuum components in the X-ray spectra of unobscured AGNs. Our analysis suggests that the soft-excess-emitting region is ∼ 50 \sim 50 % more extended than the hot corona one. However, it appears to co-exist with and is not replaced by the hot corona one in the innermost ∼ 30 ​ R g \sim 30\penalty\ R_{\rm g} . Our results highlight the potential of X-ray eclipse tomography for providing a clearer view of the innermost accretion flow geometry and of the AGN surroundings out to the torus scale.

S6.SS1: 6.1 A structured cloud: Dense core and tenuous atmosphere / block 1 · html

Our results suggest that the soft-excess-emitting region is about 50 50 % more extended than the hot corona, although with relatively large uncertainties. In Table 4 , the asymmetric uncertainties on r atm r_{\rm atm} and r SE r_{\rm SE} reflect the conditions r atm > r core r_{\rm atm}>r_{\rm core} (by definition of a structured cloud) and r SE > r HC r_{\rm SE}>r_{\rm HC} (to prevent C f , max ( SE ) > C f , max ( HC ) C_{\rm f,max}^{\rm(SE)}>C_{\rm f,max}^{\rm(HC)} at maximum coverage by the cloud core, which is not observed).

abstract · abstract

X-ray spectral variability in active galactic nuclei (AGNs) can be either intrinsic or caused by external processes, such as variable absorption. Several transient occultation events from individual clouds or X-ray eclipses have been identified in a number of AGNs in the past two decades. We report results from the analysis of two Swift monitoring campaigns of the Seyfert galaxy ESO 362-G18, a changing-look AGN that showed transitions between optical spectral types 1.5 and 1.9 in the past, most likely induced by variable absorption. We identified two X-ray eclipses during the first Swift campaign, one of which we were able to follow almost entirely from ingress to egress. We used time-resolved spectroscopy to follow the evolution of the X-ray spectrum with time in order to derive the properties of the absorbing cloud and the size of the main X-ray-emitting region. The cloud is located towards the innermost dust sublimation zone, intermediate between the dust-free broad-line region and the dusty torus. The X-ray-emitting region is confined within 35 gravitational radii (Rg) from the centre. A deeper analysis indicated that the eclipsing cloud likely comprises a denser core and a more tenuous atmosphere. We were also able to estimate the size of the soft excess and hot corona X-ray-emitting regions separately. These are two of the most relevant continuum components in the X-ray spectra of unobscured AGNs. Our analysis suggests that the soft-excess-emitting region is 50% more extended than the hot corona one. However, it appears to co-exist with and is not replaced by the hot corona one in the innermost 30 Rg. Our results highlight the potential of X-ray eclipse tomography for providing a clearer view of the innermost accretion flow geometry and of the AGN surroundings out to the torus scale.

S4.SS1: 4.1 Analysis of the low and high H/S spectra / block 1 · html

A detailed description of the spectral analysis is given in Appendix A (see, in particular, Appendix A.2 ). We briefly discuss the resulting best-fitting model. We adopted a standard Seyfert 1 X-ray spectral model comprising a soft X-ray excess described by the comptt model in xspec ( Titarchuk, 1994 ) , a hard Comptonisation component ( Zdziarski et al., 1996 ; Życki et al., 1999 , nthcomp :) , and a Gaussian emission line representing Fe K α \alpha emission with fixed rest-frame energy at 6.4 6.4 keV and width free to vary. The overall model was absorbed by the Galactic column density fixed at 1.35 × 10 20 1.35\times 10^{20} cm -2 ( HI4PI Collaboration et al., 2016 ) using the tbabs model with cross sections and abundances from Wilms et al. (2000) .

