Observation of the new emission line at ~3.5 keV in X-ray spectra of galaxies and galaxy clusters
The detection of an unidentified emission line in the X-ray spectra of cosmic objects would be a `smoking gun'signature for the particle physics beyond the Standard Model. More than a decade of its extensive searches results in several narrow faint emission lines reported at 3.5, 8.7, 9.4 and 1...
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| Zitieren: | Observation of the new emission line at ~3.5 keV in X-ray spectra of galaxies and galaxy clusters / D.A. Iakubovskyi // Advances in Astronomy and Space Physics. — 2016. — Т. 6., вип. 1. — С. 3-15. — Бібліогр.: 188 назв. — англ. |
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irk-123456789-1199512017-06-11T03:03:42Z Observation of the new emission line at ~3.5 keV in X-ray spectra of galaxies and galaxy clusters Iakubovskyi, D.A. The detection of an unidentified emission line in the X-ray spectra of cosmic objects would be a `smoking gun'signature for the particle physics beyond the Standard Model. More than a decade of its extensive searches results in several narrow faint emission lines reported at 3.5, 8.7, 9.4 and 10.1 keV. The most promising of them is the emission line at ∼3.5 keV reported in spectra of several nearby galaxies and galaxy lusters. Here I summarize its up-to-date status, overview its possible interpretations, including an intriguing connection with the radiatively decaying dark matter, and outline future directions for its studies. 2016 Article Observation of the new emission line at ~3.5 keV in X-ray spectra of galaxies and galaxy clusters / D.A. Iakubovskyi // Advances in Astronomy and Space Physics. — 2016. — Т. 6., вип. 1. — С. 3-15. — Бібліогр.: 188 назв. — англ. 2227-1481 DOI:10.17721/2227-1481.6.3-15 http://dspace.nbuv.gov.ua/handle/123456789/119951 en Advances in Astronomy and Space Physics Головна астрономічна обсерваторія НАН України |
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The detection of an unidentified emission line in the X-ray spectra of cosmic objects would be a `smoking gun'signature for the particle physics beyond the Standard Model. More than a decade of its extensive searches results in several narrow faint emission lines reported at 3.5, 8.7, 9.4 and 10.1 keV. The most promising of them is the emission line at ∼3.5 keV reported in spectra of several nearby galaxies and galaxy lusters. Here I summarize its up-to-date status, overview its possible interpretations, including an intriguing connection with the radiatively decaying dark matter, and outline future directions for its studies. |
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Article |
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Iakubovskyi, D.A. |
| spellingShingle |
Iakubovskyi, D.A. Observation of the new emission line at ~3.5 keV in X-ray spectra of galaxies and galaxy clusters Advances in Astronomy and Space Physics |
| author_facet |
Iakubovskyi, D.A. |
| author_sort |
Iakubovskyi, D.A. |
| title |
Observation of the new emission line at ~3.5 keV in X-ray spectra of galaxies and galaxy clusters |
| title_short |
Observation of the new emission line at ~3.5 keV in X-ray spectra of galaxies and galaxy clusters |
| title_full |
Observation of the new emission line at ~3.5 keV in X-ray spectra of galaxies and galaxy clusters |
| title_fullStr |
Observation of the new emission line at ~3.5 keV in X-ray spectra of galaxies and galaxy clusters |
| title_full_unstemmed |
Observation of the new emission line at ~3.5 keV in X-ray spectra of galaxies and galaxy clusters |
| title_sort |
observation of the new emission line at ~3.5 kev in x-ray spectra of galaxies and galaxy clusters |
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Головна астрономічна обсерваторія НАН України |
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2016 |
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http://dspace.nbuv.gov.ua/handle/123456789/119951 |
| citation_txt |
Observation of the new emission line at ~3.5 keV in X-ray spectra of galaxies and galaxy clusters / D.A. Iakubovskyi // Advances in Astronomy and Space Physics. — 2016. — Т. 6., вип. 1. — С. 3-15. — Бібліогр.: 188 назв. — англ. |
| series |
Advances in Astronomy and Space Physics |
| work_keys_str_mv |
AT iakubovskyida observationofthenewemissionlineat35kevinxrayspectraofgalaxiesandgalaxyclusters |
| first_indexed |
2025-07-08T16:57:06Z |
| last_indexed |
2025-07-08T16:57:06Z |
| _version_ |
1837098681915932672 |
| fulltext |
Observation of the new emission line at ∼3.5 keV
in the X-ray spe
tra of galaxies and galaxy
lusters
D.A. Iakubovskyi
∗
Advan
es in Astronomy and Spa
e Physi
s, 6, 3-15 (2016) doi: 10.17721/2227-1481.6.3-15
© D.A. Iakubovskyi, 2016
Dis
overy Center, Niels Bohr Institute, Blegdamsvej 17, Copenhagen, Denmark
Bogolyubov Institute of Theoreti
al Physi
s, Metrologi
hna Str. 14-b, 03680, Kyiv, Ukraine
The dete
tion of an unidenti�ed emission line in the X-ray spe
tra of
osmi
obje
ts would be a `smoking gun'
signature for the parti
le physi
s beyond the Standard Model. More than a de
ade of its extensive sear
hes results
in several narrow faint emission lines reported at 3.5, 8.7, 9.4 and 10.1 keV. The most promising of them is the
emission line at ∼3.5 keV reported in spe
tra of several nearby galaxies and galaxy
lusters. Here I summarize
its up-to-date status, overview its possible interpretations, in
luding an intriguing
onne
tion with the radiatively
de
aying dark matter, and outline future dire
tions for its studies.
Key words: X-rays: general, dark matter, line: identi�
ation
introdu
tion
The origin of the dark matter � the major (yet of
unknown origin) gravitating substan
e in the Uni-
verse [17, 51, 56, 59, 60, 62, 70, 71, 72, 78, 79, 84, 87,
122, 123, 137, 140, 143, 144, 151, 152, 155, 170, 179,
188℄ � still has to be revealed. If the dark matter
is made of elementary parti
les, the latter should be
massive (to form over-densities in pro
ess of gravita-
tional
ollapse), long-lived (to be stable for at least
the age of the Universe) and neutral with respe
t to
strong and ele
tromagneti
intera
tions (to be su�-
iently `dark'). The only known massive, long-lived
and neutral parti
les are the usual (left-handed) neu-
trinos, but they are too light to form small dark
matter halos [171, 184℄. As a result, the hypothe-
sis of the dark matter parti
le implies an extension
of the Standard Model of parti
le physi
s. Dozens of
the Standard Model extensions have been proposed
so far to
ontain a valid dark matter parti
le
andi-
date. However, as Fig. 1 from [80℄ demonstrates, the
masses of dark matter parti
le
andidates and their
intera
tion strengths with Standard Model parti
les
over a huge region of parameter spa
e. This results
in a large variety of observational methods developed
to sear
h for dark matter parti
les.
