New emission line at ~3.5 keV - observational status, connection with radiatively decaying dark matter and directions for future studies
Recent works of Bulbul et al. (2014) and Boyarsky et al. (2014), claiming the detection of the extra emission line with energy ∼3.5 keV in X-ray spectra of certain clusters of galaxies and nearby Andromeda galaxy, have raised a considerable interest in astrophysics and particle physics communities....
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irk-123456789-1198052017-06-10T03:03:01Z New emission line at ~3.5 keV - observational status, connection with radiatively decaying dark matter and directions for future studies Iakubovskyi, D.A. Recent works of Bulbul et al. (2014) and Boyarsky et al. (2014), claiming the detection of the extra emission line with energy ∼3.5 keV in X-ray spectra of certain clusters of galaxies and nearby Andromeda galaxy, have raised a considerable interest in astrophysics and particle physics communities. A number of new observational studies claim detection or non-detection of the extra line in X-ray spectra of various cosmic objects. In this review I summarise existing results of these studies, overview possible interpretations of the extra line, including intriguing connection with radiatively decaying dark matter, and show future directions achievable with existing and planned X-ray cosmic missions. 2014 Article New emission line at ~3.5 keV - observational status, connection with radiatively decaying dark matter and directions for future studies / D.A. Iakubovskyi // Advances in Astronomy and Space Physics. — 2014. — Т. 4., вип. 1-2. — С. 9-14. — Бібліогр.: 98 назв. — англ. 2227-1481 DOI: 10.17721/2227-1481.4.9-14 http://dspace.nbuv.gov.ua/handle/123456789/119805 en Advances in Astronomy and Space Physics Головна астрономічна обсерваторія НАН України |
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Recent works of Bulbul et al. (2014) and Boyarsky et al. (2014), claiming the detection of the extra emission line with energy ∼3.5 keV in X-ray spectra of certain clusters of galaxies and nearby Andromeda galaxy, have raised a considerable interest in astrophysics and particle physics communities. A number of new observational studies
claim detection or non-detection of the extra line in X-ray spectra of various cosmic objects. In this review I summarise existing results of these studies, overview possible interpretations of the extra line, including intriguing connection with radiatively decaying dark matter, and show future directions achievable with existing and planned X-ray cosmic missions. |
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Iakubovskyi, D.A. |
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Iakubovskyi, D.A. New emission line at ~3.5 keV - observational status, connection with radiatively decaying dark matter and directions for future studies Advances in Astronomy and Space Physics |
| author_facet |
Iakubovskyi, D.A. |
| author_sort |
Iakubovskyi, D.A. |
| title |
New emission line at ~3.5 keV - observational status, connection with radiatively decaying dark matter and directions for future studies |
| title_short |
New emission line at ~3.5 keV - observational status, connection with radiatively decaying dark matter and directions for future studies |
| title_full |
New emission line at ~3.5 keV - observational status, connection with radiatively decaying dark matter and directions for future studies |
| title_fullStr |
New emission line at ~3.5 keV - observational status, connection with radiatively decaying dark matter and directions for future studies |
| title_full_unstemmed |
New emission line at ~3.5 keV - observational status, connection with radiatively decaying dark matter and directions for future studies |
| title_sort |
new emission line at ~3.5 kev - observational status, connection with radiatively decaying dark matter and directions for future studies |
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Головна астрономічна обсерваторія НАН України |
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2014 |
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http://dspace.nbuv.gov.ua/handle/123456789/119805 |
| citation_txt |
New emission line at ~3.5 keV - observational status, connection with radiatively decaying dark matter and directions for future studies / D.A. Iakubovskyi // Advances in Astronomy and Space Physics. — 2014. — Т. 4., вип. 1-2. — С. 9-14. — Бібліогр.: 98 назв. — англ. |
| series |
