Direct evidence of non-disk optical continuum emission around an active black hole

Chelouche, Doron; Núñez, F. Pozo; Kaspi, S.

doi:10.1038/s41550-018-0659-x

Cited by 90 publications

(88 citation statements)

References 49 publications

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“…5). Similar results are also seen in the sparser data of Chelouche, Nuñez & Kaspi (2018), where they derive a spectrum of the slowly reverberating component in the optical and identify this with the BLR. Lawther et al (2018) show predictions for the diffuse BLR flux for clouds of different densities.…”

Section: The Disconnect Between the Hard X-rays And The Uv Continuumsupporting

confidence: 77%

“…Instead, the X-ray spectra are consistent with the inner disc being replaced by a hot flow, and the UV spectra and variability are consistent with being produced by reprocessing in dense gas in the broad line region as first suggested by Korista & Goad (2001). This is now strongly supported by more recent work (Chelouche, Nuñez & Kaspi 2018;Lawther et al 2018). It remains to be seen whether similar campaigns on objects at higher L/L Edd will give results which connect better to expectations of standard disc models or whether continuum reverberation from inner BLR scales always dominates the UV variability.…”

Section: Introductionsupporting

confidence: 74%

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OUP accepted manuscript

2019

MNRAS

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Recent monitoring campaigns designed to map the accretion regime in AGN show major discrepancies with models where the optical/ultraviolet (UV) is produced by X-ray-illuminated, optically thick disc material within a few hundred gravitational radii. However, these campaigns only monitored X-rays below 10 keV, whereas the bolometric luminosity for most of these AGN peaks above 50 keV. We use data from the recent multiwavelength campaign by Edelson et al. (2017) on NGC 4151-the only AGN bright enough to be monitored at higher energies with Swift BAT. We develop a spectral-timing model with a hot corona, warm Comptonisation, and outer standard disc. This fits the time-averaged spectrum well, but completely fails to match the UV variability predicted from the X-ray lightcurve. However, it reveals that NGC 4151 had a bolometric luminosity around 1.4% of the Eddington luminosity during this campaign, close to the luminosity at which AGN show a 'changing-state' transition, where the broad optical lines disappear. Stellar mass black holes show a state transition at a similarly low Eddington fraction, which is broadly interpreted as the inner disc being replaced by an optically thin flow. We find that the UV lightcurve can instead be matched by reprocessing of the X-ray flux on size scales of the broad line region (BLR; 1.5 − 20 light-days) and rule out there being optically thick material inwards of this, as expected if the thin disc is replaced by the flow below the inner radius of the BLR. These results emphasise the need for even longer-timescale, multiwavelength monitoring campaigns on variable AGN.

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Section: The Disconnect Between the Hard X-rays And The Uv Continuumsupporting

confidence: 77%

Section: Introductionsupporting

confidence: 74%

OUP accepted manuscript

2019

MNRAS

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“…State-of-the-art reverberation mapping experiments are now revealing significant wavelength-dependent continuum timedelays within the X-ray-UV-optical-IR continuum bands in several nearby AGN (e.g., Edelson et al 2015;Fausnaugh et al 2016;Cackett et al 2018, Edelson et al 2019. Whether the measured continuum inter-band delay signature is associated with reprocessing of X-ray photons in the disk remains unclear (Chelouche et al 2019). The factor few larger than expected delays remains problematic and may point to alternate disk geometries (Dexter & Agol 2011;Hall et al 2018).…”

Section: Discussionmentioning

confidence: 99%

Quantifying the impact of variable BLR diffuse continuum contributions on measured continuum interband delays

