GALEX measurements of the Big Blue Bump as a tool to study bolometric corrections

Elena MarcheseR. Della Ceca, A. Caccianiga, P. Severgnini, A. Corral

Active Galactic Nuclei 9 – Ferrara , 24-27 May 2010

Study the spectral energy distribution (SED) of type 1 AGN in the optical, Near and Far-UV and X-ray energy bands .

Constrain the luminosity of the accretion disk emission component Derive the hard X-ray bolometric correction factors for a significant

sample of Type 1 AGN spanning a large range in z and Lx.

XXX

L

L

BH LdLLKc

zX

X

log)(1

)(max,

2

Accretion rate density

dzdz

dtzz

z

BHBH max

0

)()( Total accreted mass

Active Galactic Nuclei, powered by accretion onto a Super-massive Black Hole (SMBH), emit over the entire electromagnetic spectrum with the peak of the accretion disk emission in the far-UV, a wavelength range historically difficult to investigate.

The XMM-Newton Bright Survey in pills

XMM fields used: 237

Covered Area (deg2): 28

Sources in the bright sample(BSS,0.5-4.5keV): 389 “ “ “hard” bright sample(HBSS,4.5-7-5 keV): 67

(56 sources are in common)

Total Sources: 400 (fx >~7x10-14 erg cm-2 s-1)

Della Ceca et al., 2004Caccianiga et al., 2008

The starting point of our study is a sample of 304 AGNs, counting 263 type 1 AGNs having intrinsic NH <4 ·1021 cm-2, belonging to the

XMM-Newton Bright Serendipitous Survey (XBS).

GALEX (Galaxy Evolution Explorer) is a NASA Small Explorer mission that is performing surveys of different depths/sky coverage

in the far-UV and near-UV

The GALEX mission in pills

CROSS CORRELATION WITH GALEX

160 matches 63 upper limits

263 X-ray selected AGN 1, with NH <4 ·1021 cm-2

CROSS CORRELATION WITH SDSS

82 sources having data from XBS-GALEX-SDSS

The sample

All these sources have an X-ray spectra from XMM-Newton which allows us to derive X-ray luminosities and spectral properties (e.g. Γ, Nh).

• Corral, Della Ceca, Caccianiga and Severgnini, 2010, in preparation

• A. Corral:this meeting

40 sources out GALEX field

• The data points from the SDSS and from GALEX were described using a basic accretion disk model (DISKPN model in the XSPEC package). The maximum disc temperature was chosen in the range kT≈1-64 eV , and the normalization has been left has free parameter.

The model

Corrections to measured fluxes

Observer

Our Galaxy

Amm oss ,

VB

VV E

AR

AGN

Galactic reddening:Allen law(1976) Rv =3.1 EB-V = AB – AV available from the GALEX database

Intrinsic AGN reddeningThe exact shape of the extinction curve in the Near-Far-UV is still a matter of debate

Gaskell e Benker, 2007 determined a parametrized average extinction law from

the study of 14 AGN, with FUSE and HST data.

1221108.4

magcmE

N

VB

H

Bohlin et al. 1978

Host Galaxy

Hydrogen clouds (Lymanα forest)

Corrections to measured fluxes- IVOptical emission from the host galaxy

About 20% of the sources have optical SEDs showing a hardening at the optical wavelengths. Optical spectra of these sourcescontamination by the stars in the host galaxy

Break at 4000 Å : indicator of the importance of the galaxy star-light in the total emission of the source.

Calcium- break

F

FFF+ e F- mean flux densities in the regions 4050-4250

Å and 3750-3950Å (in the source rest-frame) respectively.