Advances in Astronomy and Space Physics |
| work_keys_str_mv |
AT iakubovskyida newemissionlineat35kevobservationalstatusconnectionwithradiativelydecayingdarkmatteranddirectionsforfuturestudies |
| first_indexed |
2025-07-08T16:38:23Z |
| last_indexed |
2025-07-08T16:38:23Z |
| _version_ |
1837097502042488832 |
| fulltext |
New emission line at ∼3.5 keV � observational status,
connection with radiatively decaying dark matter
and directions for future studies
D.A. Iakubovskyi∗
Advances in Astronomy and Space Physics, 4, 9-14 (2014)
© D.A. Iakubovskyi, 2014
Bogolyubov Institute of Theoretical Physics, Metrologichna str. 14-b, 03680, Kyiv, Ukraine
Recent works of Bulbul et al. (2014) and Boyarsky et al. (2014), claiming the detection of the extra emission
line with energy ∼3.5 keV in X-ray spectra of certain clusters of galaxies and nearby Andromeda galaxy, have raised
a considerable interest in astrophysics and particle physics communities. A number of new observational studies
claim detection or non-detection of the extra line in X-ray spectra of various cosmic objects. In this review I
summarise existing results of these studies, overview possible interpretations of the extra line, including intriguing
connection with radiatively decaying dark matter, and show future directions achievable with existing and planned
X-ray cosmic missions.
Key words: X-rays: general, dark matter, line: identi�cation
introduction
We still have to explore the origin of dark matter
� gravitationally interacting substance which con-
stitutes the major fraction of non-relativistic matter
in the Universe. None of the known elementary parti-
cles constitute the bulk of dark matter. Despite this
fact, the most plausible1 hypothesis is that dark mat-
ter made of elementary particles, which implies the
extension of the Standard Model of particle physics
and is of considerable interest for particle physicists.
Dozens of Standard Model extensions proposed
so far range according to their main parameters �
mass of dark matter particles and their interaction
strength with Standard Model particles � by tens
of orders of magnitude. Astrophysical observations
of dark matter objects can probe some of them. An
interesting example is radiatively decaying dark mat-
ter. If a dark matter particle interacts with electri-
cally charged Standard Model particles, it usually
decays2 through emitting a photon. For a 2-body
radiative channel, Doppler broadening of dark mat-
ter in haloes will cause a narrow dark matter decay
line. Such a line possesses a number of speci�c prop-
erties allowing to distinguish it from astrophysical
emission lines or instrumental line-like features:
� Its position in energy is solely determined by
the mass of the dark matter particle and the
redshift of the dark matter halo (i. e. if one ne-
glects the mass of other decay product, the line
position is mdmc2
2(1+z)), having di�erent scaling with
redshift z compared with instrumental features;
� Its intensity should be proportional to dark
matter column density3 Sdm =
∫
ρdmd`; due
to di�erent 3D distributions of dark and visi-
ble matter, comparison of line intensity within
a given object and among di�erent objects al-
lows to choose between decaying dark matter
and astrophysical origin of the line;
� It is broadened with characteristic velocity of
dark matter usually di�erent from that of visi-
ble matter.
The above properties allow to reliably establish that
the line comes from decaying dark matter. In other
words, we can directly detect radiatively decaying
dark matter, relying solely on astrophysical measure-
ments.
The search for decaying dark matter in X-rays
lasts for approximately one decade, starting from
pioneering proposals in [2, 43]. The searches prior
to February 2014 are summarised in Table I of [82];
the only exception is a recent study of [57]. These
∗yakubovskiy@bitp.kiev.ua
1Viable alternative theories include modi�ed laws of gravity and/or Newtonian dynamics (see e. g. [12, 48, 77, 78]), primordial black
holes (see e. g. [26, 28]) etc.
2Known examples where this is not the case include dark matter particles holding a new quantum number conserved by Standard Model
interactions, such as R-parity for supersymmetric models, Kalutza-Klein number for extra dimensions, etc. In this case, dark matter decays
are excluded by the special structure of the theory, and the main astrophysical e�ect for such dark matter candidates is annihilation of
dark matter particles with their antiparticles.
