New Results in X-Ray Astronomy 2015
Testing Accretion Disc Theory in AGN: X-ray/Optical Inter-band Lags in NGC 4395
Sam Connolly
With Ian McHardy (University of Southampton) Brad Peterson, Curtis McCully (LCOGT), Allyson Bieryla, Martin Elvis, David Latham, Emilio Falco (McDonald), Makuto Uemura (Kanata), Hum Chand (Aries), Shai Kaspi, Hagai Netzer (Wise), Paulina Lira (Chile), Poshak Gandhi, Dimitrios Emmanoulopoulos (Soton) UV/Optical & X-Ray Variability
X-Ray Corona X-Rays
UV/Optical
Accretion fluctuations
Accretion Disc Black Hole UV/Optical & X-Ray Variability
X-Ray Corona X-Rays
UV/Optical
Accretion fluctuations
Accretion Disc Black Hole UV/Optical & X-Ray Variability
X-Ray Corona X-Rays
UV/Optical
Accretion fluctuations
Accretion Disc Black Hole Optical inter-band lags Given lag ~ wavelengthβ, Shakura-Sunyaev gives β=1.33
NGC 2617 NGC 4051
Shappee et al, 2014 Cackett et al, 2006; Sergeev et al 2005,6
• Dashed line goes through X-ray point, • Consistent with reprocessing, but β = 0.37 but no link to X-rays (β~1.33) • Solid line has β = 1.18, but is offset from X-ray point by 2.4d 5
X-Ray/Optical Time Lags
NGC 4051 MKN 79
Breedt et al. 2010 Breedt et al., 2009 Optical lags by 1.5+/- 0.5 d Strong correlation, lag uncertain (above 99% confidence) • Long timescales (years) uncorrelated - Intrinsic disc variations in optical? • Short timescales (days-weeks) well correlated - Usually a hint of optical lagging by ~day, but large uncertainty 6 NGC 4395: Short timescale CCFs Cameron et al. 2012
Lag of X-rays by B-band (days) Lag of X-rays by UVW2 (seconds)
• Very small B-band lag < 45 min • Looking within individual Swift visits • Suggesting reprocessing, but not (TOO – 12ksec) confirming • Hint that uvw2 lags X-rays by ~400s, but large uncertainty 7 Swift Monitoring of NGC 5548:
(> 500 observations)
First campaign: McHardy et al. 2014 Second campaign: Edelson et al. 2015 8 NGC 5548 Lags as function of wavelength
Lag ∝Wavelength1.23
• Good agreement with Shakura-Sunyaev (β~1.33), incl. X-rays • Observed lags are longer than expected for the Mass and accretion rate • Red line is time for 50% of light to arrive in a Shakura-Sunyaev model
(McHardy et al, 2014) 9 NGC 5548 Lags as function of wavelength
• Microlensing (e.g. Morgan et al. 2010) also require ‘larger’ disc than Shakura-Sunyaev model • Possible explanations: - Hotter than expected disc? - Inhomogeneous disc (Dexter and Agol 2011)? - Reprocessing time?
(McHardy et al, 2014) 10
NGC 4395: XMM and ground-based monitoring
• 100x lower mass than NGC5548
• OM used in very fast (sub-second) readout mode using UVW1
• First successful use of this mode( as far as we know) for AGN lag measurement
• Ground based g-band monitoring around globe.
Connolly et al. in prep.
11 NGC 4395: XMM and ground-based monitoring
100s bins 28-29 December 2014 12 NGC 4395: XMM and ground-based monitoring
100s bins 30-31 December 2014 13 NGC 4395 - DCFs
X-rays vs UVW1 X-rays vs G-band
• Clear, small lag of X-rays by both UVW2 and G-band, • using Emmanouloupolos et al. 2013 improved light curve simulation method to determine confidence curves 14 NGC 4395 – Javelin lags
Javelin Lag measurement software - Zu et al. 2011,13
UVW1 lags X-rays by g-band lags X-rays by 473 + 47 - 98 s 788 +44 -54 s Connolly et al. in prep. 15 NGC4395 – Lags vs Wavelength
Simple linear fit (red) is best fit (forced through zero). However, power law of index 4/3 (blue) is also a good fit Connolly et al. in prep. 16 NGC 4395 - Modelling
Observations
Model emission profiles
Solid lines – total disc energy release in band, including X-ray contribution Dashed lines – gravitational energy release
Observed lags correspond to peak emission radii (models from P. Lira) 17 Why don’t UV/optical disc variations drive X-ray variations? • Solid angle:
- Optical/UV variations from larger radii are seen by distant observer but few are seen by central X-ray source - Approximately half of the X-ray photons should hit accretion the disc
• Photon Conservation:
- Compton scattering within X-ray emitting corona conserves photons. - An X-ray photon heating the disc could lead to emission of many more optical/uv photons, dominating variations in intrinsic thermally produced photons. 18
CONCLUSIONS • Long timescale (years) UV-optical variability in AGN is likely intrinsic disc variation driven by inwardly propagating accretion rate fluctuations.
• Short timescale UV-optical variability in AGN is X-ray reprocessing
• Solid angle/photon conservations considerations could explain lack of X-ray lags on short timescales.
• For NGC5548 observed lags are longer than expected for a standard Shakura-Sunyaev disc; for NGC4395 Shakura-Sunyaev disc may be acceptable.
• FUTURE: Apply improved modelling, based on that in e.g. McHardy at al. 2015, to NGC 4395, to fit the UV directly using a Shakura-Sunyaev response function, and test the effects of disc flaring.
• To appear in Connolly et al. 2015 (in prep.)
19 Extra Slides Optical and X-ray PSDs: Consistent with reprocessing
X-ray
NGC3783, Arevalo et al 2009
Errorbars – Optical data Dashed line – X-rays Optical – less high frequency power: smoothed by reprocessing from large area.
Arevalo et al. 2009, Breedt et al. 2009, Breedt 2010 thesis, see also Kelly at al. 2010 21 Time Lags from Response Functions
τpeak τ50%
Flux
50%
Time