Testing Accretion Disc Theory in AGN: X-ray/Optical Inter-band Lags in NGC Sam Connolly

Transcription

1 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)

2 UV/Optical & X-Ray Variability X-Rays X-Ray Corona UV/Optical Accretion fluctuations Accretion Disc Black Hole

3 UV/Optical & X-Ray Variability X-Rays X-Ray Corona UV/Optical Accretion fluctuations Accretion Disc Black Hole

4 UV/Optical & X-Ray Variability X-Rays X-Ray Corona UV/Optical Accretion fluctuations Accretion Disc Black Hole

5 Optical inter-band lags Given lag ~ wavelength β, 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, but β = 0.37 Solid line has β = 1.18, but is offset from X-ray point by 2.4d Consistent with reprocessing, but no link to X-rays (β~1.33) 5

6 X-Ray/Optical Time Lags NGC 4051 MKN 79 Breedt et al Optical lags by 1.5+/- 0.5 d (above 99% confidence) Breedt et al., 2009 Strong correlation, lag uncertain 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

7 NGC 4395: Short timescale CCFs Cameron et al Lag of X-rays by B-band (days) Very small B-band lag < 45 min Suggesting reprocessing, but not confirming Lag of X-rays by UVW2 (seconds) Looking within individual Swift visits (TOO 12ksec) Hint that uvw2 lags X-rays by ~400s, but large uncertainty 7

8 Swift Monitoring of NGC 5548: (> 500 observations) First campaign: McHardy et al Second campaign: Edelson et al

9 NGC 5548 Lags as function of wavelength Lag Wavelength 1.23 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

10 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

11 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

12 NGC 4395: XMM and ground-based monitoring 100s bins December

13 NGC 4395: XMM and ground-based monitoring 100s bins December

14 NGC 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 improved light curve simulation method to determine confidence curves 14

15 NGC 4395 Javelin lags Javelin Lag measurement software - Zu et al. 2011,13 UVW1 lags X-rays by g-band lags X-rays by s s Connolly et al. in prep. 15

16 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

17 NGC 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

18 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

19 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 (in prep.) 19

20 Extra Slides

21 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

22 Time Lags from Response Functions τ peak τ 50% Flux 50% Time