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The Origin of UV / Optical Variability of AGN: Relationship to X-Ray Variability

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The Origin of UV / Optical Variability of AGN: Relationship to X-Ray Variability The origin of UV / Optical Variability of AGN: Relationship to X-ray Variability Ian McHardy University of Southampton 1 Southampton Main Questions • What drives UV/optical variability in AGN? • How is the X-ray band related to UV/optical? • What do X-ray/UV/optical variations tell us about AGN inner structure? 2 Southampton Possible drivers of UV/optical Variability - Reprocessing of higher energy photons - which “high” energy? X-ray? Far-UV? - reprocessing off what? Disc? BLR? - Intrinsic disc variations 3 Southampton Observational Diagnostics • Reprocessing - High energies lead uv/optical by short (hour-days) light travel time. Allows ‘reverberation’ mapping of reprocessing structures. Measure lag from different temperature regions (different λ ). • Intrinsic disc variability – High energies lag: two possibilities – Long lag (months), viscous propagation timescale for perturbations to reach X-ray region from optical in disc – Short lag (hour-day), light travel time of UV seed photons to corona 4 REPROCESSING Southampton Wavelength dependence of lags For standard Shakura-Sunyaev DISC, 4 −1 −3 L(R)=σT ∝MBH . m! E R dissipating gravitational potential energy ! ( R in gravitational radii) −1/4 1/4 −3/4 i.e. T ∝ M m! R BH E Disc illumination from point source, height H above disc, also falls off as H R-3 So for reprocessing 4/3 Lag ∝Wavelength (eg Cackett et al 2007) from disc, we expect and Lag ∝M2/3m! 1/3 ! E For illumination of a shell-type structure, eg the BLR or torus, Illumination falls of as R-2 giving Lag ∝Wavelength2 5 Southampton Optical interband lags NGC4051 Consistent with reprocessing from a disc but no link to high energies (Cackett et al, 2007; Sergeev et al 2005,6) 6 Southampton The X-ray / Optical lag Many RXTE + ground based optical programmes; eg Breedt et al 2010 7 Southampton NGC 4051 8 Southampton NGC4051 99% confidence 95% confidence Optical lags by 1.5 +/- 0.5 d Breedt et al 2010 (Possible secondary longer (40d) lag – torus?) 9 Southampton MKN 79 (Breedt et al, 2009, MNRAS) CCFs often asymmetric -more than one variability component Long timescales (years) – poorly correlated behaviour. Intrinsic disc variations in optical? Short timescales (days-weeks) - well correlated. Usually a hint of optical lagging by ~day, but large uncertainty 10 Southampton Problems with reprocessing from a disc MR2251-178 Observed B-band lc (black dots) is smoother than model lc (purple) Arevalo et al 2008 Kasanas+Nayakshin 2001 Arevalo et al 2008, 2009 Gardner+Done 2016 Need illuminating source scale height ~100 Rg for adequate DISC illumination – much larger than measured for X-ray corona (eg Emmanoulopoulos et al 2014; Cackett et al 2014) 11 Southampton Caveats interpreting CCFs Arevallo et al 2008 X-ray DCFs B band – from propagating disc fluctuations B band– including reprocessing DCF X-ray vs B from propagating disc fluctuations (B leads) DCF X-ray vs B including reprocessing (zero peaked) From Arevalo et al 2008: a small contribution to optical light from reprocessing of X-rays pulls the peak of the DCF close to zero lag, but DCF is asymmetric due to 2nd component See also Gardner and Done 2016 12 Southampton Better Short Timescale Sampling: NGC4051 XMM and RXTE X-rays vs. XMM OM UVW1 OM data Continuous line – X-rays UV lightcurve reasonably (85% confidence) described by reflection from broad ring at 0.2 light days. (M=1.7x106) Mason et al 2002 OM in imaging mode. ~1200 s resolution 13 . Southampton NGC4051 XMM-Newton Alston et al 2013 OM Scaled counts PN Large UVW1 variations on short timescales. Tentative conclusion: UVW1 lags X-rays by 3 ks 14 Southampton NGC4395 Swift (Cameron et al 2012) Strong X-ray/UV/optical correlation (2d sampling) (M=3.6x105 , low ≈ 0.1%) m! E ! 15 Southampton Swift: NGC4395 Blue B-band Black 2-10 keV 16 Southampton Swift X-ray/B-band CCF No measurable lag of peak (but asymmetry towards B-band lead) 17 Better sampled data: Southampton Swift NGC4395 X-RAYS B-BAND Cameron et al 2012, MN, 422, 902 18 Southampton NGC4395: Short timescale CCF Swift orbital sampling (96min) still not good enough to measure very short lag accurately. suggesting reprocessing, but not confirming Lag of X-rays by B-band (days) (days) 19 Southampton NGC4395: Very Short timescale CCF Looking within individual Swift visits (~1ks observations) Hint that uvw2 lags X-rays by ~400s but large uncertainty Will return to this lag with XMM-Newton observations later.. 