A deep view of the iron line and spectral variability in NGC 4051

James ReevesCollaborators:- Jane Turner, Lance Miller, Andrew

Lobban, Valentina Braito, Steve Kraemer, Mike Crenshaw

NGC 4051

• Nearby Narrow Lined Seyfert 1 Galaxy (z = 0.002336, 15.2 Mpc)• Low black hole mass (~ 106 Msolar - e.g. Denney et al. 2009)• Low luminosity (L2-10 ~ 5 x 1041 erg s-1; Lbol ~ 1043 erg s-1) but

relatively high L/LEdd (~10%)• Highly variable on both long and short timescales (low BH mass)

- Occasionally falls into “low-flux state”

• Suzaku observations in 2005 (90ks) and 2008 (350ks), Chandra HETG observation in 2008 (350ks)

• Questions - what drives the spectral variability? - what are the properties of the iron K line profile and hard X-ray excess? - where is the reprocessing material - evidence for a hard X-ray lag?

Suzaku Observations of NGC 4051

• NGC 4051 Suzaku/XIS light curve across the full 0.5 – 10 keV band

• Highly variable – by up to a factor of up to ~ 10, doubling timescales ~ 4ks

• Low flux state during 2005 (obs1) with F2-10 ~ 8x10-12 erg cm-2 s-1.

2005 (obs1)

2008 (obs 2) 2008 (obs3)

obs 1

Broad-Band Spectra

obs 2

obs 3

Fe K Line Complex with Suzaku (Turner et al. 2010, ApJ in press + poster by Andrew Lobban)

Strong narrow K line at 6.4 keV. Constant flux over 3 years.

However redshifted but relatively narrow line emission is observed at 5.4 keV and 5.9 keV in 2005 low state.

Obs 1 vs Obs2

“Redshifted” emission lines at ~ 5.44 keV and ~ 5.95 keV.

-Coincident with K fluorescent lines of Cr and Mn. Line equivalent widths are 180 eV (Fe K), 40 eV (Mn K) and 40 eV (Cr K), i.e. 4:1:1. Solar abundance ratio 130:1:2 (Grevesse & Sauval 1998)

-Spallation? (Skibo 1996, Turner & Miller 2010) - destruction of Fe nuclei into lighter elements (Sc to Mn)

But would require Cr and Mn abundances ~25 times higher than standard solar abundances to produce observed equivalent widths plus predicts high proton / -ray flux.

- Potentially gravitationally redshifted iron:

1

)2(1

1

2

−−

=

rcGM

z

5.44 keV line: R = 8 Rg 5.95 keV line: R = 16 Rg

Alternatively:

-“Hotspot” of Fe K close to black hole forming an annulus due to a long exposure.

- Combination of gravitational redshift and transverse doppler shift produces broad, skewed line profile

- Disk annulus at ~141 Rg with inclination of 122 deg, assuming neutral Fe.

Here Torb ~ 2ks so stable for many orbital timescales (200ks)..

Or special (truncation?) radius

Fe K at High resolution (Chandra/HETG)

Unresolved (FWHM <2000 km/s) narrow K core (EW = 50 eV). Underlying broad line is required (FWHM = 17000 km/s or =160 eV).

Relativistic line is NOT required, however BL consistent with originating within a few 100 Rg of nucleus. Variable?

Fe KFe XXV (f)

Fe XXVI

Spectral Variability of NGC 4051

Suzaku XIS+HXD spectra (2005-2008) sliced according to flux.Low/hard spectra dominated by narrow iron K line, with constant flux. Deeper soft X-ray absorption also occurs during low flux.Broader iron K line component more apparent at high flux.

Fe KFe XXV (f)

Fe XXVI

PCA analysis (Miller et al. 2010)

Fe KFe XXV (f)

Fe XXVI

Offset (constant)Eigenvector 1 (variable)

Detection of hard X-ray lag - evidence for reverberation? (Miller et al. 2010)

Fe KFe XXV (f)

Fe XXVI

Hard X-ray lag detected increasing with time period. Higher energy bands have greater lag compared to soft.

Could be explained by reverberation (via reflection) of continuum photons off material at a distance of a few 100 Rg from the black hole.

Time lags measured between the hard X-ray band (6-10 keV) lagging the soft X-ray (0.3-1.0 keV) band.

Dotted lines show the lag between medium X-rays (2-4 keV) and soft X-rays.

Reconstructing the broad-band spectrum

Long term spectral variability between obs 1, 2 and 3.Variable continuum (=2.4), modified by multiple soft X-ray layers of absorption, as measured by Chandra HETG spectrum.Reflector 1 - low ionization, invariant to continuum, likely distant reflectorReflector 2 - high ionization, variable, associated with disk (or disk wind).Variable absorber - with changing covering factor between 2005 - 2008

Fe KFe XXV (f)

Fe XXVI

Best-fit to Obs1 (2005) and Obs 2,3 (2008)

Conclusions• Substantial spectral variability in NGC 4051 from low/hard “state” to high/soft “state”. Intrinsic variability (flares), however soft X-ray absorption is more pronounced in low flux spectrum.

• Iron K line profile from Suzaku and HETG show multiple components. The narrow unresolved K core does not vary with time (hard offset spectrum), originating from distant (e.g. pc scale) matter.

• A weak broad component is seen in Chandra HETG and Suzaku (at high fluxes) which appears to be variable (associated with eigenvector 1). Origin in matter closer to BH on scales of 100 Rg?

• Hard X-rays lag soft X-rays in Suzaku data - possible signature of reverberation of matter a few 100 Rg from the black hole.

• Narrow redshifted lines (at high significance >99.99%) are present in the Suzaku low spectrum at 5.4 and 5.9 keV.

• Origin of redshifted lines either in spallation of Fe into lower Z elements or from disk hot-spot/annulus ~14 Rg from black hole.