AGN

Host galaxy

AGN + host galaxy

Calcium break FL

UX

ENERGY (kev)

SDSS GALEX

SED OF ONE OF THE SOURCES

Model formulated to correct for the emission of the host galaxy

Model of a normal galaxy: Heaviside function such that Δ=50%

AGN: αν =-0.44 (αλ =-1.56).(Vanden Berk et al. 2001)

We calculated the resulting Δ from the combined emission of the AGN (with different normalizations) and host galaxy

FLU

X RA

TIO

AG

N/G

ALAX

Y

Break at 4000 Å

Ca break≈40%

HOST GALAXY

AGN

H.GALAXY+AGN

FLU

X

λ (Å)

HOST GALAXY

AGN

H. GALAXY+AGN

FLU

X λ (Å)Ca break≈2%

Corrections to measured fluxes- IVOptical emission from the host galaxy

HOST GALAXY

AGN

GAL+AGN

AGN

HOST GALAXY

GAL+AGN

Δ=17.3%

Δ=33.2%

0.

0.

ENERGIA (keV)

FLU

X FL

UX

ENERGY (keV)

ENERGY(keV)

SDSS

SDSS

GALEX

GALEX FLU

X FL

UX 0.

ENERGY (keV)

ENERGY (keV)

SDSS

SDSSGALEX

GALEX

AFTER CORRECTIONBEFORE CORRECTION

The presence of emission lines within the filter bandpass can contribute significantly to the observed magnitudes of an AGN. Since this effect is a strong function of redshift, we need to take it into account to derive the continuum…

Assuming R ~1

Average spectra and equivalent widths of the emission lines present in the energy bands we are studying, calculated by Telfer et al. 2002, from the spectra of 184 quasars with z>0.33 .

Corrections to measured fluxesEmission lines contribution

FLU

X

WAVELENGTH (Å)

TIPICAL SPECTRA OF AN AGN

FLUSSI OTTICO-UV

FLUSSI X

ENERGIA (keV)

FLUSSI OTTICO-UV

FLUSSI X

ENERGIA (keV)

FLUSSI OTTICO-UV

FLUSSI X

ENERGIA (keV)

OPTICAL-UV

X-rayXMM

ENERGIA (keV)

SPECTRAL ENERGY DISTRIBUTIONS

OPTICAL-UV

X-ray (XMM)

ENERGIA (keV)

Median maximum temperature : kT ≈ 4 eV

2-10 keV luminosityAvailable for every object from the X-ray spectral analysis (Corral et al. 2010)

0.1-100 keVluminosity Extrapolated from the 2-10 keV luminosity, using the spectal index measured for every sorce

Accretion disk luminosity

Calculated as the integral of the SEDs in the optical-UV bands.

L bol =L disc + L 0.1-100 keV

82 sources with XBS-GALEX-SDSS data discussed so far

78 sources with XBS-GALEX data

63 sources with XBS-GALEX upper limits

UV fluxes or their upper limits were fitted with the same model with a Tmax

fixed to kT ≈ 4 eV

Strong dependence of the accretion disk luminosities to the X-ray luminosities the

two emission mechanisms are highly correlated

Results - I CORRELATION LDISK – L2-10keV

Best-fit bisector relation:

913.8log)056.0232.1(log 102 keVdisc LL

In good agreement with previous results on X-ray selected sources: Lusso et al. 2010: β=1.31±0.038

78 sources XBS-GALEX

82 sources XBS-GALEX-SDSS

63 sources XBS-GALEX upper limits

keV

bolbol L

Lk

102

We don’t find any significant correlation between bolometrc

correction and X-ray luminosities

This is probably due to a very large spread in the distributions of

the hard X-ray bolometric corrections, going from ~5 up to few hundred, implying a large dispersion in the mean SED

Results - II BOLOMETRIC CORRECTION AGAINST HARD X-RAY LUMINOSITY

63 sources XBS-GALEX upper limits

78 sources XBS-GALEX

82 sources XBS-GALEX-SDSS

We used a sample of 223 sources spanning a large range in X-ray luminosities (LX ≈1041 – 1046 erg/s) and redshift (z≈0-2.4) and we find :

• A high correlation between the accretion disk luminosity and the X-ray luminosity, in agreement with previous works on X-ray selected AGNs.

• A very large spread in the distributions of the hard X-ray bolometric corrections, going from ~5 up to few hundred -> a large intrinsic dispersion in the mean SED;

Conclusions

Thank you