The spe
i�
example
onsidered in this review is
the radiatively de
aying dark matter. If a dark mat-
ter parti
le intera
ts with ele
tri
ally
harged parti-
les, it may
1
possess a radiative de
ay
hannel. If a
non-relativisti
dark matter parti
le de
ays to a pho-
ton and another parti
le, a slight (v/c . 5 × 10−3
)
Doppler broadening due to non-zero velo
ities of
dark matter parti
les in halos would
ause a narrow
dark matter de
ay line. Su
h a de
ay line possesses
several spe
i�
features allowing to robustly distin
t
it from the emission lines of astrophysi
al origin (see
e. g. [64, 164℄) or from instrumental line-like features:
� its position in energy is solely determined by
the mass of the dark matter parti
le and the
redshift of the dark matter halo (i. e. if one
negle
ts the mass of other de
ay produ
ts, the
line position is
m
dm
c2
2(1 + z)
), having di�erent s
al-
ing with the halo redshift z as the instrumental
line-like features;
� its intensity is proportional to the dark mat-
ter
olumn density S
dm
=
∫
ρ
dm
dℓ; due to the
di�erent 3D distributions of the dark and vis-
ible matter,
omparison of the new line in-
tensity within the given obje
t � as well as
among among di�erent obje
ts � would allow
to
hoose between its de
aying dark matter and
astrophysi
al origins;
� it is broadened with the
hara
teristi
velo
ity
of the dark matter � di�erent from that of vis-
ible matter.
∗
iakubovskyi�nbi.ku.dk
1
The widely-known examples where this is not the
ase are the dark matter parti
les as the lightest parti
les holding a new quantum
number
onserved by the Standard Model intera
tions, su
h as R-parity for super-symmetri
models, Kaluza-Klein number for extra di-
mensions, et
. In this
ase, the dark matter de
ays are stri
tly forbidden by the spe
ial stru
ture of the theory, and the main astrophysi
al
e�e
t for the dark matter parti
les is their annihilation with their antiparti
les.
3
Advan
es in Astronomy and Spa
e Physi
s D. A. Iakubovskyi
5
6
7
F
lu
x
(
c
n
ts
s
-1
k
e
V
-1
)
-0.2
-0.1
0
0.1
0.2
0.3
R
e
s
id
u
a
ls
3 3.2 3.4 3.6 3.8 4
Energy (keV)
300
305
310
315
E
ff
.
A
re
a
(
c
m
2
)
XMM - MOS
Perseus
(with core)
317 ks
Fig. 1: The
ombined MOS spe
trum of the Perseus
lus-
ter s
aled to the 3-4 keV energy range. On top of the their
best-�t model, the series of the single-bin residuals
orre-
sponding to the extra emission line at 3.57 keV are shown
in red. (Adapted from Figure 7 in [45℄).
0.22
0.24
0.26
0.28
0.30
0.32
0.34
0.36
N
o
r
m
a
l
i
z
e
d
c
o
u
n
t
r
a
t
e
[
c
t
s
/
s
e
c
/
k
e
V
]
M31 ON-center
No line at 3.5 keV
-4⋅10-3
-2⋅10-3
0⋅100
2⋅10-3
4⋅10-3
6⋅10-3
8⋅10-3
1⋅10-2
3.0 3.2 3.4 3.6 3.8 4.0
D
a
t
a
-
m
o
d
e
l
[
c
t
s
/
s
e
c
/
k
e
V
]
Energy [keV]
No line at 3.5 keV
Line at 3.5 keV
Fig. 2: The same as in Figure 1 but for the
ombined
spe
trum of Andromeda galaxy. (Adapted from Figure 1
in [39℄).
The above-mentioned
hara
teristi
s allow to di-
re
tly dete
t the radiatively de
aying dark matter re-
lying on the astrophysi
al measurements. This moti-
vates the extensive sear
h for the new lines in X-ray
spe
tra of
osmi
obje
ts proposed about 15 years
ago [2, 3, 67℄, see Table 1. An example is the analy-
sis of the line
andidate at ∼2.5 keV initially reported
in [112℄ in the X-ray spe
trum of the Willman 1 dwarf
spheroidal at 2.5σ level. Further non-observation of
this line
andidate in the
entral part and outskirts of
the Andromeda galaxy, Fornax and S
ulptor dwarf
spheroidal galaxies [41℄ ex
ludes the de
aying dark
matter origin of the ∼2.5 keV signal at a high sig-
ni�
an
e level (above 14σ). This result is further
strengthened by the authors of [128℄ who reanalysed
the same observations of Willman 1 as [112℄ (and did
not �nd the ∼2.5 keV line) and the authors of [127℄
who analysed another dwarf spheroidal, Segue 1. Fi-
nally, [113℄ ruled out the dark matter origin of the
∼2.5 keV feature by observing Willman 1 with better
statisti
s. The probable origin of the ∼2.5 keV line,
a
ording to [41℄, is purely instrumental, being the
result of under-modelling of the time-variable soft
proton ba
kground (see e. g. [106℄) in some observa-
tions
ombined with an apparent dip at ∼2.5 keV in
the e�e
tive area of existing X-ray instruments.
observational eviden
e
for the line at ∼3.5 keV
The new emission line at ∼3.5 keV was reported
by two di�erent groups [39, 45℄ in February 2014.
In [45℄, the authors
ombine X-ray emission from the
sample of nearby galaxy
lusters observed by the Eu-
ropean Photon and Imaging Camera (EPIC) [167,
172℄ on-board the XMM-Newton X-ray
osmi
ob-
servatory [95℄ with the largest number of
ounts
(>105
ounts for redshifts z < 0.1 and >104
ounts
for redshifts 0.1 < z < 0.4). The sta
king is made
in the
luster's rest frame. As a result, the emis-
sion from the instrumental lines is smeared out,
while
osmi
lines appear more prominent. This
method allows [45℄ to dete
t 28 emission lines of as-
trophysi
al origin in 2-10 keV band, mu
h more than
in individual galaxy
lusters, see e. g. [61℄. Apart
from them, [45℄ identi�es the new line lo
ated at
3.57±0.02 keV in XMM-Newton/MOS [167℄
ameras
and at 3.51 ± 0.03 keV in XMM-Newton/PN [172℄
amera at the level & 10 larger than predi
ted from
the two
omplexes of nearby astrophysi
al emis-
sion lines lo
ated at 3.51 keV (Kxviii) and 3.62 keV
(Arxvii). The new line is also dete
ted at > 3σ
lo
al signi�
an
e in several di�erent sub-samples of
their
ombined XMM-Newton/EPIC
luster dataset,
see e. g. Fig. 1, and in Chandra/ACIS spe
trum of
Perseus
luster, see Table 2 for details.