3Dark matter column densities for di�erent dark matter-dominated objects are compiled in [22].
9
Advances in Astronomy and Space Physics D.A. Iakubovskyi
searches have not revealed the presence of viable
candidate lines from decaying dark matter, and ob-
tained only upper bounds on radiative decay lifetime
of dark matter particles. The only exception is the
claim of [88] about the excess of Fexxvi Lyγ line at
8.7 keV in Suzaku spectrum of Galactic Center [67]
compared with the standard ionization and recombi-
nation processes. Results of existing X-ray telescope
observations are inconclusive with regards to the na-
ture of this excess, so the claim of [88] should be
tested with new instruments having better spectral
resolution, discussed in e. g. [19].
observational status
of ∼3.5 keV line
In February 2014, the situation changed dramat-
ically: two groups [21, 24] have claimed the pres-
ence of an extra line at ∼3.5 keV. Stacking X-ray
spectra of central parts of 81 galaxy clusters (ob-
served by XMM-Newton and Chandra) in emitter's
rest frame enabled [24] to reach unprecedented sen-
sitivity, as compared with previous line searches in
galaxy clusters. As a result, a new line has been de-
tected in independent subsets � Perseus galaxy clus-
ter, the sum of three nearby galaxy clusters (Coma,
Centaurus and Ophiuchus), and the rest of galaxy
clusters of their sample. On the other hand, [21]
presented analysis of several independent datasets
� the nearby Andromeda galaxy, outskirts of the
Perseus cluster and the new blank-sky dataset � ob-
served by XMM-Newton, and claimed the detection
of new line in Perseus outskirts (using set of observa-
tions completely di�erent from that of [24]). Follow-
ing this, the same group [17] has presented another
evidence for extra line at ∼3.5 keV by looking at the
central part of our Galaxy. Another recent study [96]
detected the 3.5 keV line in the central part of the
Perseus cluster4 observed by Suzaku. These studies
have been accompanied by claims of several other
groups [6, 63, 75, 90] which have not detected the
extra line at ∼3.5 keV in several di�erent datasets of
dark matter objects. Basic properties of all of these
datasets are summarised in Table 1.
possible explanations
The following possibilities for the origin of the
new line have been considered:
(1) The possibility that the new line is not from
astrophysical emission has �rst been studied in pio-
neering papers [21, 24]. Using detailed computations
of line intensities in thermal plasma hosted by galaxy
clusters based on atomic line database ATOMDB
v.2.0.2 [51], [24] argued that possible contributions
from astrophysical lines near 3.5 keV are a factor
& 30 smaller than the detected �ux of the extra line.
In addition, [21] showed that the angular distribu-
tion of ∼ 3.5 keV line in the Perseus galaxy outskirts
is much more consistent with decaying dark matter
distribution than with astrophysical emission.
However, these conclusions of [21, 24] were ques-
tioned in [63], in which the authors argue that (a)
it is possible to explain the new line in the central
part of our Galaxy and in combined dataset of [24]
by the contribution of Kxviii and Clxvii lines5, and
(b) the extra line from M31 centre seen by [21] can
be lowered to <90% con�dence by adjusting the X-
ray continuum over a narrow energy range near the
line (3�4 keV).
The criticism of [63] has stimulated the imme-
diate comment of [18]. Here, claim (b) of [63] is
repudiated by showing both that the X-ray con-
tinuum of [63] selected at 3�4 keV is signi�cantly
overestimated at larger energies, and that the ex-
tra line �ux is at least an order of magnitude less
than expected from astrophysical lines near 3.5 keV.
