Photometric Reverberation
Mapping of NGC 4395
Stephen Rafter – The Technion, Haifa Israel
The Restless Nature of AGNs
22 May, 2013
Collaborators
• Haim Edri – Masters Student - the Technion
• Doron Chelouche – Tel Aviv University/Univ. of Haifa
• Shai Kaspi – Tel Aviv University/the Technion
• Ehud Behar – the Technion
Broad Band Photometric RM
• Use broad band filters to detect time lags between continuum
and emission lines when their signal is mixed in a given band
NGC 4395 & SDSS filters
The Photometric RM Method
• We follow the method developed by Chelouche & Daniel
(2012) as follows:
Take 2 filters, one which covers a ‘continuum only’
region, and a second which covers a region with lines
and continuum such that we can define light curves for
filters X and Y such that
(1) FX (t) =
c
fX (t)
c
l
(2) FY (t) = fY (t) + fY (t)
The Photometric RM Method
• The CCF between the lines and continuum is given by:
l
c
(3) CCFlc(τ) = fY (t + τ) * fY (t)
c
c
(4) CCFlc(τ) = [FY (t + τ) - fY (t + τ)] * fY (t)
• Since the continuum is the dominant variable source
contributing to the band (> 80%) we assume:
c
(5) fY (t) ≈ FX (t)
(6) CCFlc(τ) = [FY (t + τ) - FX (t + τ)] * FX (t)
(7) CCFlc(τ) = CCFXY(τ) - ACFX(τ)
Simulations
• Generate a ‘continuum’ light curve (LC)
• Shift by hand in time (τ) and scale down amplitude of
variability to simulate line response to continuum variations
• Add the line and continuum LCs
FX (t)
FY (t)
Simulations
• Compute the CCF and ACF and look for a peak in the
difference. We recover the input time lag of ~4 hours
Units in the
lower panel
have no
‘physical’
significance
𝜏 = 4.0+1.5
−1.1 ℎ𝑜𝑢𝑟𝑠
Simulations
• Flux randomization (FR) within measured error bars in a
Monte Carlo simulation (also random subsets, RSS)
• Final measurement is τ = 3.8
+1.5
-1.1 hours
Why NGC 4395?
• Nearby, so it’s bright
and well studied
DL ≈ 5 Mpc
g` ≈ 14.5 mag
• Lowest luminosity of
any confirmed AGN:
Lbol = 7 x 1039 erg s-1
• Low luminosity implies RBLR
on the order of a few hours
• HST CIV RM gives
RBLR ≈ 1 hour (Peterson et al. 2005)
Photometric Observations and LCs
• 9 nights using the Wise Observatory's 1 meter telescope
• SDSS g', r' and i' broad band filters, 5 minute exposure
NGC 4395 – g` & r` Bands
• The g` band is ‘continuum’
• τ = 3.68 +0.70
-0.84 hours
NGC 4395 – i` & r` Bands
• The i` band is continuum
• τ = 3.46 +1.59
-0.36 hours
NGC 4395 Velocity
• Hβ is fit atypically here with 3 components:
Narrow ~ 65 km/s (modeled using [OIII] lines)
Intermediate ~ 250 km/s (put in by hand)
Broad ~ 1500 km/s (free parameter)
Hi-Res Keck Spectrum 2011
from Ari Laor
Mass Estimation
• To calculate the mass we take:
ΔV = 1500 ± 500 km/s
τ = 3.6 ± 0.8 hours
f = 0.75 (isotropic circular velocity field)
MBH = 1.464 x 105 (RBLR/days) (ΔV/1000 km s-1)2
MBH = 4.9 ± 2.6 x 104 M
Based on
this study
NGC 4395 is
about here
Comparing to Previous Work
The Next Step:
The Multivariate Correlation Function
• An extension of the CCF-ACF method outlined in
Chelouche & Zucker (2013, accepted by ApJ.)
• Adds an extra parameter, α, which is the fractional line
contribution to the total flux in the band where:
α = 1 is a pure line emission light curve
α = 0 is a pure continuum light curve
• Alleviates the need for spectral decomposition to obtain a
pure continuum light curve, which can be subjective due to
broad line wings and line blends in spectroscopic RM.
SDSS NLS1 Candidate “SL01”
& The Multivariate Correlation Function
MCF Time Lag for Hα
Maximum at τ = 18+3
-7 days and α = 0.12
Warmer colors represent a
higher correlation coefficient
Comparison of Methods
ICCF and ZDCF determined spectroscopically
Comparing RM Methods
Spectroscopy
Photometry
Resolved lines
Broad band filters
Accurately measure flux in the lines and
continuum
Continuum and emission lines are mixed
Observationally time consuming but more
accurate
Relatively quick but less accurate
Only about 50 objects have been mapped
Broad Band Photometric Reverberation
Possible to use current and future large area
surveys to estimate the masses of numerous
AGNs (LSST will monitor ~107 AGNs)
Conclusions
• We use broad band filters and the CCF – ACF difference
method to estimate RBLR for NGC 4395 (see Edri et al. 2012 for formal results)
• We introduce the MCF method which adds an addition
parameter to characterize the contribution of variable line
emission to a broad band light curves
• The MCF method can be applied to moderate/low resolution
spectra to determine time lags as well as broad band light
curves
• These methods can be applied to large area time series
surveys like LSST to estimate RBLR for a very large number of
AGN
• Still a lot of work to do and there will be more results to
follow…