In [39℄ the new line at 3.53± 0.03 keV in the
en-
tral part of Andromeda galaxy (see Fig. 2) and in the
outskirts of Perseus
luster is dete
ted, see Table 2.
[39℄ ex
luded the
entral part of the Perseus
luster
(analysed in [45℄) be
ause of its rather
omplex stru
-
ture in X-rays, so the two datasets used in [39, 45℄
are totally independent enhan
ing the statisti
al sig-
ni�
an
e for the new line. Another important result
of [39℄ is the radial dependen
e of the new line �ux in
Perseus that appears more
onsistent with the de
ay-
ing dark matter pro�le than with the astrophysi
al
emission.
4
Advan
es in Astronomy and Spa
e Physi
s D. A. Iakubovskyi
1
10
0.01 0.1
Li
ne
fl
ux
, 1
0-6
p
ho
to
ns
c
m
-2
s
-1
Projected mass density, MSun/pc2
GC
M31
Perseus
Blank-sky
τ DM
= 6 x
10
27 s
τ DM
= 8 x
10
27 s
τ DM
= 2 x
10
27 s
τ DM
= 1.8 x
10
28 s
1
10
0.01 0.1
Li
ne
fl
ux
, 1
0-6
p
ho
to
ns
c
m
-2
s
-1
Projected mass density, MSun/pc2
GC
M31
Perseus
Blank-sky
τ DM
= 6 x
10
27 s
τ DM
= 8 x
10
27 s
τ DM
= 2 x
10
27 s
τ DM
= 1.8 x
10
28 s
1
10
0.01 0.1
Li
ne
fl
ux
, 1
0-6
p
ho
to
ns
c
m
-2
s
-1
Projected mass density, MSun/pc2
GC
M31
Perseus
Blank-sky
τ DM
= 6 x
10
27 s
τ DM
= 8 x
10
27 s
τ DM
= 2 x
10
27 s
τ DM
= 1.8 x
10
28 s
1
10
0.01 0.1
Li
ne
fl
ux
, 1
0-6
p
ho
to
ns
c
m
-2
s
-1
Projected mass density, MSun/pc2
GC
M31
Perseus
Blank-sky
τ DM
= 6 x
10
27 s
τ DM
= 8 x
10
27 s
τ DM
= 2 x
10
27 s
τ DM
= 1.8 x
10
28 s
Fig. 3: The �ux of the ∼3.5 keV line from the Gala
ti
Centre, the Perseus
luster outskirts, the Andromeda
galaxy, and the `blank sky' dataset [39℄ as a fun
tion of
the dark matter proje
ted mass. Diagonal lines show the
expe
ted behaviour of the de
aying dark matter signal
for a given dark matter parti
le lifetime. The verti
al
sizes of the boxes are ±1σ statisti
al error on the line's
�ux � or the 2σ upper bound for the blank-sky dataset.
The blue shaded regions show a parti
ular Navarro-
Frenk-White [132, 133℄ pro�le of the Milky Way [163℄,
its horizontal size indi
ates un
ertainties in the gala
ti
disk modelling. The lifetime τ
dm
∼ (6−8)×1027 s is
on-
sistent with all datasets. New results from a prolonged
Dra
o XMM-Newton/EPIC observation [98, 153℄ give
ontroversial results: while [98℄ reports an ex
lusion of
dark matter hypothesis at 99% level, the results of [153℄
laim that the values of τ
dm
≃ (7− 9)× 1027 se
are still
onsistent with all existing observations. (Adapted from
Figure 2 in [27℄).
The en
ouraging results of [39, 45℄ have stimu-
lated several groups to look at the other dark matter-
dominated obje
ts. The following sear
hes report
the presen
e of the line at ∼3.5 keV, see Table 3:
1. The identi�
ation of the line at ∼3.5 keV from
the region of the Gala
ti
Centre [27, 49, 97,
145℄. Although it is un
lear whether the de-
te
ted line has an astrophysi
al origin (see the
next se
tion for detailed dis
ussion), its expla-
nation in terms of de
aying dark matter is
on-
sistent with the previous new line dete
tions,
see [27, 115℄ for details.
2. The dete
tion of the new line in Suzaku/XIS
observations of the Perseus, Coma and Ophi-
u
hus galaxy
lusters [173℄. While the subse-
quent study of Suzaku/XIS spe
tra in [169℄ did
not reveal the new line at ∼3.5 keV in the
en-
tral part of the Perseus
luster, another re
ent
study in [77℄ does; however, its apparent dis-
repan
y with the negative result of [169℄ is still
un
lear and has to be resolved further.
3. The dete
tion of the new line at 3.52±0.08 keV
observed in the X-ray spe
tra of 8 individual
nearby galaxy
lusters in
luding Perseus and
Coma [93℄.
1
10
0.01 0.1
Li
ne
fl
ux
, 1
0-6
p
ho
to
ns
c
m
-2
s
-1
Projected mass density, MSun/pc2
τ DM
= 6 x
10
27 s
τ DM
= 2 x
10
27 s
τ DM
= 1.8 x
10
28 s
Fig. 4: The same as in Fig. 3 but over-plotted are the
ranges for the > 2σ dete
tions in MOS (green) and PN
(magenta)
ameras, see [93℄. (Adapted from Figure 2
in [93℄).
In summary, positive dete
tions of the new line
listed in Tables 2 and 3 support the hypothesis of the
radiatively de
aying dark matter implying its life-
time is τ
dm
≃ (6− 8)× 1027 s [27, 93, 153℄.
On the
ontrary, the following studies do not de-
te
t the ∼3.5 keV line putting the upper bounds on
its �ux:
1. The
entral part of the Virgo
luster observed
by Chandra/ACIS [45℄, Suzaku/XIS[173℄ and
XMM -Newton/EPIC [93℄, as well as other 10
galaxy
lusters from [93℄.
2. Combined spe
trum from dwarf spheroidal
galaxies [120℄.
3. Outskirts of galaxies [9, 39, 91℄.
4. Combined blank-sky observations [39, 159℄.
5. Prolonged XMM-Newton/EPIC observations of
the Dra
o dwarf spheroidal galaxy [98, 153℄;
although [153℄ reports a line-like ex
ess at
3.54±0.06 keV with ∆χ2 = 5.3 in PN
amera,
see Table 2, this �nding is not supported by an
independent analysis of [98℄ and is not a
om-
panied with a similar ex
ess in Dra
o spe
tra
seen by MOS
amera [98, 153℄.
