Photometric Reverberation Mapping of NGC 4395

The Restless Nature of AGNs22 May, 2013Stephen Rafter – The Technion, Haifa Israel

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:

(1) FX (t) = fXc(t)

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

(2) FY (t) = fYc(t) + fY

l(t)

The Photometric RM Method• The CCF between the lines and continuum is given by:

(3) CCFlc(τ) = fYl(t + τ) * fY

c(t)

• Since the continuum is the dominant variable source contributing to the band (> 80%) we assume:

(5) fYc(t) ≈ FX (t)

(4) CCFlc(τ) = [FY (t + τ) - fY

c(t + τ)] * fYc(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)

𝜏=4 .0−1 .1+1 .5 h𝑜𝑢𝑟𝑠

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

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 hours-1.1

Why NGC 4395?

• Lowest luminosity of any confirmed AGN: Lbol = 7 x 1039 erg s-1

• Nearby, so it’s bright and well studied

DL ≈ 5 Mpc g` ≈ 14.5 mag

• 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 hours -0.84

NGC 4395 – i` & r` Bands

• The i` band is continuum

• τ = 3.46 +1.59 hours-0.36

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 2011from Ari Laor

Mass Estimation• To calculate the mass we take:

ΔV = 1500 ± 500 km/sτ = 3.6 ± 0.8 hoursf = 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 studyNGC 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 curveα = 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 HαMaximum at τ = 18+3 days and α = 0.12

-7

Warmer colors represent a higher correlation coefficient

Comparison of Methods

ICCF and ZDCF determined spectroscopically

Comparing RM Methods

Broad Band Photometric Reverberation

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 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…