S4.SS1: 4.1 Analysis of the low and high H/S spectra / block 1 · html

This simple model, in which the only parameters that are allowed to vary independently are the intrinsic X-ray flux and the covering fraction C f C_{\rm f} of the ionised absorber, resulted in an excellent description of the data with C = 1206 C=1206 for 1318 1318 degrees of freedom. The best-fitting models are shown as solid lines in the upper panel of Fig. 4 and the corresponding residuals, normalised by the uncertainties, are shown in the lower panel. The soft excess can be described by a warm optically thick corona with (common) temperature k ​ T e = 110 ± 10 kT_{\rm e}=110\pm 10 eV and poorly constrained optical depth τ ≳ 25 \tau\gtrsim 25 . The (common) slope of the hard Comptonisation component is Γ = 1.75 ± 0.06 \Gamma=1.75\pm 0.06 . A broad ( σ ≃ 300 \sigma\simeq 300 - 700 700 eV) Fe K line at 6.4 6.4 keV is detected and forcing the line to be unresolved resulted in a worse fit by Δ ​ C = + 11 \Delta C=+11 for 1 degrees of freedom. The quality of the short-exposure XRT spectra does not allow us to study the Fe line profile in detail nor to confidently claim the detection of a relativistically broadened Fe line. However, the line is resolved which is consistent with results by Agís-González et al. (2014) who have analysed much higher quality data of ESO 362-G18, suggesting that a relativistic reflection component is present.

S4.SS1: 4.1 Analysis of the low and high H/S spectra / block 1 · html

Figure 4: Top : Unfolded low (blue) and high (red) H/S X-ray spectra accumulated during the time-intervals highlighted in Fig. 3 shown together with their respective best-fitting models. Bottom : Resulting residuals normalised by the uncertainties. The data were re-binned for visual clarity.

S4.SS2: 4.2 Time-resolved spectroscopy / block 1 · html

The smooth shape of the H/S light curve in Fig. 3 , coupled with results from the low and high H/S spectral analysis, suggests that the variable H/S during the campaign is due occultation events (or eclipses) progressively covering and uncovering the X-ray-emitting region with maximum coverage around ≃ 12 \simeq 12 d, and possibly also ≃ 56 \simeq 56 d. We then considered a time-resolved X-ray spectral analysis considering the 12 X-ray spectra corresponding to the data points shown in Fig. 3 which are constructed by stacking three consecutive of the original Swift /XRT observations to increase the signal-to-noise ratio.

S5: 5 Cloud properties and size of the X-ray-emitting region / block 1 · html

The results from the X-ray time-resolved spectral analysis in Sect. 4.2 can be used to derive estimates of the absorber and X-ray-emitting regions properties. We made a series of standard simplifying assumptions following earlier work, for instance by Lamer et al. (2003) ; Risaliti et al. (2005) ; Risaliti et al. (2007) ; Sanfrutos et al. (2013) ; Markowitz et al. (2014) ; Lian et al. (2025) , and we derive estimates of the eclipsing cloud size, density, transverse velocity, and location as well as of the X-ray-emitting region size. We focus here on the first eclipse only due to its much better sampling.

S5: 5 Cloud properties and size of the X-ray-emitting region / block 1 · html

In summary, fits to the C f C_{\rm f} evolution were performed with a flat-top Gaussian function with t plat = [ 2 , 12 ] t_{\rm plat}=[2,12] d in steps of 1 1 d. We defined the total eclipse duration t tot t_{\rm tot} as the Gaussian full width at tenth of maximum, and we obtained C f , max C_{\rm f,max} , t tot t_{\rm tot} , and t ingr = 1 / 2 ​ ( t tot − t plat ) t_{\rm ingr}=1/2\left(t_{\rm tot}-t_{\rm plat}\right) for each t plat t_{\rm plat} . One realisation of such fits is shown in Fig. 6 with t plat = 6 t_{\rm plat}=6 d. Table 1 reports the best-fitting values to the C f C_{\rm f} evolution for t plat = ( 2 , 8 , 12 ) t_{\rm plat}=(2,8,12) d. All parameters for different t plat t_{\rm plat} are within the range of values defined by the extremes in t plat t_{\rm plat} .

S5: 5 Cloud properties and size of the X-ray-emitting region / block 1 · html

Table 1: Best-fitting results for the C f C_{\rm f} evolution using a flat-top Gaussian function. 3 3 3 Notes. We only show results for t plat = ( 2 , 8 , 12 ) t_{\rm plat}=(2,8,12) d.