At the moment, it is un
lear whether these negative
sear
hes rule out the de
aying dark matter hypoth-
esis for this new line. While the bounds obtained
in [120℄ are mildly
onsistent with the de
aying dark
5
Advan
es in Astronomy and Spa
e Physi
s D. A. Iakubovskyi
matter origin of the dete
tions in [39, 45℄, the re-
sults of [9℄ formally ex
lude the de
aying dark mat-
ter hypothesis of the origin of the ∼3.5 keV line im-
posing a very stri
t 3σ bound, τ
dm
> 1.8 × 1028 s.
However, taking into a
ount the systemati
e�e
ts
in the spe
tra (e. g.
ausing signi�
ant negative
residuals) obtained in [9℄ and the apparent un
er-
tainty in the used dark matter distributions [40℄
would result in mu
h weaker bound. For instan
e,
τ
dm
& 3.5 × 1027 s is reported in [94℄ using the
sta
ked dataset of nearby galaxies with
ompara-
ble exposure from [91℄ � still
onsistent with ex-
isting dete
tions. The un
ertainty in the dark mat-
ter distributions also helps to re
on
ile the results
of the other negative sear
hes [89, 159, 182℄ with
the ∼3.5 keV line dete
tions using the de
aying dark
matter paradigm. There is also no
larity with
the new prolonged (∼ 1.4 Ms) XMM-Newton/EPIC
observation of the Dra
o dwarf spheroidal galaxy
� the obje
t having both well-measured dark mat-
ter distribution [82℄ and proven low X-ray ba
k-
ground [99, 115, 120, 146℄. While [98℄ reports an ex-
lusion of dark matter hypothesis at 99% level having
2σ upper bound on the radiative dark matter de
ay
lifetime of τ
dm
> 2.7 × 1028 s, the results of [153℄
suggest τ
dm
≃ (7− 9)× 1027 s � the value still
om-
patible with all existing observations.
�standard� explanations
of the line at ∼3.5 keV
There are three possible �standard� explanations
of the new line dete
tions at ∼3.5 keV:
1. statisti
al �u
tuations;
2. general-type systemati
e�e
ts;
3. astrophysi
al emission line.
With re
ent in
rease of positive dete
tions re-
ported by [93℄, it is very hard to explain all of the
dete
tions with pure statisti
al �u
tuations. Nev-
ertheless, statisti
al �u
tuations may be responsible
for the new line dete
tions or non-dete
tions in some
individual obje
ts, as well as for variations of the de-
te
ted line position up to ∼110 eV [93℄, see Fig. 5 �
the e�e
t that should be properly taken into a
ount
when sear
hing for the new line (unlike [9, 120, 173℄).
The systemati
origin of the line is
arefully in-
vestigated be
ause of the previous study of the line-
like residual at ∼2.5 keV in the Willman 1 dwarf
spheroidal, see the `Introdu
tion' se
tion for details.
However, the explanation of the ∼3.5 keV line with
the general-type systemati
s suggested in [97℄ is un-
likely. For example, its position (in the frame of
emitting obje
t) remains remarkably
onstant with
the redshift [39, 45, 93℄, see Fig. 5, whi
h
annot be
explained by simple systemati
s. The line is also
independently dete
ted by �ve dete
tors on-board
three
osmi
missions, see Tables 2 and 3. Finally,
similar feature of systemati
origin should have been
dete
ted in the blank-sky dataset [39℄, and should
have di�erent radial behaviour in the outskirts of the
Perseus
luster [39, 77℄.
3.35
3.4
3.45
3.5
3.55
3.6
3.65
3.7
3.75
0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045 0.05 0.055 0.06
Li
ne
p
os
iti
on
[k
eV
]
NED redshift
MOS
PN
Fig. 5: The position of new line dete
ted in [93℄ (in the
frame of emitting galaxy
luster) as a fun
tion of the
luster redshift. The red and bla
k dashed lines show
the expe
ted behaviour in
ase of purely systemati
and
osmi
line origins (assuming the line position 3.52 keV
in the dete
tor frame expe
ted from [39, 27℄), respe
-
tively (Adapted from Figure 3 in [93℄).
On the other hand, the explanation of the new
line with the Kxviii line
omplex at ∼3.5 keV sug-
gested by [97℄ (see also an extensive dis
ussion
in [28, 45, 46, 96℄) is still possible, at least for
the Gala
ti
Centre region and galaxy
lusters,
on-
trary to the initial
laims of [39, 45℄. The rea-
son is that the emission �ux from the Kxviii line
omplex at ∼3.51 keV suggested by [97℄ is highly
un
ertain due to large un
ertainties of the Potas-
sium abundan
e, see e. g. [139, 150℄ for a potential
2
level of un
ertainty. Moreover, unlike other possible
emission lines of astrophysi
al origin near ∼3.5 keV
(su
h as Clxvii lines at 3.51 keV found largely sub-
dominant in the Gala
ti
Centre region [97℄ and in
galaxy
lusters [46℄), Kxviii line
omplex does not
have stronger
ounterparts at other energies and
an
hardly be ex
luded by the measurements of other
lines, the strongest of them is the Kxix line
om-
2
The results of [139℄ indi
ate an order of magnitude over-abundan
e of Potassium in the solar
orona
ompared to the solar photo-
sphere. Based on this result, [139℄ suggested that the Potassium abundan
e in hot plasma in galaxies and galaxy
lusters may have also
been enhan
ed
ompared to the solar photospheri
values. However, be
ause at the moment there is no established me
hanism that
ould
e�e
tively provide su
h an enhan
ement, the results of [139℄ only indi
ate the potential level of un
ertainty, similar to the measurements
in [150℄.
6
Advan
es in Astronomy and Spa
e Physi
s D. A. Iakubovskyi
plex at 3.71 keV of
omparable strength [92℄. The
same is true about the
harge ex
hange of Sxvi ions
re
ently suggested by [86℄.
An alternative approa
h is to study the line mor-
phology. At the moment, two di�erent methods have
been used. The �rst method [39, 45℄ is to split
the region
overed by some astrophysi
al sour
es
into several independent subregions, large enough
to dete
t the line in ea
h of them, and to model
their spe
tra separately looking for a line-like ex-
ess in ea
h of subregions. As a result, in [45℄ it
was shown that the ∼3.5 keV line in the Perseus
luster is somewhat more
on
entrated
ompared to
de
aying dark matter distributed a
ording to the
Navarro-Frenk-White [132, 133℄ pro�le. By studying
the ∼3.5 keV line emission from the Perseus
luster
outskirts, in [39℄ it was obtained that su
h distribu-
tion is better
onsistent with the radiatively de
ay-
ing dark matter distributed a
ording to the well-
established Navarro-Frenk-White pro�le than with
the astrophysi
al
ontinuum emission distributed a
-
ording to the isothermal β-model of [50℄. The re
ent
detailed study [77℄
on�rms this result and expands
it to the
entral region of the Perseus
luster.
