Abstract
The AGN STORM 2 campaign is a large, multiwavelength reverberation mapping project designed to trace out the structure of Mrk 817 from the inner accretion disk to the broad emission line region and out to the dusty torus. As part of this campaign, Swift performed daily monitoring of Mrk 817 for approximately 15 months, obtaining observations in X-rays and six UV/optical filters. The X-ray monitoring shows that Mrk 817 was in a significantly fainter state than in previous observations, with only a brief flare where it reached prior flux levels. The X-ray spectrum is heavily obscured. The UV/optical light curves show significant variability throughout the campaign and are well correlated with one another, but uncorrelated with the X-rays. Combining the Swift UV/optical light curves with Hubble Space Telescope UV continuum light curves, we measure interband continuum lags, τ(λ), that increase with increasing wavelength roughly following τ(λ) ∝ λ 4/3, the dependence expected for a geometrically thin, optically thick, centrally illuminated disk. Modeling of the light curves reveals a period at the beginning of the campaign where the response of the continuum is suppressed compared to later in the light curve—the light curves are not simple shifted and scaled versions of each other. The interval of suppressed response corresponds to a period of high UV line and X-ray absorption, and reduced emission line variability amplitudes. We suggest that this indicates a significant contribution to the continuum from the broad-line region gas that sees an absorbed ionizing continuum.
| Original language | English |
|---|---|
| Article number | 195 |
| Journal | Astrophysical Journal |
| Volume | 958 |
| Issue number | 2 |
| DOIs | |
| State | Published - Dec 1 2023 |
Bibliographical note
Publisher Copyright:© 2023. The Author(s). Published by the American Astronomical Society.
Funding
The difference between the lag centroid and lag peak from the unadjusted light curves indicates an asymmetric transfer function—that there is significant response on longer timescales. This is supported by the decrease in the lag centroid once the light curves are detrended. An excess lag in the U band is seen in analysis of the unadjusted light curves in the lag centroids using the ICCF technique and in pyROA fits. This U-band excess is almost universally seen in other continuum reverberation mapping and has been attributed to diffuse continuum emission from the BLR (Korista & Goad , ; Lawther et al. ; Netzer , ). While it peaks in the U band at the Balmer jump, and also at the Paschen jump, the BLR diffuse continuum should affect all wave bands. Since the BLR emitting region is presumably more extended than the UV/optical part of the accretion disk, the timescale of the response from the BLR should be longer than from the accretion disk. It is interesting, that when long-timescale variations are removed the U-band excess disappears. This would again support the idea that significant continuum emission originates from the BLR. Of course, the Swift filters are broadband and so include emission lines too. Figure shows a broadband HST/STIS spectrum from our campaign compared to the Swift filter bandpasses. Prominent emission lines C iii ] λ1909, Mg ii λ2800, and H β fall within the UVW2, UVW1, and B filters, respectively. This will lengthen the lags in those bands, though simulations during other campaigns show this does not dominate (e.g., Fausnaugh et al. ). See a similar discussion for the case of Fairall 9 in R. Edelson et al. (2023, in preparation). On the other hand, the HST continuum light curves are calculated over line-free integration windows (Paper ). A full analysis of the lags using frequency-dependent methods (Cackett et al. ) and power spectral analysis is left to future work, as is a detailed modeling of the spectra to determine the strength of the expected BLR continuum lags following the methods of Korista & Goad () and Netzer (). Our project began with the successful Cycle 28 HST proposal 16196 (Peterson et al. 2020). E.M.C. gratefully acknowledges support from NASA through grant 80NSSC22K0089. Y.H. acknowledges support from the Hubble Space Telescope program GO-16196, provided by NASA through a grant from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. Research at UC Irvine was supported by NSF grant AST-1907290. J.G. gratefully acknowledges support from NASA through grant 80NSSC22K1492. C.S.K. is supported by NSF grant AST-2307385. M.C.B. gratefully acknowledges support from the NSF through grant AST-2009230. H.L. acknowledges a Daphne Jackson Fellowship sponsored by the Science and Technology Facilities Council (STFC), UK. D.I., A.B.K, and L.Č.P. acknowledge funding provided by the University of Belgrade—Faculty of Mathematics (contract 451-03-68/2022-14/200104), Astronomical Observatory Belgrade (contract 451-03-68/2022-14/200002), through grants by the Ministry of Education, Science, and Technological Development of the Republic of Serbia. D.I. acknowledges the support of the Alexander von Humboldt Foundation. A.B.K. and L.Č.P. thank the support by Chinese Academy of Sciences President’s International Fellowship Initiative (PIFI) for visiting scientists. A.V.F. is grateful for financial assistance from the Christopher R. Redlich Fund and numerous individual donors. M.V. gratefully acknowledges support from the Independent Research Fund Denmark via grant No. DFF 8021-00130. Y.R.L. acknowledges financial support from NSFC through grant Nos. 11922304 and 12273041 and from the Youth Innovation Promotion Association CAS. This work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester.
| Funders | Funder number |
|---|---|
| Science and Technology Facilities Council | |
| Christopher R. Redlich Fund | |
| University of Leicester | |
| Youth Innovation Promotion Association of the Chinese Academy of Sciences | |
| Chinese Academy of Sciences | |
| Alexander von Humboldt-Stiftung | |
| Danmarks Frie Forskningsfond | DFF 8021-00130 |
| National Aeronautics and Space Administration | GO-16196, 80NSSC22K0089 |
| Space Telescope Science Institute | NAS5-26555 |
| National Science Foundation Arctic Social Science Program | AST-1907290, AST-2009230, 1907290, AST-2307385, 80NSSC22K1492 |
| Astronomical Observatory Belgrade | 451-03-68/2022-14/200002 |
| National Natural Science Foundation of China (NSFC) | 12273041, 11922304 |
| UK Industrial Decarbonization Research and Innovation Centre | 53706 |
| ???publication-publication-funding-organisation-not-added??? | 200104 |
| University of Belgrade | 451-03-68/2022-14/200104 |
ASJC Scopus subject areas
- Astronomy and Astrophysics
- Space and Planetary Science