The other concern (a) of [63] has recently been com-
mented in [25] showing that the analysis of [63] suf-
fers from their use of the approximate atomic data
from ATOMDB [51] website. In contrast, the full
version of ATOMDB used by [17, 18, 24] leads to
a signi�cant lack of astrophysical emission to ex-
plain the observed line at ∼ 3.5 keV in the combined
dataset of galaxy clusters of [24]. These comments,
in turn, have been replied to in [62]. By using a wider
energy range proposed by [17, 21], [62] reproduced
the initial result of [21] with regards to the promi-
nence of the 3.5 keV line; however, unlike [21], [62]
obtained a much more prominent line-like negative
residual below 3 keV. The origin of this discrepancy
remains unsolved. A probable reason is the slope
of the instrumental component seen in Fig. 2 of [62]
from its �ducial value E−0.2, see e. g. [73], which can,
due to ∼ 5% dip in the e�ective area (see e. g. Fig. 1
of [62]), mimic such a large negative residual. On the
other hand, the use of the full ATOMDB version [62]
presented an additional argument of the initial claim
of [63] based on new Caxix/Caxx line ratios, so a
more detailed investigation (including e. g. system-
atic uncertainties on ion emissivities) is required to
�nally resolve this issue.
An alternative approach is to study the line mor-
4[96] also analysed Suzaku observations of Coma, Ophiuchus and Virgo galaxy clusters. In fact, the faint extra line at ∼3.45 keV
(rest-frame) was found in Coma and Ophiuchus spectra, see their Fig. 3. Because the position of this extra line line coincides with other
detections within Suzaku energy resolution (' 150 eV), we included the detections in Coma and Ophiuchus into Table 1. Taking into
account this fact and the level of actual uncertainty between X-ray and weak lensing modelling at a virial radius [83], the results of [96]
are consistent with the decaying dark matter hypothesis.
5The fact that emission near 3.5 keV from Galactic Centre region is consistent with adding Kxviii lines at 3.47 and 3.51 keV was �rst
mentioned in [90]. [21] also mentioned that fact and showed that using Galactic Centre data alone it was not possible to neither claim the
existence an unidenti�ed spectral line on top of the element lines, nor constrain it.
10
Advances in Astronomy and Space Physics D.A. Iakubovskyi
phology. In [27], XMM-Newton observations of the
central part of the Perseus cluster and the Galactic
Centre have been analysed. [27] collected all events
(either cosmic or instrumental origin) in narrow en-
ergy ranges (roughly corresponding to the energy res-
olution), and looked for the best-�t approximation
with the rescaled continuum obtained from several
adjacent line-free bands. The main result of [27] is
that adding decaying dark matter distribution from a
smooth DM pro�le (Navarro-Frenk-White, Einasto,
Burkert) does not improve the �t quality in both ob-
jects. In addition, [27] demonstrated that the distri-
bution of the events in 3.45�3.6 keV bands correlates
with that in the energy bands of strong astrophysi-
cal emission, rather than with that of line-free energy
bands. Based on these �ndings, [27] claimed the ex-
clusion of decaying dark matter origin of 3.5 keV in
the Galactic Centre and the Perseus cluster.
(2) Linear scaling of line position with the red-
shift observed by [21, 24] is an important argument
against the instrumental origin of ∼ 3.5 keV line.
The fact that this line has not been found in the
long blank-sky dataset of [21] provides additional ev-
idence against its instrumental origin.
(3) The authors of [17, 21, 24] argue that all ba-
sic properties of the detected ∼ 3.5 keV line � its
position, line strength, scaling with redshift, angular
distribution inside extended objects (Perseus clus-
ter outskirts, centre vs. o�-centre of Andromeda
galaxy, Galactic Centre vs. blank-sky dataset),
scaling among di�erent objects, and even its non-
observation in some datasets of [21, 24] � are all
consistent with an explanation in terms of a radia-
tively decaying dark matter line.