0
20
40
60
80
100
3400 3450 3500 3550 3600 3650 3700 3750 3800
B
ro
ad
en
ed
li
ne
e
m
is
si
vi
ty
[a
rb
. u
ni
ts
]
Energy [eV]
3 x K lines, GC model 3, Astro-H/SXS
1 x Ar lines, GC model 3, Astro-H/SXS
1/3 x S lines, GC model 3, Astro-H/SXS
1 x Cl lines, GC model 3, Astro-H/SXS
Fig. 6: Line emissivities (in arbitrary units) broadened
with energy resolution of Soft X-ray Spe
trometer (SXS)
on-board Hitomi (former Astro-H ), σSXS = 5 eV, as
fun
tions of energy for three-
omponent model of [97℄
of Gala
ti
Centre. The relative S, Ar, Cl and K abun-
dan
es are set to 1/3 : 1 : 1 : 3, a
ording to Se
. 2.2
of [97℄. Thin dashed line shows the total line emissivity
(Adapted from Figure 2 in [92℄).
The se
ond method to study the line morphol-
ogy [49℄ deals with the spatial distribution of the
`line plus
ontinuum' X-ray emission in the Perseus
luster and the Gala
ti
Centre region with further
eliminating the
ontinuum
omponent by either as-
suming it is spatially smooth or
ross-
orrelating
the `line plus
ontinuum' images in several energy
bands (in
luding those dominated by the astrophys-
i
al line emission). By using the se
ond method,
the authors of [49℄ show that adding the de
aying
dark matter distribution from a smooth dark mat-
ter pro�le (Navarro-Frenk-White, Einasto, Burkert)
does not improve the �t quality in both obje
ts, and
demonstrate that distribution of the events in the
3.45�3.6 keV bands
orrelates with that in the en-
ergy bands of strong astrophysi
al emission, rather
than with that in line-free energy bands. Based on
these �ndings, [49℄
laims the ex
lusion of the de-
aying dark matter origin of 3.5 keV in the Gala
ti
Centre and the Perseus
luster.
To ultimately
he
k the astrophysi
al origin of the
∼3.5 keV line, new observations with high-resolution
imaging
3
spe
trometers su
h as Soft X-ray Spe
-
trometer (SXS) [130℄ on-board the re
ently laun
hed
Hitomi
4
(former Astro-H ) mission [168℄, Mi
ro-X
sounding ro
ket experiment [73℄ and the X-ray In-
tegral Field Unit (X-IFU) [14, 142℄ on-board the
planned Athena mission [15, 131℄, are planned. If the
position of the new line in
identally
oin
ides with
that of the Kxviii line
omplex, a more detailed
study of the ratios of the Potassium line emissivities
will be essential to �nally
he
k the astrophysi
al ori-
gin of the new line, see Fig. 6 for details.
other extra line
andidates in X-ray range
Although the line at ∼3.5 keV re
eives the largest
attention of the
ommunity, there are three other line
andidates in X-rays whi
h origin is also not estab-
lished:
1. A
ording to [141℄, intensity of the Fexxvi Lyγ
line at 8.7 keV observed in Suzaku/XIS spe
-
trum of the Milky Way
entre [103℄
annot be
explained by the standard ionization and re-
ombination pro
esses and dark matter de
ay
may be a possible explanation of this ex
ess.
2. A
ording to Se
. 1.4 of [104℄, two faint extra
line-like ex
esses at 9.4 and 10.1 keV are de-
te
ted in the
ombined Suzaku/XIS spe
trum
of the Gala
ti
Bulge region. Notably, posi-
tions of these ex
esses do not
oin
ide with any
bright
5
astrophysi
al or instrumental line and
their intensities
an be explained in frames of
de
aying dark matter origin, see right Fig. 8
of [104℄.
3
Grating spe
trometers su
h as Chandra/HETGS [48℄ have ex
ellent spe
tral resolution for point sour
es; however, for extended
(&1 ar
min) sour
es their spe
tral resolution usually degrades to that of existing imaging spe
trometers, see e. g. [65℄.
4
Although Hitomi is now broken apart, it had observed Perseus
luster before be
oming non-operational [88, 101℄.
5
The newest available atomi
database AtomDB v.3.0.2 [75℄
ontains several faint Nixxvi � Nixxviii emission lines at 10.02-10.11 keV.
7
Advan
es in Astronomy and Spa
e Physi
s D. A. Iakubovskyi
possible impli
ations
for new physi
s
If none of the �
onventional� explanations dis-
ussed in the previous se
tions were valid, the exis-
ten
e of the new line at ∼3.55 keV would be an indi-
ation of a new physi
s beyond the Standard Model.
Histori
ally, the �rst model dis
ussed in
onne
-
tion with ∼3.5 keV dete
tion is the neutrino minimal
extension of the Standard Model with three right-
handed (sterile) neutrinos (the νMSM) [12, 43℄. In
this model, the lightest sterile neutrino with the mass
in keV range forms the bulk of dark matter while the
two heavier sterile neutrinos are responsible for the
two other established phenomena beyond the Stan-
dard Model � neutrino os
illations and generation
of asymmetry between baryons and anti-baryons in
early Universe. Sterile neutrinos de
ay possesses the
2-body radiative
hannel N → γ + ν, so the obser-
vation of ∼3.5 keV de
ay line would imply the ex-
isten
e of light sterile neutrino dark matter parti-
les with mass ∼7.1 keV. The simplest produ
tion
s
enario of sterile neutrino dark matter � via the
non-resonant os
illations of the usual (a
tive) neu-
trinos in the early Universe [1, 2, 3, 11, 66, 67℄ �
is already ex
luded by the
ombination of the X-
ray measurements [30℄, measurements of the Lyα
forest [31, 32, 160, 176, 177, 178℄ and the phase-
spa
e bound from dwarf spheroidal galaxies [10, 38,
85, 161, 171℄. The realisti
s
enario of the dark
matter produ
tion within the νMSM now involves
the resonant os
illations of a
tive neutrinos in hot
primordial plasma with the signi�
ant lepton asym-
metry generated by de
ays of heavier sterile neutri-
nos [4, 83, 108, 162, 174℄. The parameters of the ob-
served ∼3.5 keV line are
onsistent with the νMSM
predi
tions, see Fig. 7 for details. Be
ause the inter-
a
tion of sterile neutrino dark matter with the Stan-
dard Model parti
les is orders of magnitude weaker
than that of ordinary neutrinos, its prospe
ts for di-
re
t dete
tion in a parti
le physi
s experiment are
very far from the existing experimental te
hnique,
see [6, 68, 110, 111, 126℄. To
on�rm the νMSM,
a sear
h for heavier sterile neutrinos in the GeV
range is needed, handled by e. g. the planned Sear
h
for Hidden Parti
les (SHiP) experiment [7, 21℄ and
the Future ele
tron-positron e
+
e
−
Cir
ular Collider
(FCC-ee) [19℄.