20 Southampton Multiwaveband Lags Southampton NGC2617 – Swift + Ground Shappee et al, 2014 M~4x107 ~60 observations per band Longer wavelengths smoothed as well For lag wavelength β as lagged Dashed line goes through X-ray point but β = 0.37 , inconsistent with reprocessing Solid line has β = 1.18 but is offset from X-ray point by 2.4d Is this offset real? 22 Southampton Swift Monitoring of NGC5548: First Campaign: (> 500 observations) Good correlation, but not perfect, eg large W2 rise after day 6480 (McHardy et al, 2014, MN) 23 Southampton X-ray / W2 Correlations All of the data ICCF DCF Lag close to 0 day, but hard to be certain. Possibly W2 lags slightly. 24 Southampton Lag of X-rays by UVW2 Mean-subtracted lightcurves Lag distribution Intensively sampled period (Javelin – Zu et al 2011,13) Complex long timescale variations, which are different in different bands, can distort short timescale lags (eg Welsh1999) so are removed. 25 NGC5548: In period when X-ray is not identical to UVW2 on long timescales, all UV and optical bands are similar – McH et al 2014 26 Southampton Lags as function of wavelength Expect 4/3 power for 1.23 Shakura-Sunyaev disc. Lag ∝Wavelength So good agreement. Fit goes through X-ray point BUT … observed lags are longer than expected for the Mass and m ! ! E Red line is time for HALF of (McHardy et al, 2014) reprocessed light to arrive. Microlensing obs (eg Morgan et al 2010) also require larger disc than SS model Hotter than expected disc (eg higher m ! , higher Lx)? ! E Inhomogeneous disc (Dexter and Agol 2011)? 27 Southampton NGC5548: Edelson et al 2015; Fausnaugh et al 2016 SWIFT CCF rel to HST FR/RSS HX HST 137nm UVW2 V Similar long term trends in UV/optical not seen in X-rays Using all data, HX may lead SX 28 Southampton Fausnaugh et al NGC5548 lags For disc model, disc either too big or too hot. Excess in u and i from Balmer and Paschen continua (Korista+Goad 2001) 29 Southampton NGC5548: X-ray / UV link Raw HST (1367A) and Swift X-rays above 0.8 keV 30 Southampton NGC5548: Detrended HX-ray and UV Consistent with lags in McH etal 2014 Removal of boxcar mean of full width 10d 31 Southampton NGC5548: lag vs wavelength Fit does go through the X-ray point with β=4/3. 32 V B U UVW1 UVM2 UVW2 X1 X2 X3 X4 BAT 5468 A 4392 A 3465 A 2600 A 2246 A 1928 A 20 A 10 A 4 A 2 A 0.5 A 3.6 3.8 4.0 4.23.8 4.2 4.0 4.4 4.8 5.2 5.6 6.0 6.45.0 5.5 6.0 5.5 6.5 0.00 0.15 0.300.00 0.10 0.200.0 0.4 0.8 1.20.0 0.4 0.80.000 0.006 57440 0.3-1.25keV 1.25-2.5 keV 2.5-5 keV 5-10 keV UVW1 UVM2 UVW2 57450 B U V 57460 BAT 15-150 keV BAT Modified Julian Date Modified Julian NGC4151: Swift 57470 57480 57490 V / X3 B / X3 U / X3 UVW1 / X3 UVM2 / X3 UVW2 / X3 X1 / X3 X2 / X3 X3 / X3 X4 / X3 BAT / X3 prep in etal, Edelson − −0.5 0.5 −0.5 0.5 −0.5 0.5 −0.5 0.5 −0.5 0.5 −0.5 0.5 −0.5 0.5 −0.5 0.5 −0.5 0.5 −0.5 0.5 −0.5 0.5 10 − 5 ICCF Lag (days) result$tau result$tau result$tau result$tau result$tau result$tau result$tau result$tau result$tau result$tau result$tau 0 rel to 2.5-5rel to keV 5 10 − 0 300 6000 400 10000 300 6000 400 10000 400 0 1000 0 300 6000 600 14000 2000 0 3000 0 1000 10 FR/RSS − 5 iccf.result$cent.dist iccf.result$cent.dist iccf.result$cent.dist iccf.result$cent.dist iccf.result$cent.dist iccf.result$cent.dist iccf.result$cent.dist iccf.result$cent.dist iccf.result$cent.dist iccf.result$cent.dist iccf.result$cent.dist Lag (days) 0 5 10 Question: Do the short timescale X-ray variations correlate similarly with the UV/optical in all AGN and, if so, what is the lag? Swift can study one, or maybe 2 AGN per year. Fastest (orbital) sampling is ~ 96 minutes. Hard to measure lags less than a few hours, ie restricted to AGN with M ≥ few x 106 - 107 For shorter lags (lower M, m ! ) we need XMM-Newton ! E Southampton XMM and ground based monitoring of NGC4395 (x100 lower mass than NGC5548) OM used in very fast (sub-second) readout mode using UVW1 Ground based g-band monitoring around globe. (McHardy et al 2016 and Connolly et al in prep) 35 Southampton XMM and ground based monitoring of NGC4395 100s bins 28-29 December 2014 36 Southampton XMM and ground based monitoring of NGC4395 30-31 December 2014 37 Southampton NGC4395 - DCFs X-rays vs UVW1 X-rays vs g-band (Using Emmanouloupolos et al 2013 improved lightcurve simulation method for simulations) 38 Southampton NGC4395 – Javelin lags Javelin - Zu et al 2011,13 UVW1 lags X-rays by g-band lags X-rays by 473 (+47, -98) s 788 (+44, -54) s 39 Southampton NGC4395 – Lags vs Wavelength Simple linear fit (red) is best fit (forced through zero).
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