S8: VIII Summary / block 1 · html

For sources with available multi-epoch spectroscopy, we performed a variability analysis for our sample. We found that 11 out of 13 sources ( 85 − 15 + 5 % 85^{+5}_{-15}\% ) exhibit variability either in flux (10/13, 77 − 15 + 8 % 77^{+8}_{-15}\% ) or N H N_{\rm H} (4/12, 33 − 10 + 15 % 33^{+15}_{-10}\% ). These results are in excellent agreement with recent findings on lower-luminosity ( log ⁡ L X / erg ​ s − 1 ∼ 43 \log L_{\rm X}/\mathrm{erg\,s^{-1}}\sim 43 ), Compton-thin AGNs.

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For 20/21 sources with adequate optical spectroscopy, we measured the [ Ne v ] λ ​ 3426 \lambda 3426 emission line and found a detection rate of 85 − 11 + 5 % 85^{+5}_{-11}\% (17/20). This confirms that the [ Ne v ] λ ​ 3426 \lambda 3426 emission line is a strong tracer of AGN activity even in cases of heavy obscuration, in agreement with previous studies;

S3: 3 Results / block 1 · html

Considering the three epochs of the Chandra monitoring program individually, J1641 was not detected in any of them in the soft band, while it was detected in the hard and full band in the 2022 epoch only (Tab. 1 ) with significance ( 1 − P B ) = 0.9996 (1-P_{B})=0.9996 and 0.9975 0.9975 , respectively. The significance of the hard-band and full-band detections increases to 0.9998 0.9998 and 0.9999 0.9999 , respectively, when stacking the three epochs, while no significant counterpart is detected in the soft band. We estimate the spectral properties of J1641 from this stacked dataset by considering the hardness ratio H ​ R = H − S H + S HR=\frac{H-S}{H+S} , where H and S are the net counts in the hard and soft bands, respectively. Following the method used in 54 , we obtain H ​ R > − 0.04 HR>-0.04 , that, assuming simple power-law emission, accounting for the Galactic absorption ( Kalberla et al., 2005 ) , and using the merged Chandra response files at the position of J1641, corresponds to an effective photon index Γ < 0.4 \Gamma<0.4 . 4 4 4 We used XSPEC v.12.13 ( Arnaud, 1996 ) for these calculations. This value is much flatter than the photon index obtained from spectral analysis in 2018 (i.e., Γ = 2.4 ± 0.5 \Gamma=2.4\pm 0.5 ), and suggests that the soft-band emission became heavily absorbed.

S3: 3 Results / block 1 · html

To estimate the obscuration level, we fixed Γ = 2 \Gamma=2 and added to our assumed spectral model an absorption component (i.e., model pha*zpha*pow in XSPEC). The lower limit on the hardness ratio translates into a column density log N H cm − 2 > 24.2 \frac{N_{H}}{\mathrm{cm^{-2}}}>24.2 . Similar values are obtained by assuming the photon index measured in 2018 or the more physically motivated MYTorus model ( Murphy & Yaqoob, 2009 ) . Tab. 1 reports the flux of J1641 estimated assuming this column density. The corresponding absorption-corrected, rest-frame 2–10 keV luminosity is L X = 7.7 − 4.2 + 4.2 × 10 44 ​ erg ​ s − 1 L_{X}=7.7^{+4.2}_{-4.2}\times 10^{44}\,\mathrm{erg\,s^{-1}} . This value is lower than the luminosity measured for the 2018 dataset (i.e., 33.4 − 5.1 + 5.6 × 10 44 ​ erg ​ s − 1 33.4^{+5.6}_{-5.1}\times 10^{44}\,\mathrm{erg\,s^{-1}} ), but it should be treated as a lower limit, as we assumed the lower limit on N H N_{H} estimated above for the absorption correction.