However, the
on�rmation of the de
aying dark
matter origin of the new line does not imply the ex-
isten
e of νMSM sterile neutrinos as there are plenty
of other alternatives whi
h
an potentially explain
the ∼3.55 keV line, see e. g. [6, 27, 94℄ and the refer-
en
es therein. Di�eren
es among these models
an
be further probed by:
�
hanges in the new line morphology be
ause of
the non-negligible initial dark matter velo
ities,
see e. g. [117, 119℄;
� other astrophysi
al and
osmologi
al tests, see
e. g. [4, 23, 24, 25, 38, 44, 90, 100, 109, 116, 118,
124, 154, 156, 157, 175, 180℄;
� sear
h for the �smoking gun� signatures in the
future dedi
ated parti
le physi
s experiments,
su
h as SHiP [7, 21℄ and FCC-ee [19℄ experi-
ments.
Re
ently proposed alternatives to the radiatively
de
aying dark matter in
lude: the de
ay of ex
ited
dark matter states [18, 20, 53, 54, 55, 63, 74, 138,
158℄, annihilating dark matter [13, 69, 76, 121℄,
dark matter de
aying into the axion-like parti
les
with further
onversion to photons in a magneti
�eld [8, 16, 52, 57, 58℄. These models predi
t the
substantial di�eren
e in the ∼3.5 keV line morphol-
ogy
ompared to the radiatively de
aying dark mat-
ter. For example, the spatial distributions of the new
line in these models should be more
on
entrated to-
wards the
entres of the dark matter-dominated ob-
je
ts
ompared to radiatively de
aying dark matter,
e. g. due to larger dark matter density (for ex
ited
and annihilating dark matter) or larger magneti
�elds (for magneti
�eld
onversion of axion-like par-
ti
les). Further non-observation of the ∼3.5 keV line
in the outskirts of the dark matter-dominated ob-
je
ts would argue in favour of these models.
I
n
t
e
r
a
c
t
i
o
n
s
t
r
e
n
g
t
h
S
i
n
2
(
2
θ)
Dark matter mass MDM [keV]
10-13
10-12
10-11
10-10
10-9
10-8
10-7
2 5 50 1 10
DM overproduction
Not enough DM
T
r
e
m
a
i
n
e
-
G
u
n
n
/
L
y
m
a
n
-
α Excluded by X-ray observations
I
n
t
e
r
a
c
t
i
o
n
s
t
r
e
n
g
t
h
S
i
n
2
(
2
θ)
Dark matter mass MDM [keV]
10-13
10-12
10-11
10-10
10-9
10-8
10-7
2 5 50 1 10
DM overproduction
Not enough DM
T
r
e
m
a
i
n
e
-
G
u
n
n
/
L
y
m
a
n
-
α Excluded by X-ray observations
I
n
t
e
r
a
c
t
i
o
n
s
t
r
e
n
g
t
h
S
i
n
2
(
2
θ)
Dark matter mass MDM [keV]
10-13
10-12
10-11
10-10
10-9
10-8
10-7
2 5 50 1 10
DM overproduction
Not enough DM
T
r
e
m
a
i
n
e
-
G
u
n
n
/
L
y
m
a
n
-
α Excluded by X-ray observations
I
n
t
e
r
a
c
t
i
o
n
s
t
r
e
n
g
t
h
S
i
n
2
(
2
θ)
Dark matter mass MDM [keV]
10-13
10-12
10-11
10-10
10-9
10-8
10-7
2 5 50 1 10
DM overproduction
Not enough DM
T
r
e
m
a
i
n
e
-
G
u
n
n
/
L
y
m
a
n
-
α Excluded by X-ray observations
I
n
t
e
r
a
c
t
i
o
n
s
t
r
e
n
g
t
h
S
i
n
2
(
2
θ)
Dark matter mass MDM [keV]
10-13
10-12
10-11
10-10
10-9
10-8
10-7
2 5 50 1 10
DM overproduction
Not enough DM
T
r
e
m
a
i
n
e
-
G
u
n
n
/
L
y
m
a
n
-
α Excluded by X-ray observations
I
n
t
e
r
a
c
t
i
o
n
s
t
r
e
n
g
t
h
S
i
n
2
(
2
θ)
Dark matter mass MDM [keV]
10-13
10-12
10-11
10-10
10-9
10-8
10-7
2 5 50 1 10
DM overproduction
Not enough DM
T
r
e
m
a
i
n
e
-
G
u
n
n
/
L
y
m
a
n
-
α Excluded by X-ray observations
Fig. 7: Constraints on sterile neutrino dark matter
within the νMSM model [12, 43℄. In every point in
the white region sterile neutrinos
onstitute 100% of
dark matter and their properties agree with the existing
bounds. The blue point
orresponds to the observed line
from the Andromeda galaxy, while the error bars indi-
ate the statisti
al errors (thi
k) and un
ertainty in the
dark matter distribution at the
entral part of the An-
dromeda galaxy (thin) (Adapted from Figure 4 in [39℄).
on
lusion and future dire
tions
The origin of the new emission line at ∼3.5 keV
reported in [27, 39, 45, 93, 173℄ remains unexplained.
The observed properties of the new line are
onsis-
tent with the radiatively de
aying dark matter as
well as the other interesting s
enarios (su
h as ex
it-
ing dark matter, annihilating dark matter and the
8
Advan
es in Astronomy and Spa
e Physi
s D. A. Iakubovskyi
dark matter de
aying into axion-like parti
les fur-
ther
onverted in
osmi
magneti
�elds) motivated
by various parti
le physi
s extensions of the Stan-
dard Model. In
ase of the radiatively de
aying dark
matter, further dete
tions would lead to the dire
t
dete
tion of the new physi
s. Spe
ially dedi
ated ob-
servations using the existing X-ray missions (su
h as
XMM-Newton, Chandra, Suzaku) still allow for su
h
dete
tions although one should take spe
ial
are of
the various systemati
e�e
ts that
ould mimi
or
hide the new line.