Korista

Goad

2019

Monthly Notices of the Royal Astronomical Society

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We investigate the contribution of reprocessed continuum emission (1000Å -10,000Å) originating in broad line region (BLR) gas, the diffuse continuum (DC), to the wavelength-dependent continuum delays measured in AGN disk reverberation mapping experiments. Assuming a spherical BLR geometry, we adopt a Local Optimallyemitting Cloud (LOC) model for the BLR that approximately reproduces the broad emission-line strengths of the strongest UV lines (Lyα and Civ) in NGC 5548. Within this LOC framework, we explore how assumptions about the gas hydrogen density and column density distributions influence flux and delay spectra of the DC. We find that: (i) models which match well measured emission line luminosities and time delays also produce a significant DC component, (ii) increased n H and/or N H , particularly at smaller BLR radii, result in larger DC luminosities and reduced DC delays, (iii) in a given continuum band the relative importance of the DC component to the measured inter-band delays is proportional (though not 1:1) to its fractional contribution to the total light in that band, (iv) the measured DC delays and DC variability amplitude depends also on the variability amplitude and characteristic variability timescale of the driving continuum, (v) the DC radial surface emissivity distributions F(r) approximate power-laws in radius with indices close to −2 (≈1:1 response to variations in the driving continuum flux), thus their physics is relatively simple and less sensitive to the unknown geometry and uncertainties in radiative transfer. Finally, we provide a simple recipe for estimating the DC contribution in disk reverberation mapping experiments.

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“…This discrepancy may indicate an inhomogeneous disc (Dexter & Agol 2011). The Swift observations also provide evidence of reprocessing of high energy emission from a larger reprocessor than just the accretion disc, probably the broad line region (BLR) clouds (Korista and Goad 2019;Chelouche et al 2019;Lawther et al 2018;McHardy et al 2018;Cackett et al 2018;Sun et al 2018;Pal and Naik 2018). This evidence is in the form of an excess lag in the U-band (Edelson et al 2017;Fausnaugh et al 2016;Edelson et al 2015), which contains the Balmer continuum, (Kotov et al 2001) and an excess lag at 3634Å (known as the Balmer jump), and also in the fact that the reprocessing function required to explain the optical emission as reprocessing of X-ray emission, has a tail to long delays (a few days) as well as a sharp peak at short timescales (∼hours) from the disc.…”

Section: Introductionmentioning

confidence: 99%

Evidence for variability time-scale-dependent UV/X-ray delay in Seyfert 1 AGN NGC 7469

Pahari

McHardy

Vincentelli

et al. 2020

Monthly Notices of the Royal Astronomical Society

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Using a month-long X-ray light curve from RXTE/PCA and 1.5 month-long UV continuum light curves from IUE spectra in 1220–1970 Å, we performed a detailed time-lag study of the Seyfert 1 galaxy NGC 7469. Our cross-correlation analysis confirms previous results showing that the X-rays are delayed relative to the UV continuum at 1315 Å by 3.49 ± 0.22 d, which is possibly caused by either propagating fluctuation or variable Comptonization. However, if variations slower than 5 d are removed from the X-ray light curve, the UV variations then lag behind the X-ray variations by 0.37 ± 0.14 d, consistent with reprocessing of the X-rays by a surrounding accretion disc. A very similar reverberation delay is observed between Swift/XRT X-ray and Swift/UVOT UVW2, U light curves. Continuum light curves extracted from the Swift/GRISM spectra show delays with respect to X-rays consistent with reverberation. Separating the UV continuum variations faster and slower than 5 d, the slow variations at 1825 Å lag those at 1315 Å by 0.29 ± 0.06 d, while the fast variations are coincident (0.04 ± 0.12 d). The UV/optical continuum reverberation lag from IUE, Swift, and other optical telescopes at different wavelengths are consistent with the relationship: τ ∝ λ4/3, predicted for the standard accretion disc theory while the best-fitting X-ray delay from RXTE and Swift/XRT shows a negative X-ray offset of ∼0.38 d from the standard disc delay prediction.

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Direct evidence of non-disk optical continuum emission around an active black hole

Cited by 90 publications

References 49 publications

OUP accepted manuscript

OUP accepted manuscript

Quantifying the impact of variable BLR diffuse continuum contributions on measured continuum interband delays

Evidence for variability time-scale-dependent UV/X-ray delay in Seyfert 1 AGN NGC 7469

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