Spectral analysis of four ‘hypervariable’ AGN: a micro-needle in the haystack?

Bruce, A.; Lawrence, A.; MacLeod, C.; Elvis, M.; Ward, M.J.; Collinson, J.S.; Gezari, S.; Marshall, P.J.; Lam, M.C.; Kotak, R.; Inserra, C.; Polshaw, J.; Kaiser, N.; Kudritzki, R-P.; Magnier, E.A.; Waters, C.

Abstract

We analyse four extreme active galactic nuclei (AGN) transients to explore the possibility that they are caused by rare, high-amplitude microlensing events. These previously unknown type-I AGN are located in the redshift range 0.6–1.1 and show changes of >1.5 mag in the g band on a time-scale of ∼years. Multi-epoch optical spectroscopy, from the William Herschel Telescope, shows clear differential variability in the broad line fluxes with respect to the continuum changes and also evolution in the line profiles. In two cases, a simple point-source, point-lens microlensing model provides an excellent match to the long-term variability seen in these objects. For both models, the parameter constraints are consistent with the microlensing being due to an intervening stellar mass object but as yet there is no confirmation of the presence of an intervening galaxy. The models predict a peak amplification of 10.3/13.5 and an Einstein time-scale of 7.5/10.8 yr, respectively. In one case, the data also allow constraints on the size of the C III] emitting region, with some simplifying assumptions, to be ∼1.0–6.5 light-days and a lower limit on the size of the Mg II emitting region to be >9 light-days (half-light radii). This C III] radius is perhaps surprisingly small. In the remaining two objects, there is spectroscopic evidence for an intervening absorber but the extra structure seen in the light curves requires a more complex lensing scenario to adequately explain.

Citation

Bruce, A., Lawrence, A., MacLeod, C., Elvis, M., Ward, M., Collinson, J., …Waters, C. (2017). Spectral analysis of four ‘hypervariable’ AGN: a micro-needle in the haystack?. Monthly Notices of the Royal Astronomical Society, 467(2), 1259-1280. https://doi.org/10.1093/mnras/stx168

• Journal Article Type: Article
• Acceptance Date: Jan 18, 2017
• Online Publication Date: Jan 20, 2017
• Publication Date: May 21, 2017
• Deposit Date: Jul 25, 2017
• Publicly Available Date: Jul 25, 2017
• Journal: Monthly Notices of the Royal Astronomical Society
• Print ISSN: 0035-8711
• Electronic ISSN: 1365-2966
• Publisher: Royal Astronomical Society
• Peer Reviewed: Peer Reviewed
• Volume: 467
• Issue: 2
• Pages: 1259-1280
• DOI: https://doi.org/10.1093/mnras/stx168
• Public URL: https://durham-repository.worktribe.com/output/1372891