The alternative is to use new better instruments.
The basi
requirements for su
h instruments �
higher grasp (the produ
t of �eld-of-view and ef-
fe
tive area) and better spe
tral resolution � were
�rst formulated in [26℄. Both the soft X-ray Spe
-
trometer [130℄ on-board the new X-ray mission Hit-
omi (former Astro-H ) [102, 168℄ and the planned
Mi
ro-X sounding ro
ket experiment [73℄ meet only
the se
ond requirement having the energy resolution
by an order of magnitude better (∼ 5 eV) than ex-
isting imaging spe
trometers. Before being broken
apart, Hitomi has already observed the Perseus
lus-
ter [101℄. It was expe
ted [45℄ that su
h an observa-
tion would have allowed Hitomi to pre
isely deter-
mine the new line position in the brightest obje
ts
with the prolonged observations and to dete
t the
Kxix emission line
omplex at ∼3.71 keV. Another
possible option is to resolve the intrinsi
width of
the new line be
ause of its Doppler broadening in
galaxies and galaxy
lusters [45, 166℄. As a result,
Hitomi/SXS is able to
he
k whether the new line
omes from the new physi
s or from the (anoma-
lously enhan
ed) astrophysi
al emission. The same
is expe
ted from the Mi
ro-X ro
ket-based mi
ro-
alorimeter (to be laun
hed in 2017) whi
h will ob-
serve the
entral region of our Galaxy. Another pos-
sibility is to use the planned eROSITA instrument
on-board Spektrum-Röntgen-Gamma mission [125℄
and the planned LOFT mission [187℄ whose high
grasp and moderate energy resolution would allow
to dete
t the new line at mu
h smaller intensi-
ties [134, 186℄. Finally, an �ultimate� imaging spe
-
trometer proposed in e. g. [29℄ (an example is the X-
ray Integral Field Unit (X-IFU) [14, 142℄ on-board
the planned Athena mission [15, 131℄) would reveal
the detailed morphology stru
ture of the ∼3.5 keV
line [135℄.
a
knowledgement
This work was supported by a resear
h grant from
VILLUM FONDEN. The author also a
knowledges
partial support from the Swiss National S
ien
e
Foundation grant SCOPE IZ7370-152581, the Pro-
gram of Cosmi
Resear
h of the National A
ademy
of S
ien
es of Ukraine, the State Fund for Fundamen-
tal Resear
h of Ukraine and the State Programme of
Implementation of the Grid Te
hnology in Ukraine
during the early stages of this work.
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Table 1: Summary of sear
hes for dark matter de
ay line in X-ray observations
ondu
ted so far. This Table is an
update of Table 1 in [134℄.
Ref. Obje
t Instrument Cleaned exposure, ks
[34℄ Di�use X-ray ba
kground HEAO-1, XMM-Newton/EPIC 224, 1450
[35℄ Coma, Virgo XMM-Newton/EPIC 20, 40
[36℄ Large Magellani
Cloud XMM-Newton/EPIC 20
[147℄ Milky Way Chandra/ACIS-S3 Not spe
i�ed
[181℄ M31 (
entral 5′) XMM-Newton/EPIC 35
[148℄ Abell 520 Chandra/ACIS-S3 67
[37℄ Milky Way, Ursa Minor XMM-Newton/EPIC 547, 7
[5℄ Milky Way Chandra/ACIS 1500
[42℄ 1E 0657-56 (�Bullet
luster�) Chandra/ACIS-I 450
[26℄ Milky Way X-ray mi
ro-
alorimeter 0.1
[185℄ Milky Way INTEGRAL/SPI 5500
[30℄ M31 (
entral 5− 13′) XMM-Newton/EPIC 130
[33℄ Milky Way INTEGRAL/SPI 12200
[114℄ Ursa Minor Suzaku/XIS 70
[146℄ Dra
o Chandra/ACIS-S 32
[112℄ Willman 1 Chandra/ACIS-I 100
[41℄ M31, Fornax, S
ulptor XMM-Newton/EPIC , Chandra/ACIS 400, 50, 162
[128℄ Willman 1 Chandra/ACIS-I 100
[127℄ Segue 1 Swift/XRT 5
[22℄ M33 XMM-Newton/EPIC 20-30
[182℄ M31 (12− 28′ o�-
entre) Chandra/ACIS-I 53
[113℄ Willman 1 XMM-Newton/EPIC 60
[107℄ Ursa Minor, Dra
o Suzaku/XIS 200, 200
[91℄ Sta
ked galaxies XMM-Newton/EPIC 8500
[89℄ M31 Chandra/ACIS-I 404
[120℄ Sta
ked dSphs XMM-Newton/EPIC 410
[9℄ Sta
ked galaxies XMM-Newton/EPIC, Chandra/ACIS-I 14600, 15000
[169℄ Perseus Suzaku/XIS 520
[90, 136℄ Milky Way Fermi/GBM 4600
[159℄ Milky Way Suzaku/XIS 31500
[165℄ Dra
o XMM-Newton/EPIC 87
[149℄ 1E 0657-56 (�Bullet
luster�) NuSTAR 266
[98℄ Dra
o XMM-Newton/EPIC 1660
13
Advan
es in Astronomy and Spa
e Physi
s D. A. Iakubovskyi
Table 2: Properties of the ∼3.5 keV line reported by [39, 45℄. For their analysis, the authors of [39, 45℄ use di�erent
X-ray datasets observed by MOS [172℄ and PN [167℄
ameras on-board XMM-Newton observatory [95℄ and ACIS
instrument [81℄ on-board Chandra observatory [183℄. All error bars are at 1σ (68%) level.