The predictive power of the decaying dark matter
scenario motivated several groups of researchers [6,
27, 63, 75, 90, 96] to study X-ray spectra of var-
ious dark matter-dominated objects. Their stud-
ies are summarised in Table 1. At the moment, no
further con�rmation of decaying dark matter ori-
gin after the papers [17, 21, 24] has been presented.
While [27, 63, 90, 96] found a line at ∼ 3.5 keV
(though interpreted it as a sum of astrophysical
lines), [75] and [6] have not detected the line in com-
bined datasets of dwarf spheroidal galaxies (dSphs)
and spiral galaxies, respectively, placing only upper
bounds on dark matter lifetime. Non-detection of
the line at 3.55 keV by [75] is still consistent with de-
caying dark matter hypothesis; to rule it out, even
quoting [75]6, one needs to increase the sensitivity by
a factor ∼ 2 (which means a factor ∼ 4 increase of
exposure assuming similar dark matter column den-
sity). On the other hand, non-observation of the
∼ 3.5 keV line in the dataset of [6] (see their Fig. 4)
may be interpreted as a tension with decaying dark
matter hypothesis and therefore motivates more de-
tailed study. According to [58], where combined
dataset of galaxies with comparable exposure has
been analysed, at exposures larger than ∼ 10Msec
line-like systematic errors start to dominate over sta-
tistical errors. As a result, the usual method of de-
termination of continuum level (by simply minimiz-
ing χ2, as [6] did) is no longer appropriate. Previ-
ous studies of line in M31 centre [18, 21, 63] show
that precise determination of the continuum level is
important to quantify the intensity of 3.5 keV line.
Therefore, the only way to put robust exclusions to
line intensity is to perform continuum modelling in
a way similar to [58]: to decrease the level of contin-
uum below the best-�t in order to ensure absence of
signi�cant negative residuals; and to add systematic
errors to account non-Gaussian distribution of pos-
itive residuals. According to Fig. 5.26 of [58], such
analysis would produce 3σ upper bounds for 3.5 keV
line �ux close to ∼ 1.5×10−6 ph/sec/cm2 per XMM-
Newton �eld-of-view, still consistent with line obser-
vation in M31 centre [21].
Although the results of [21, 24] are formulated
for a speci�c dark matter candidate � right-handed
(sterile) neutrinos (see [23, 19] for recent reviews),
they can be applied for any type of radiatively de-
caying dark matter, see e.g. [1, 3, 4, 9, 10, 11, 14, 29,
30, 31, 32, 39, 40, 42, 45, 46, 47, 49, 52, 53, 54, 55,
56, 59, 60, 61, 64, 65, 66, 68, 69, 70, 79, 80, 81, 84,
87, 86, 89, 91, 92, 94]. The di�erence among these
models can be further probed by:
� Changes in line morphology due to non-
negligible initial dark matter velocities, see
e. g. [72, 74];
� Other astrophysical tests such as Lyα method,
see e. g. [3, 76, 97];
� Search of �smoking gun� signatures in accel-
erator experiments, see e. g. [15] for the mini-
mal neutrino extension of the Standard Model,
νMSM [7, 8, 23].
(4) Recently proposed alternatives to radiatively
decaying dark matter currently include decay of ex-
cited dark matter states [13, 34, 35, 36, 50, 85, 93],
annihilating dark matter [10, 44, 52], dark matter
decaying into axion-like particles with further con-
version to photons in magnetic �eld [5, 33, 37, 38].
These models predict substantial di�erence in the
∼ 3.5 keV line morphology, compared with the ra-
diatively decaying dark matter. For example, their
line pro�les should be more concentrated towards
the centres of dark matter-dominated objects due
to larger dark matter density (for exciting and an-
nihilating dark matter) and larger magnetic �elds
(for magnetic �eld conversion of axion-like particles).
Further non-observation of the ∼ 3.5 keV line in out-
skirts of dark matter-dominated objects would there-
fore an argument in favour of these models.
6Given large uncertainties in dark matter modelling, the obtained bounds are usually (see e. g. [19, 20]) diluted by an extra factor of 2,
contrary to [75].
