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Figure 2 : XIS spectra of the main-cluster

XIS0 0.4-10keV

Main-Cluster

Sub-Cluster

Search for Bulk Motion in A2256 with Suzaku and ASTRO-H

UEDA Shutaro (shutaro@ess.sci.osaka-u.ac.jp)

HAYASHIDA Kiyoshi, NAGAI Masaaki, and TAWA Noriaki (Osaka University)

3. Bulk Motion of the Hot Gas

4. ASTRO-H Simulation1. IntroductionMerging process plays a key role in evolution of clusters of galaxies. The

nearby rich cluster A2256 is a well-studied example of merging clusters,

showing 1) distinctive two peaks in the X-ray image1), 2) two (or three)

components in the radial velocity distribution of galaxies 2), 3) a cold front in

the gas temperature map 3). If the merging event in A2256 is at its early stage,

as was suggested by authors, the hot gas in the main-cluster and that in the

sub-cluster may have different velocities by their bulk motions. We aim to

measure those velocities from X-ray spectra observed with Suzaku XIS.

A2256 is also known as one of the clusters in which non-thermal hard X-ray

emission was detected with Beppo-SAX and RXTE 4),5). Furthermore, radio

relic was observed in this cluster. These might be related to the merging event,

in which non-thermal electrons are produced. Search for non-thermal emission

with Suzaku XIS/HXD is also the purpose of our observation.

4.2. SXS Measurement of Turbulent Flow

2. Observation & AnalysisSuzaku AO-1 observation of A2256 was performed on Nov. 11-12, 2006.

Exposure time was 96ks for the XIS and 97ks for the HXD. Unfortunately,

since the observation was just after the XIS2 trouble on Nov 9, XIS2 was

not available and the SCI was off for other three sensors during the

observation.

We re-processed the XIS data with the latest CALDB at 2010.7.1, as the

energy scale error is crucial for our analysis. We estimated Non-X-ray and

Cosmic X-ray Background by utilizing the method we developed6).

MWH

LHB

CXB

ICM

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5. SummaryVelocity Difference of 1030+570

-640 km/s is detected between the X-ray

emitting hot gas at the main-cluster and that at the sub-cluster. It

supports the idea that A2256 is in the early stage of a merging event of

(at least) two clusters.

SXS simulations indicates the redshifts will be measured as accurate

as a few tens km/s when we the turbulent velocity dispersion is 100km/s

or less. If the turbulent velocity is as large as 10eV(~450km/s), the

uncertainty in the bulk velocity gets larger but small enough for positive

detection. The velocity dispersion itself will be measured with an accuracy

of about 100km/s.

As shown in Fig. 1, we extracted two circular

regions (main-cluster, sub-cluster), each with

2.5’ radius. The spectra were fitted with APEC

model for the ICM component, with particular

focus on the redshifts determined in the fits.

Although detailed examinations on the

systematic errors will be reported in our

upcoming paper, we confirmed that the results

are consistent with/without additional corrections

to the CCD energy scale. Statistically significant

difference of the redshifts in the main and the sub

is found, and its direction is consistent with that

measured for optical galaxies.

References1)Briel, U.G. et al. 1991, A&A, 246, L10

2)Berrington, R.C., Lugger, P., & Cohn, H.N. 2002, AJ, 123, 2261

3)Sun, M. et al. 2002, ApJ, 565, 867

4)Fusco-Femiano, R., Landi, R., & Orkandini, M., 2005 ApJ, L69

5)Rephaeli, Y., & Gruber, D.E. 2003, ApJ, 595, 137

6)Tawa, N. et al. 2008, PASJ, 60 S11-S24

4.1. SXS Simulation of Bulk Motion in A2256

Optical galaxies

(Berrington et al., 2002)

Redshift Difference of 0.342+0.189-0.215 x10-2 (Velocity Difference of 1030+570

-640 km/s )is detected between the hot gas in the main-cluster and that in the sub-cluster.

(Errors are 90%CL. We have not yet corrected for the PSF leakage between the two regions.)

APEC model

kT=7.33+0.13-0.14keV

Z=0.30+0.02-0.02

Sub-Cluster

MWH

LHB

CXB

ICM

APEC model

kT=6.20+0.11-0.06keV

Z=0.31+0.02-0.02

SXS Simulation (APEC model)

We performed SXS simulation by utilizing response file(SXS baseline design, FWHM

is 7eV) that is distributed with ASTRO-H official website. The exposure time is set as

78ks. We normalized the count rate of SXS by referring the spectral fits with XIS; the

count rate of SXS is ~0.4 cts/s for the entire energy band and 2.2x10-3cts/s in the iron

lines. This simulation indicates the redshfits are determined with accuracy of a few

tens km/s, enough for our pourpose. Note , however , that the extract region of the

SXS (3’x3’) is much smaller than the regions for the XIS shown in Fig.1..

We further consider the spectral model in which turbulent motion of the gas is not

negligible. Specifically, we convolve the APEC model with gsmooth model. It is

found that the SXS can measure the velocity dispersions with an accuracy of a few

eV, i.e., 100km/s. Even with the turbulent velocity of 10eV(~450km/s), the redshift

difference between the main and the sub clusters will be detected significantly.

Optical galaxies Chandra results from

Sun et al., 2002

Turbulent flow:2eV(~90km/s) Turbulent flow:10eV(~450km/s)

SXS Simulation (APEC convolved with gsmooth)

He-like Iron line

~2.6x10-3 cts/s

He-like Iron line

~3.0x10-3 cts/s

Figure1 : XIS image of A2256

Figure 3 : XIS spectra of the sub-cluster

Figure 4 : Redshift Difference between main and sub clusters

SXS SimulationSuzaku XIS

Fig 5 : XIS spectrum in 6-8keV Fig 6 : SXS spectrum in among 6-

8keVTable 1: Redshifts obtained with XIS and SXS

Figure 7 : SXS simulations in which turbulent flow is taken into account

Redshift [ x10-2 ]

main sub

XIS 5.972+0.166-0.164 5.630+0.090

-0.139

SXS 5.974+0.005-0.005 5.634+0.002

-0.007

Velocity Difference [km/s]

XIS 1025+566-644

SXS 1022+10-25

He-like Iron line

~2.2x10-3cts/s

Fig 8 : Redshift obtained

in SXS simulationsFig 9 : Turbulent velocity obtained

in SXS simulations

Velocity Difference [km/s]

XIS 1025+566-644

SXS(0eV) 1022+10-25

SXS(2eV) 1032+20-30

SXS(10eV) 1088+270-101

Table 2: Redshifts obtained with SXS

simulation