Ref. Obje
t Redshift Instrument Exposure, Line position, Line �ux,
Ms keV 10−6
ph/s/
m
2
[45℄ Full sta
ked sample 0.009-0.354 MOS 6 3.57±0.02 4.0±0.8
[45℄ Full sta
ked sample 0.009-0.354 PN 2 3.51±0.03 3.9
+0.6
−1.0
[45℄ Coma+Centaurus+Ophiu
hus 0.009-0.028 MOS 0.5 3.57
a
15.9
+3.4
−3.8
[45℄ Coma+Centaurus+Ophiu
hus 0.009-0.028 PN 0.2 3.57
a < 9.5 (90%)
[45℄ Perseus (< 12′) 0.016 MOS 0.3 3.57
a
52.0
+24.1
−15.2
[45℄ Perseus (< 12′) 0.016 PN 0.05 3.57
a < 17.7 (90%)
[45℄ Perseus (1− 12′) 0.016 MOS 0.3 3.57
a
21.4
+7.0
−6.3
[45℄ Perseus (1− 12′) 0.016 PN 0.05 3.57
a < 16.1 (90%)
[45℄ Rest of the
lusters 0.012-0.354 MOS 4.9 3.57
a
2.1
+0.4
−0.5
[45℄ Rest of the
lusters 0.012-0.354 PN 1.8 3.57
a
2.0
+0.3
−0.5
[45℄ Perseus (> 1′) 0.016 ACIS-S 0.9 3.56±0.02 10.2
+3.7
−3.5
[45℄ Perseus (< 9′) 0.016 ACIS-I 0.5 3.56
a
18.6
+7.8
−8.0
[45℄ Virgo (< 500′′) 0.003-0.004 ACIS-I 0.5 3.56
a < 9.1 (90%)
[39℄ M31 (< 14′) -0.001
b
MOS 0.5 3.53±0.03 4.9
+1.6
−1.3
[39℄ M31 (10− 80′) -0.001
b
MOS 0.7 3.50-3.56 < 1.8 (2σ)
[39℄ Perseus (23− 102′) 0.0179
b
MOS 0.3 3.50±0.04 7.0±2.6
[39℄ Perseus (23− 102′) 0.0179
b
PN 0.2 3.46±0.04 9.2±3.1
[39℄ Perseus, 1st bin (23− 37′) 0.0179
b
MOS 0.2 3.50
a
13.8±3.3
[39℄ Perseus, 2nd bin (42− 54′) 0.0179
b
MOS 0.1 3.50
a
8.3±3.4
[39℄ Perseus, 3rd bin (68− 102′) 0.0179
b
MOS 0.03 3.50
a
4.6±4.6
[39℄ Blank-sky � MOS 7.8 3.45-3.58 < 0.7 (2σ)
a
The line position is �xed at given value.
b
The redshift is �xed at NASA Extragala
ti
Database (NED) value.
14
Advan
es in Astronomy and Spa
e Physi
s D. A. Iakubovskyi
Table 3: Properties of∼3.5 keV line sear
hed after February 2014 in di�erent X-ray datasets observed by MOS [172℄ and
PN [167℄
ameras on-board XMM-Newton observatory [95℄, ACIS [81℄ instrument on-board Chandra observatory [183℄
and XIS instrument [105℄ on-board Suzaku observatory [129℄. All error bars are at 1σ (68%) level.
Ref. Obje
t Redshift Instrument Exposure, Line position, Line �ux,
Ms keV 10−6
ph/s/
m
2
[145℄ Gala
ti
entre (2.5− 12′) 0.0 ACIS-I 0.8 3.51 ≃ 10a
[97℄ Gala
ti
entre (0.3− 15′) 0.0 MOS 0.7 3.51 45± 4a
[97℄ Gala
ti
entre (0.3− 15′) 0.0 PN 0.5 3.51 39± 7a
[97℄ M31 0.0 MOS 0.5 3.53±0.07 2.1±1.5c
[27℄ Gala
ti
entre (< 14′) 0.0 MOS 0.7 3.539±0.011 29±5
[173℄ Perseus
ore (< 6′) 0.0179
b
XIS 0.74 3.510
+0.023
−0.008 32.5+3.7
−4.3
[173℄ Perseus
on�ned (6− 12.7′) 0.0179
b
XIS 0.74 3.510
+0.023
−0.008 32.5+3.7
−4.3
[173℄ Coma (< 12.7′) 0.0231
b
XIS 0.164 ≃ 3.45d ≃ 30d
[173℄ Ophiu
hus (< 12.7′) 0.0280
b
XIS 0.083 ≃ 3.45d ≃ 40d
[173℄ Virgo (< 12.7′) 0.0036
b
XIS 0.09 3.55
a < 6.5 (2σ)
[93℄ Abell 85 (< 14′) 0.0551
b
MOS 0.20 3.44
+0.06
−0.05 6.3
+3.9
−3.6
[93℄ Abell 2199 (< 14′) 0.0302
b
MOS 0.13 3.41
+0.04
−0.04 10.1
+5.1
−4.8
[93℄ Abell 496 (< 14′) 0.0329
b
MOS 0.13 3.55
+0.06
−0.09 7.5
+6.1
−4.4
[93℄ Abell 496 (< 14′) 0.0329
b
PN 0.08 3.45
+0.04
−0.03 16.8
+5.9
−6.4
[93℄ Abell 3266 (< 14′) 0.0589
b
PN 0.06 3.53
+0.04
−0.06 8.7
+5.1
−4.5
[93℄ Abell S805 (< 14′) 0.0139
b
PN 0.01 3.63
+0.05
−0.06 17.1
+9.3
−7.4
[93℄ Coma (< 14′) 0.0231
b
MOS 0.17 3.49
+0.04
−0.05 23.7
+10.7
−9.0
[93℄ Abell 2319 (< 14′) 0.0557
b
MOS 0.08 3.59
+0.05
−0.06 18.6
+10.7
−7.4
[93℄ Perseus (< 14′) 0.0179
b
MOS 0.16 3.58
+0.05
−0.08 25.2
+12.5
−12.6
[93℄ Virgo
e
(< 14′) 0.0036
b
PN 0.06 � < 9.3
[153℄ Dra
o (< 14′) 0.0 PN 0.65 3.54
+0.06
−0.05 1.65
+0.67
−0.70
[77℄ Perseus (< 8.3′) 0.0179
b
XIS 1.67 3.54±0.01 27.9
+3.5
−3.5
[77℄ Perseus (< 2′) 0.0179
b
XIS 1.67 3.51±0.02 9.3
+2.6
−2.7
[77℄ Perseus (2′ − 4.5′) 0.0179
b
XIS 1.67 3.55±0.02 16.7
+2.9
−3.0
[77℄ Perseus (4.5′ − 8.3′) 0.0179
b
XIS 1.67 3.58±0.02 16.1
+3.2
−3.4
[47℄ Sta
ked
lusters 0.01-0.45 XIS 8.1 3.54
f
1.0
+0.5
−0.5
a
Best-�t line �ux at the �xed position 3.51 keV
oin
ides with the brightest Kxviii line.
b
Redshift was �xed at the NASA Extragala
ti
Database (NED) value.
c
The line is dete
ted at < 90%
on�den
e level. Su
h a low �ux (
ompared with [39℄) is be
ause of non-physi
ally
enhan
ed level of
ontinuum in the 3-4 keV band used in [97℄, see [28℄ for details.
d
Parameters estimated from Fig. 3 of [173℄.
e
Gives an example of the new line non-dete
tion, see Table II of [93℄ for more details.
f
Line position is �xed at the best-�t energy dete
ted in Suzaku observations of the Perseus
luster by [77℄.
15
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