11
Advances in Astronomy and Space Physics D.A. Iakubovskyi
conclusions
and future directions
A new emission line at ∼3.5 keV in spectra of
galaxy clusters and central parts of the Andromeda
galaxy recently reported by [21, 24] remains unex-
plained in terms of astrophysical emission lines or
instrumental features (see however recent works [27,
62]). Its properties are consistent with radiatively
decaying dark matter and other interesting scenarios
(such as exciting dark matter, annihilating dark mat-
ter, and dark matter decaying into axion-like parti-
cles further converted in cosmic magnetic �elds) mo-
tivated by various particle physics extensions of the
Standard Model. In the case of radiatively decaying
dark matter, further detection of the new emission
line in other objects would lead to direct detection of
new physics. Specially dedicated observations by ex-
isting X-ray missions (such as XMM-Newton, Chan-
dra, Suzaku) still allow such detection (see e. g. [71])
although one should take detailed care on various
systematic e�ects that could mimic or hide the new
line.
The alternative is to use newer better instru-
ments. The basic requirements for such instruments
� higher grasp (the product of �eld-of-view and ef-
fective area) and better spectral resolution 7 � have
�rst been formulated in [16]. The imaging spectrom-
eter on-board the new X-ray mission Astro-H [95]
scheduled to launch in 2015 meets only the second
requirement, having an energy resolution of an order
of magnitude better (∼ 5 eV) compared with that
of existing instruments. This will allow Astro-H to
precisely determine the line position in the bright-
est objects, with prolonged observations (according
to [24], a 1 Msec observation of the Perseus cluster is
required) and thus Astro-H will �nally close the ques-
tion whether the new line is from new physics or from
(anomalously enhanced) astrophysical emission. An-
other interesting possibility is proposed in [21]: one
should see the decaying dark matter signal in the
Milky Way halo in every Astro-H observation, there-
fore their combination could also reveal the radia-
tively decaying dark matter nature of new line. Yet
another possibility is to use the planned LOFT mis-
sion [98] whose high grasp and moderate energy res-
olution would enable to detect the new line at much
smaller intensities [82]. Finally, an �ultimate� imag-
ing spectrometer proposed in e. g. [19] would reveal
the detailed structure of ∼3.5 keV line.
acknowledgement
This work was supported by part by the Swiss
National Science Foundation grant SCOPE IZ7370-
152581, the Program of Cosmic Research of the Na-
tional Academy of Sciences of Ukraine, the State
Programme of Implementation of Grid Technology
in Ukraine, and the grant of President of Ukraine for
young scientists.
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13
Advances in Astronomy and Space Physics D.A. Iakubovskyi
Ref. Object Redshift Instrument Exposure, Line position, Line �ux,
Msec keV 10−6 ph/sec/cm2