abstract · abstract

V902 Mon is one of a few eclipsing Intermediate Polars (IPs), and show deep eclipses in the optical lightcurves. The presence of a strong Fe K$α$ fluorescence line in its X-ray spectrum and its low X-ray flux compared to other IPs suggests significant absorption, most likely from an accretion disk. In an observation carried out using the Nuclear Spectroscopic Telescope Array (NuSTAR), we confirm the presence of an X-ray eclipse in the energy resolved lightcurves, coincident with the optical AAVSO/CV-band lightcurves. Broadband X-ray spectral analysis using NuSTAR and XMM-Newton observations confirm a strong absorption N$_{H}$ $\sim 10^{23}$ cm$^{-2}$ local to the source, along with a high equivalent width of about 0.7 keV for a Fe K$α$ fluorescence line. We interpret this using a model similar to an Accretion Disk Corona source, which have a very high inclination and the compact object is heavily obscured by the body of the accretion disk. We propose that the primary X-rays from the accretion column in V902 Mon is hidden from our direct view at all times by the accretion disk. In this scenario, the observed scattered X-rays indicate substantial absorption of direct X-rays by the accretion disk. Additionally, a strong Fe fluorescence line suggests reprocessing of the radiation by a more extended region, such as the pre-shock region, which could be located a few white dwarf radii above the orbital plane.

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Extreme sudden changes in the flow of accreting gas onto SMBHs manifest themselves via large-amplitude continuum variability and changes to broad Balmer emission profiles, driving changing-look AGN. X-ray flux monitoring with SRG/eROSITA revealed that in the Seyfert AGN HE 1237-2252 the soft X-ray flux dipped abruptly, by a factor of 17 within 18 months. We initiated a follow-up campaign that caught the luminosity recovery after the dip, and enabled us to study how the various accretion components responded during this flux recovery. Our campaign included multiband photometry, X-ray spectroscopy, and optical spectroscopy. We tracked as the accretion rate relative to Eddington increased by a factor of 7 in 3 years. Based on broad Hbeta variability, HE 1237-2252 was subtype 1.0-1.2 in 2002, transitioned to subtype 1.8 by the time of the luminosity dip, and then transitioned back to subtype 1.0 within 3 months as luminosity recovered. Both transitions saw broad Hbeta integrated line flux change by factors of 4-6. The broad Balmer profile is decomposed into a broad Gaussian consistent with virialized gas at 27+/-3 lt-dy, plus a double-peaked profile, consistent with a diskline structure at more than roughly 5 lt-dy. The diskline component's relative contribution to the total profile increases as continuum flux rises. The lack of obscuration in the X-ray spectra, as well as the IR continuum dip, point to an intrinsic pause in the accretion rate as opposed to variable line-of-sight obscuration. Candidates for the underlying mechanisms include propagating cold and warm fronts in the accretion disk. The increased prominence of the diskline BLR component's emission could be due to evolution in the physical extent of the X-ray corona, and in the fraction of >13.6 eV photons intercepted by the diskline, as the accretion rate increases.

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Active galactic nuclei (AGN) showing dramatic spectral and flux variations, either due to changes in the accretion rate (changing-state, CS-AGN) of the supermassive black hole or in the line-of-sight column density (changing-obscuration, CO-AGN), have been classified as changing-look (CL) AGN. Here we present a peculiar source, 2MASX J0413-0050, first identified as a narrow-line Seyfert 1 (NLS1s) galaxy in 2004. When re-observed twice in 2021, it showed a transition in the spectral type (towards a Seyfert 1.9) and the complete and mysterious disappearance of the Hbeta line while source was in a high accretion state. In the meantime, the X-ray flux decreased between observations taken in 2020 and 2022, and again in the most recent spectrum of 2023. Shortly after this, another optical spectrum revealed the re-emergence of both the narrow and broad Hbeta components (Seyfert 1.8). Despite the fact that it was not possible to retrieve the line-of-sight column density from the X-ray spectra, which would have helped in assessing whether this event could be attributed to a CO AGN scenario, the observational evidence does not necessarily support such an interpretation. J0413-0050 may have undergone several switch-on switch-off phases over the past 20 years, on an unknown timescale, which could have affected the accretion power and, consequently, the optical continuum and so the emission lines coming from the broad-line region (BLR). For these reasons, it is reasonable to classify this source as a CS-AGN. The case of J0413-0050 supports the hypothesis that NLS1s can indeed experience CL phenomena.

