RIKEN Accel. Prog. Rep. 48 (2015)

Ⅱ-7. Astrophysics and Astro-Glaciology

X-ray and Optical/UV Correlation Studies of Active Galactic Nuclei
H. Noda,∗1 and T. Minezaki,∗2 on behalf of the AGN Optical–X-ray Monitoring Collaboration
Almost all galaxies in the universe are considered
to harbor, at their center, a supermassive Black Hole(BH) with a mass of ∼ 105−10 M , where M is Solarmass. In Active Galactic Nuclei (AGNs), a considerable amount gas accretes onto the BH, and multi wavelength signals, including optical, UV, and X-ray, aregenerated with bolometric luminosity of ∼ 1040−46 erg
s−1 . The X-ray is presumably generated, via inverse
comptonization in high-temperature electron clouds(coronae) formed near the BH, while the optical/UVemissions via black body processes of an optically thickand geometrically thin accretion disk extending out tofarther radii from the BH. Although a combinationof the accretion disk and coronae has been known as“the central engine” creating a large amount of energy
in AGNs, its properties including geometries, physicalconditions, and heating mechanisms are still unclear.
Applying a timing method1) to Suzaku 2) data of an
AGN called NGC 3227, we found that a Hard PrimaryComponent (HPC) dominated X-ray emission in thefaint phase, while a Broad-band Primary Component(BPC) appeared in addition to the HPC in the brightphase (Fig. 1a). This demonstrates that at least two
X-ray emitting regions with distinct properties exist(a)

Faint phase

Bright phase
HPCHPC

νFν

BPC
Reprocessed X-ray

2

Reprocessed X-ray

45 2
Energy (keV)

45

UV (~1500 Å) count rate (cnt/s)

109.5(b)98.5
8BPC + HPC
Only HPC

7.5
76.5
60

0.5 1
1.5 2
2.5 3
2-10 keV count rate (cnt/s)

Fig. 1. (a) Spectral components of NGC 3227 in the faint(left) and bright (right) phase3) . (b) A plot between
fluxes in the 2–10 keV and UV bands. Systematic errorsof the Swift detectors were not included in the errors.
∗1∗2RIKEN Nishina Center
Institute of Astronomy, School of Science, The University ofTokyo

near a BH, and one emitting the HPC is always visible,
while the other radiating the BPC shows up only when
the amount of accreting gas is large3) . However, thegeometries of the HPC and BPC creating regions canbe hardly identiﬁed with only the X-ray information.
To study the geometries, we focus on correlations
between the ﬂuxes of primary X-ray and optical/UV,
because a better correlation possibly reﬂects a stronger
geometrical connection of an X-ray radiating region
with the accretion disk. In 2013–2014, we performed
optical and X-ray simultaneous monitoring on anAGN called NGC 3516 with Suzaku and ﬁve Japaneseground-based telescopes. As a result, a signiﬁcant correlation was discovered between ﬂuxes of HPC and theoptical signals. The result will be discussed elsewhere.
Optical–UV data derived by Swift 4) are useful aswell. Almost all Suzaku observations of NGC 32273)
were simultaneously followed up by Swift; therefore
X-ray ﬂux in the faint and bright phase, identiﬁed by
Suzaku (Fig. 1a), can be individually compared withthose in optical/UV. First, we extracted UV countrates from a 5”-radius circle centering at the nucleus on
UV images obtained with the UVW2 ﬁlter. Next, weextracted 2–10 keV count rates from the Suzaku andSwift datasets derived at the same time as the UV images. Figure 1(b) shows a count-count plot between theUV and X-ray count rates with diﬀerent colors between
the faint (purple) and bright (green) phases. When
we ﬁtted the faint-phase plots (Fig. 1b purple) witha linear function, including systematic errors of 3%
into the UV count rates, the ﬁt almost succeeded withχ2 /ν = 13.4/6. However, in ﬁtting all the plots (Fig.
1b purple plus green) again with a linear function, after
including the 3% systematic errors into the UV countrates, the result degraded giving χ2 /ν = 61.5/9. Thismight indicate that the HPC ﬂux is strongly correlatedwith UV, while the BPC ﬂux is little correlated.
Because the HPC was found to deeply relate withblack body photons from the accretion disk, it may
come from an hot accretion ﬂow, and if so, the faintphase corresponds to the low/hard state5) . On theother hand, the origin of the BPC still remains unclear, because it did not appear for long enough in theprevious observations of NGC 3516 and NGC 3227. To
examine the bright phase in detail, we perform another
X-ray and optical monitoring with Suzaku, and the systematic analyses of AGN data in the Swift archive.
References
1) H. Noda et al.: ApJ, 771, 100 (2013)
2) K. Mitsuda et al.: PASJ, 59, 1 (2007)
3) H. Noda et al.: ApJ, 794, 2 (2014)4) N. Gehrels et al.: ApJ, 611, 1005 (2004)
5) F. Yuan & R. Narayan: ARAA, 52, 529 (2014)- 170 -完全版2014_本文.indd 170

15/10/16 17:51