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XRISM constraints on unidentified X-ray emission lines, including the 3.5 keV line, in the stacked spectrum of ten galaxy clusters

XRISM Collaboration, Marc Audard, Hisamitsu Awaki, Ralf Ballhausen, Aya Bamba, Ehud Behar, Rozenn Boissay-Malaquin, Laura Brenneman, Gregory V. Brown, Lia Corrales, Elisa Costantini, Renata Cumbee, Maria Diaz Trigo, Chris Done, Tadayasu Dotani, Ken Ebisawa, Megan E. Eckart, Dominique Eckert, Satoshi Eguchi, Teruaki Enoto, Yuichiro Ezoe, Adam Foster, Ryuichi Fujimoto, Yutaka Fujita, Yasushi Fukazawa, Kotaro Fukushima, Akihiro Furuzawa, Luigi Gallo, Javier A. García, Liyi Gu, Matteo Guainazzi, Kouichi Hagino, Kenji Hamaguchi, Isamu Hatsukade, Katsuhiro Hayashi, Takayuki Hayashi, Natalie Hell, Edmund Hodges-Kluck, Ann Hornschemeier, Yuto Ichinohe, Daiki Ishi, Manabu Ishida, Kumi Ishikawa, Yoshitaka Ishisaki, Jelle Kaastra, Timothy Kallman, Erin Kara, Satoru Katsuda, Yoshiaki Kanemaru, Richard Kelley, Caroline Kilbourne, Shunji Kitamoto, Shogo Kobayashi, Takayoshi Kohmura, Aya Kubota, Maurice Leutenegger, Michael Loewenstein, Yoshitomo Maeda, Maxim Markevitch, Hironori Matsumoto, Kyoko Matsushita, Dan McCammon, Brian McNamara, François Mernier, Eric D. Miller, Jon M. Miller, Ikuyuki Mitsuishi, Misaki Mizumoto, Tsunefumi Mizuno, Koji Mori, Koji Mukai, Hiroshi Murakami, Richard Mushotzky, Hiroshi Nakajima, Kazuhiro Nakazawa, Jan-Uwe Ness, Kumiko Nobukawa, Masayoshi Nobukawa, Hirofumi Noda, Hirokazu Odaka, Shoji Ogawa, Anna Ogorzałek, Takashi Okajima, Naomi Ota, Stephane Paltani, Robert Petre, Paul Plucinsky, Frederick S. Porter, Katja Pottschmidt, Kosuke Sato, Toshiki Sato, Makoto Sawada, Hiromi Seta, Megumi Shidatsu, Aurora Simionescu, Randall Smith, Hiromasa Suzuki, Andrew Szymkowiak, Hiromitsu Takahashi, Mai Takeo, Toru Tamagawa, Keisuke Tamura, Takaaki Tanaka, Atsushi Tanimoto, Makoto Tashiro, Yukikatsu Terada, Yuichi Terashima, Yohko Tsuboi, Masahiro Tsujimoto, Hiroshi Tsunemi, Takeshi Tsuru, Aysegül Tümer, Hiroyuki Uchida, Nagomi Uchida, Yuusuke Uchida, Hideki Uchiyama, Shutaro Ueda, Yoshihiro Ueda, Shinichiro Uno, Jacco Vink, Shin Watanabe, Brian J. Williams, Satoshi Yamada, Shinya Yamada, Hiroya Yamaguchi, Kazutaka Yamaoka, Noriko Yamasaki, Makoto Yamauchi, Shigeo Yamauchi, Tahir Yaqoob, Tomokage Yoneyama, Tessei Yoshida, Mihoko Yukita, Irina Zhuravleva, Jean-Paul Breuer, Priyanka Chakraborty, Stefano Ettori, Andrew Fabian, Annie Heinrich, Marie Kondo, Julie HLavacek-Larrondo, Hannah McCall, Paul Nulsen, Tom Rose, Helen Russell, Arnab Sarkar, Evan Scannapieco, Kazunori Suda, Ming Sun, Prathamesh Tamhane, Nhut Truong, Norbert Werner, Congyao Zhang

TL;DR

XRISM Resolve observations stack 3.748 Ms of data from ten galaxy clusters to search for unidentified X-ray emission lines in rest-frame 2.5–15 keV. The analysis uses DM mass modeling to predict a DM decay signal and rest-frame spectral stacking with high-resolution spectroscopy, followed by a systematic line search using Gaussian features within three energy bands. No unidentified lines are detected, yielding a 3σ upper limit on the DM decay rate of $\Gamma \leq 0.97 \times 10^{-27}$ s$^{-1}$ for a sterile-neutrino-like DM particle of mass $m_s=7.1$ keV, which is 3–4× stronger than Hitomi's Perseus limit but about 5× above the XMM-Newton detected signal. The results demonstrate XRISM's capability to test the XMM-Newton 3.5 keV line hypothesis with future deeper observations, particularly by targeting cool-core clusters and improving mass-concentration modeling.

Abstract

We stack 3.75 Megaseconds of early XRISM Resolve observations of ten galaxy clusters to search for unidentified spectral lines in the $E=$ 2.5-15 keV band (rest frame), including the $E=3.5$ keV line reported in earlier, low spectral resolution studies of cluster samples. Such an emission line may originate from the decay of the sterile neutrino, a warm dark matter (DM) candidate. No unidentified lines are detected in our stacked cluster spectrum, with the $3σ$ upper limit on the $m_{\rm s}\sim$ 7.1 keV DM particle decay rate (which corresponds to a $E=3.55$ keV emission line) of $Γ\sim 1.0 \times 10^{-27}$ s$^{-1}$. This upper limit is 3-4 times lower than the one derived by Hitomi Collaboration et al. (2017) from the Perseus observation, but still 5 times higher than the XMM-Newton detection reported by Bulbul et al. (2014) in the stacked cluster sample. XRISM Resolve, with its high spectral resolution but a small field of view, may reach the sensitivity needed to test the XMM-Newton cluster sample detection by combining several years worth of future cluster observations.