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X-ray surveys of active galactic nuclei (AGNs) provide direct constraints on the properties of individual AGNs, such as their emission, obscuration, and accretion rate. Previous AGN population synthesis models have not addressed such properties self-consistently. Here, we use a simulation-based inference (SBI) approach to constrain the geometrical and physical properties of the AGN population. We perform numerical simulations with our ray-tracing code, RefleX, which allows the self-consistent modelling of the X-ray emission of AGNs with flexible circumnuclear and source geometries. We create our synthetic population by sampling the intrinsic active black hole mass function (BHMF) and Eddington ratio distribution function (ERDF) of local AGNs, and we construct a geometry based on the radiation-regulated model, along with Eddington-ratio-dependent emission spectra. Using the RefleX-simulated emission of the AGN population, we aim to simultaneously reproduce the cosmic X-ray background (CXB), differential AGN number counts, and several observed absorption properties of local AGNs, such as the fraction of $N_\mathrm{H}$ in bins of log($N_\mathrm{H}$), the Compton-thick fraction as a function of limiting flux, and the number of obscured and unobscured AGNs as a function of Eddington ratio. With this approach, we test the consistency of the radiation-regulated model with a very comprehensive set of X-ray observables, while constraining the size and density of the dusty torus and the evolution of the local AGN population. We derive an intrinsic Compton-thick fraction of $40\pm3$%, and find that a simple evolutionary prescription controlling the active fraction of supermassive black holes is sufficient for our synthetic population to reproduce the CXB.

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To investigate the physical origin of the Balmer decrement in the broad-line region of active galactic nuclei (AGNs), we measure the temporal variability of the fluxes of the broad H$β$ and H$α$ emission lines using multi-epoch spectroscopic data of low-redshift AGNs from the Sloan Digital Sky Survey. The analysis of the mean spectra reveals that the Balmer decrement shows no correlation with AGN luminosity, while it is inversely correlated with the Eddington ratio. However, the temporal variation of the Balmer decrement in individual objects exhibits an even stronger anti-correlation with AGN luminosity, suggesting that the change in AGN luminosity plays a dominant role in determining the Balmer decrement. By comparing the temporal evolution of the Balmer decrement with the continuum color, we find that reddening due to the AGN itself may not be the primary factor. Instead, radiative transfer effects and excitation mechanisms, which deviate from the Case B recombination, appear to be critical for the variation of the Balmer decrement. These results provide useful insights into the underlying physics of changing-look AGNs and high-$z$ AGNs, such as the ``little red dots'', which exhibit extreme values of the Balmer decrement that can be misinterpreted as evidence for dust.

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The multi-cycle JWST Treasury program NEXUS will obtain cadenced imaging and spectroscopic observations around the North Ecliptic Pole during 2024-2028. Here we report a systematic search for nuclear variability among $\sim 25\,$k sources covered by NIRCam (F200W+F444W) imaging using the first two NEXUS epochs separated by 9 months in the observed frame. Difference imaging techniques reach $1σ$ variability sensitivity of 0.18~mag (F200W) and 0.15~mag (F444W) at 28th magnitude (within 0".2 diameter aperture), improved to $0.01$~mag and $0.02$~mag at $<25$th magnitude, demonstrating the superb performance of NIRCam photometry. The difference imaging results represent significant improvement over aperture photometry on individual epochs (by $>30\%$). We identify 465 high-confidence variable sources among the parent sample, with 2-epoch flux difference at $>3σ$ from the fiducial variability sensitivity. Essentially all these variable sources are of extragalactic origin based on preliminary photometric classifications, and follow a similar photometric redshift distribution as the parent sample up to $z_{\rm phot}>10$. While the majority of these variability candidates are likely normal unobscured AGNs, some of them may be rare nuclear stellar transients and tidal disruption events that await confirmation with spectroscopy and continued photometric monitoring. We also constrain the photometric variability of ten spectroscopically confirmed broad-line Little Red Dots (LRDs) at $3\lesssim z \lesssim 7$, and find none of them show detectable variability in either band. We derive stringent $3σ$ upper limits on the F444W variability of $\sim 3-10\%$ for these LRDs, with a median value of $\sim 5\%$. These constraints imply weak variability in the rest-frame optical continuum of LRDs.