Dark Matter SearchwithHigh-resolution X-ray Spectroscopy
Tetsu Kitayama (Toho University)

Outline• Why DM search with X-ray observations• Previous results on an unidentified emission
line at E=3.5 keV by X-ray CCD• New results with the Hitomi satellite
Hitomi collaboration (2016a) Nature, 535, 117
Hitomi collaboration (2016b) arXiV: 1607.07420

Motivation
・DM makes up >80% of total matter in the Universe.

・No firm detection in laboratory experiments orin γ-rays, so far.
・There are robust (nearly model-independent)
bounds for the mass of dark matter particles
in the keV range.

(1) Mass bound for “fermion DM”
If DM is a fermion, there exits arobust lower bound of the DM mass,
below which the Fermi velocity(2εF/m)1/2 exceeds the escape velocity(2GM/R)1/2.

DM particles

e.g, dwarf spheroidal galaxies(DM dominated objects)

Boyarsky et al. (2009)
cf. Tremaine and Gunn (1979)

(2) Mass bound for “nearly thermal DM”
If DM particles were moving at nearly(not necessarily exactly) thermalvelocity, they were relativistic whenand smoothed density perturbations up
to the free-streaming scale (comoving)Thus, galaxy formation is possible
only if mc2 > keV.
※ Mpc=3.1 ×1024 cm

Why is keV DM interesting?
Plausible candidate of the lightest fermion DM
and/or Warm DM (free-streaming ～ galaxy scale)that may resolve several shortcomings of the Cold Dark
Matter Model at sub-galactic scales:e.g., ・Substructure problem
・Angular momentum problem
・Flat density profile of dwarf galaxies(Weinberg et al. 2015, arXiv:1306.0913)

i.e., CDM tends to over-predict mass concentration
at sub-galactic scales, while the roles of baryonicprocesses are also uncertain.

To what extent is CDM tested?
CMBz～1000

SDSS galaxies
z～0galaxyclusterz～0
Lyαforestz～3Power spectrum ofdensity fluctuations(Tegmark et al. 2004)

To date, observational
evidences for CDM are“above” the galaxy
scale (～Mpc）.

Line: ΛCDM
1010 Msun

Large scale

Small scale

Can we “see” DM in X-rays?
DM particles must be stable for >1010 yr, but they are detectable
if they decay to X-ray photons in ～1020 yr

Decay rate

Surface mass density
within the FOV

Candidate: right-handed (sterile) neutrino
Pal and Wolfenstein(1982)The mass can also be measured from the line energy.

Unidentified emission line at 3.5 keV?
Bulbulet al.
(2014)
best-fit model + Gaussian
best-fit model without excess

Perseuscenter (< 12’)5.2+2.6-1.6
×10-5
ph/cm2/s
cf.
73 clusters
stacked0.40±0.08
×10-5ph/cm2/s

Compiled by Iakubovskyi arXiv:1510.00358[33] Bulbul et al. (2014)[34] Boyarsky et al. (2014)[64] Tamura et al. (2014)[75] Riemer-Sorensen (2014)[76] Jeltema, Profumo (2015)[77] Boyarsky et al. (2015)[80] Urban et al. (2015)[82] Iakubovskyi et al. (2015)10 5

X-ray CCD results (ΔE～100 eV)
1. >3σ signatures are reported from the Perseus galaxy cluster,M31 galaxy, and a stacked sample of 73 galaxy clusters.
Other individual systems have < 3σ significance.
(e.g., Bulbul et al. 2014; Boyarsky et al. 2014)
2. The signal towards the Perseus center is >10 times brighter than
the average of the other massive clusters. (Bulbul et al. 2014)
3. The results are sensitive to how continuum and neighboring lines
are modeled. Non-detections or controversial results are also
reported. (e.g., Tamura et al. 2014; Urban et al. 2014)
Note: 3.5 keV line is NOT resolved by X-ray CCDs with ΔE～100eV.
Systematics effects are not fully excluded.

How can we reduce systematics?
1. Excluding plasma origins
Line width
DM: velocity dispersion of collisionless particles( >1000 km/s for massive galaxy clusters)
plasma ions: thermal + turbulent motion (～200 km/s)
2. Excluding instrumental effects
Line centroid energy
should vary with redshifts (distances) of the target
objects, if the line originates from them.
Both require spectral resolution of ΔE < 10eV.

Hitomi satelliteFirst X-ray satellite to have achieved
ΔE=5 eV (x20 better than CCD)
for extended sources.

Hitomi (ASTRO-H)

cf. Expected FWHM of a DM line
14m, 2.7t

Launched in February, 2016,
observed a few targets,and lost control in March, 2016.

Suzaku (6m, 1.7t)
13Hitomi observed
the Perseus cluster
at z=0.0179

FOV3’x3’(Ohashi’s talk tomorrow)He-like Fe(triplet)

Suzaku CCD
ΔE～100 eV

Chandra X-ray image
H-like Fe(doublet)

Hitomi calorimeter
ΔE=5 eV
14Hitomi collaboration (2016a)

Hitomi spectrum of the Perseus center around 3.5 keVBest-fit thermal plasma model⇒ turbulence 179±16 km/s
with no excess at 3.5 keV
XMM range for stacked sample
XMM range for Perseus

Hitomi collaboration (2016b)

XMM CCD
Perseus

StackedHitomi collaboration (2016b)

Best-fit (solid)±3σ (shaded)
fluxes for anadditional
Gaussian line
broadened by1) plasma
turbulence(180 km/s)
2) DM velocity
dispersion(1300 km/s)
Red and black error
bars show systematic
uncertainty of theeffective area of thetelescope.

Significance ofthe difference
between the best-fit
XMM and Hitomi
results for a Gaussianline with varying
widths (not including
the errors of theXMM result).

3σ

Hitomi collaboration (2016b)

Red error bars show
systematic uncertainty ofthe effective area of thetelescope. Red dashed line
shows the difference fromthe zero flux.

Summary
1. An unidentified line at 3.5 keV was inferred in a number ofsystems based on unresolved spectra taken by X-ray CCD.
2. Hitomi obtained the first and only resolved spectrum toward
the center of the Perseus galaxy cluster and excluded the previous
CCD result (the brightest signal) for this system at >99% confidence
level, taking account of uncertainty of the previous result.
3. An order of magnitude improvement in sensitivity is required totest the previous result for the fainter average signal froma sample of galaxy clusters.