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.

doi:10.1093/mnras/stx168

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.

Authors

A. Bruce

A. Lawrence

C. MacLeod

M. Elvis

Professor Martin Ward martin.ward@durham.ac.uk
External Examiner (PGR)

J.S. Collinson

S. Gezari

P.J. Marshall

M.C. Lam

R. Kotak

C. Inserra

J. Polshaw

N. Kaiser

R-P. Kudritzki

E.A. Magnier

C. Waters

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

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This article has been accepted for publication in Monthly notices of the Royal Astronomical Society ©: 2017 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.