XRISM constraints on unidentified X-ray emission lines, including the 3.5 keV line, in the stacked spectrum of ten galaxy clusters

TL;DR

XRISM Resolve observations stack 3.748 Ms of data from ten galaxy clusters to search for unidentified X-ray emission lines in rest-frame 2.5–15 keV. The analysis uses DM mass modeling to predict a DM decay signal and rest-frame spectral stacking with high-resolution spectroscopy, followed by a systematic line search using Gaussian features within three energy bands. No unidentified lines are detected, yielding a 3σ upper limit on the DM decay rate of s for a sterile-neutrino-like DM particle of mass keV, which is 3–4× stronger than Hitomi's Perseus limit but about 5× above the XMM-Newton detected signal. The results demonstrate XRISM's capability to test the XMM-Newton 3.5 keV line hypothesis with future deeper observations, particularly by targeting cool-core clusters and improving mass-concentration modeling.

Abstract

We stack 3.75 Megaseconds of early XRISM Resolve observations of ten galaxy clusters to search for unidentified spectral lines in the 2.5-15 keV band (rest frame), including the keV line reported in earlier, low spectral resolution studies of cluster samples. Such an emission line may originate from the decay of the sterile neutrino, a warm dark matter (DM) candidate. No unidentified lines are detected in our stacked cluster spectrum, with the upper limit on the 7.1 keV DM particle decay rate (which corresponds to a keV emission line) of s. This upper limit is 3-4 times lower than the one derived by Hitomi Collaboration et al. (2017) from the Perseus observation, but still 5 times higher than the XMM-Newton detection reported by Bulbul et al. (2014) in the stacked cluster sample. XRISM Resolve, with its high spectral resolution but a small field of view, may reach the sensitivity needed to test the XMM-Newton cluster sample detection by combining several years worth of future cluster observations.

Paper Structure

This paper contains 8 sections, 4 figures, 2 tables.

Table of Contents

  1. Introduction
  2. XRISM observations and data reduction
  3. Analysis and discussion
  4. Cluster mass modeling
  5. Spectral stacking
  6. Searching for unidentified lines
  7. Discussion and Conclusions
  8. The stacked spectra

Figures (4)

  • Figure 1: The stacked XRISM spectra in the rest-frame 3--4 keV. Black shows the full cluster sample (ten clusters, 3.75 Ms total exposure), red shows the hot subsample (six clusters with $M_{\rm 200c} > 10^{14.5}$ M$_{\odot}$, 2.49 Ms total exposure), and blue for cool clusters in the sample (four clusters with $M_{\rm 200c} < 10^{14.5}$ M$_{\odot}$, 1.26 Ms total). The green curve shows the best fit with a two bapec model to the full sample. For clarity, the hot cluster spectrum is lowered by a factor of two and the cool cluster spectrum is lowered by a factor of three. Detected atomic lines in this energy range are marked. The atomic lines have a velocity dispersion of 150--160 km s$^{-1}$, which is $\sim$ 6 times smaller than the velocity dispersion we adopted for the DM line search in the full sample. For the cool and hot subsamples, the difference is $\sim$ 4 times and $\sim$ 7 times respectively. Thus, the expected DM line in these spectra should be 4--7 times broader than the shown atomic lines. The green bracket shows the 90% confidence interval on the unidentified 3.5 keV line energy for the most-restrictive XMM-Newton MOS stacked-clusters sample in B14.
  • Figure 2: Best-fit flux for an additional line as a function of its energy across the 2.5--15 keV rest-frame band (split into three intervals for clarity). The red solid line shows the best-fit line flux for a line width of 950 km s$^{-1}$, the weighted velocity dispersion expected for DM in this sample, while the red dotted lines show the $3\sigma$ range. The black solid line shows the best-fit line flux for a width of 160 km s$^{-1}$ (an ICM line), while the black dotted lines show its $3\sigma$ range. The vertical grey shaded areas mark strong ICM emission lines, where the search for faint lines is not possible. The right vertical axis shows the approximate corresponding DM particle decay rate $\Gamma$, assuming all DM is comprised of the decaying particle. The green cross shows the energy range and decay rate (and 1-$\sigma$ error) for the B14 3.5 keV line, which is still a factor 5 below our $3\sigma$ limit on the broad line in the same energy range. The blue dashed line shows a polynomial fit to approximate the $3\sigma$ upper bound. We also mark the positions of 6.40 keV Fe fluorescent line likely from X-ray AGN, the possible Fe charge exchange feature at $\sim$ 8.8 keV and three bumps at $E>$ 9.8 keV likely from residual NXB lines.
  • Figure 3: The stacked spectra in the 1.97--3 keV and 4--6.5 keV ranges, with the best-fit two bapec model shown in the green curve. The same as Fig. \ref{['fig:spec']}, black, red and blue show the full cluster sample, hot subsample and cool subsample respectively. For clarity, the hot cluster spectrum is lowered by a factor of two and the cool cluster spectrum is lowered by a factor of three. Detected atomic lines are also labeled.
  • Figure 4: Similar to Fig. \ref{['fig:spe1']} but in the 6.5--7.0 keV and 7.0--9.4 keV ranges.