[24] Full stacked sample 0.009-0.354 MOS 6 3.57±0.02 4.0±0.8
[24] Full stacked sample 0.009-0.354 PN 2 3.51±0.03 3.9+0.6
−1.0
[24] Coma+Centaurus+Ophiuchus 0.009-0.028 MOS 0.5 3.57a 15.9+3.4
−3.8
[24] Coma+Centaurus+Ophiuchus 0.009-0.028 PN 0.2 3.57a < 9.5 (90%)
[24] Perseus (< 12') 0.016 MOS 0.3 3.57a 52.0+24.1
−15.2
[24] Perseus (< 12') 0.016 PN 0.05 3.57a < 17.7 (90%)
[24] Perseus (1-12') 0.016 MOS 0.3 3.57a 21.4+7.0
−6.3
[24] Perseus (1-12') 0.016 PN 0.05 3.57a < 16.1 (90%)
[24] Rest of the clusters 0.012-0.354 MOS 4.9 3.57a 2.1+0.4
−0.5
[24] Rest of the clusters 0.012-0.354 PN 1.8 3.57a 2.0+0.3
−0.5
[24] Perseus (> 1') 0.016 ACIS-S 0.9 3.56±0.02 10.2+3.7
−3.5
[24] Perseus (< 9') 0.016 ACIS-I 0.5 3.56a 18.6+7.8
−8.0
[24] Virgo (< 500�) 0.003-0.004 ACIS-I 0.5 3.56a < 9.1 (90%)
[21] M31 (< 14') -0.001b MOS 0.5 3.53±0.03 4.9+1.6
−1.3
[21] M31 (10-80') -0.001b MOS 0.7 3.50-3.56 < 1.8 (2σ)
[21] Perseus (23-102') 0.0179b MOS 0.3 3.50±0.04 7.0±2.6
[21] Perseus (23-102') 0.0179b PN 0.2 3.46±0.04 9.2±3.1
[21] Perseus, 1st bin (23-37') 0.0179b MOS 0.2 3.50a 13.8±3.3
[21] Perseus, 2nd bin (42-54') 0.0179b MOS 0.1 3.50a 8.3±3.4
[21] Perseus, 3rd bin (68-102') 0.0179b MOS 0.03 3.50a 4.6±4.6
[21] Blank-sky � MOS 7.8 3.45-3.58 < 0.7 (2σ)
[90] Galactic center (2.5-12') 0.0 ACIS-I 0.8 ' 3.5 . 25 (2σ)
[63] Galactic center (0.3-15') 0.0 MOS 0.7 ' 3.5 < 41
[63] Galactic center (0.3-15') 0.0 PN 0.5 ' 3.5 < 32
[63] M31 0.0 MOS 0.5 3.53±0.07 2.1±1.5c
[17] Galactic center (< 14') 0.0 MOS 0.7 3.539±0.011 29±5
[75] Combined dSphs 0.0 MOS+PN 0.4+0.2 3.55a < 0.254 (90%)
[6] Combined galaxies
(& 0.01Rvir)
0.0 MOS 14.6 ' 3.5 unknownd
[6] Combined galaxies
(& 0.01Rvir)
0.0 ACIS-I 15.0 ' 3.5 unknownd
[96] Perseus core (< 6') 0.0179b XIS 0.74 3.510+0.023
−0.008 32.5+3.7
−4.3
[96] Perseus con�ned (6-12.7') 0.0179b XIS 0.74 3.510+0.023
−0.008 32.5+3.7
−4.3
[96] Coma (< 12.7') 0.0231b XIS 0.164 ' 3.45e ' 30e
[96] Ophiuchus (< 12.7') 0.0280b XIS 0.083 ' 3.45e ' 40e
[96] Virgo (< 12.7') 0.0036b XIS 0.09 3.55a < 6.5 (2σ)
Table 1: Properties of ∼3.5 keV line searched in di�erent X-ray datasets observed by MOS and PN cameras on-board
XMM-Newton observatory, ACIS instrument on-board Chandra observatory and XIS instrument on-board Suzaku
observatory. All error bars are at 1σ (68%) level.
a Line position was �xed at given value.
b Redshift was �xed at NASA Extragalactic Database (NED) value.
c The line was detected at < 90% con�dence level. Such a low �ux (compared with [21]) was due to unphysically
enhanced level of continuum at 3-4 keV band used in [63], see [18] for details.
d [6] only qouted the �minimal probed values� of sterile neutrino mixing angle sin2(2θ) . 2 × 10−11 by XMM-
Newton/MOS and . 5 × 10−11 by Chandra/ACIS-I. For XMM-Newton dataset with average dark matter column
density in �eld-of-view equal to 100 M�/pc
2, these values would correspond to upper bound on ∼3.5 keV line �ux
∼ 3.0× 10−7 and ∼ 7.5× 10−7 ph/sec/cm2, respectively.
e Parameters estimated from Fig. 3 of [96], see text.
14
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