Dark matter annihilation in ! Centauri: astrophysical implications derived from the MWA radio data Arpan Kara,∗, Biswarup Mukhopadhyayab, Steven Tingayc, Ben McKinleyc,d, Marijke Haverkorne, Sam McSweeneyc, Natasha Hurley-Walkerc, Sourav Mitraf, Tirthankar Roy Choudhuryg aRegional Centre for Accelerator-based Particle Physics, Harish-Chandra Research Institute, HBNI, Chhatnag Road, Jhunsi, Allahabad - 211 019, India bIndian Institute of Science Education and Research Kolkata, Mohanpur, West Bengal 741246, India cInternational Centre for Radio Astronomy Research, Curtin University, Bentley, WA 6102, Australia dARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), Bentley, Australia eDepartment of Astrophysics/IMAPP, Radboud University, P.O. Box 9010, 6500 GL Nijmegen, The Netherlands fSurendranath College, 24/2 M. G. ROAD, Kolkata, West Bengal 700009, India gNational Centre for Radio Astrophysics, TIFR, Post Bag 3, Ganeshkhind, Pune 411007, India Abstract We present an analysis of Murchison Widefield Array radio telescope data from ! Cen, possibly a stripped dwarf spheroidal galaxy core captured by our Galaxy. Recent interpretations of Fermi-LAT γ-ray data by Brown et al. (2019) and Reynoso-Cordova et al. (2019) suggest that ! Cen may contain significant Dark Matter. We utilise their best-fit Dark Matter annihilation models, and an estimate of the magnetic field strength in ! Cen, to calculate the expected radio synchrotron signal from annihilation, and show that one can usefully rule out significant parts of the magnetic field - diffusion coefficient plane using our current observational limits on the radio emission. Improvement by a factor of 10-100 on these limits could constrain the models even more tightly. Keywords: ! Cen, γ-ray, Dark matter annihilation, Radio observation, MWA 1. Introduction the MWA [2] and the Giant Metre-wave Radio Telescope (GMRT [3]) to place limits on diffuse synchrotron emission ! Cen is possibly a stripped dwarf spheroidal galaxy from these galaxies for various models. While this repre- core captured by our Galaxy, in which the dark matter sented a significant step toward much deeper observations (DM) density may be high. Two groups have recently by the future Square Kilometre Array (SKA) [4, 5, 6, 7], analysed the γ-ray data from Fermi-LAT for this object, the results placed only limited constraints on DM models. to suggest best-fit values of the DM mass and annihilation Similar results have recently been reported by a team us- rates in various dominant channels. We take advantage ing another low frequency interferometer, LOFAR, for the of the proximity of ! Cen and calculate the expected ra- dSph galaxy Canes Venatici I [8]. One fundamental factor dio synchrotron surface brightness arising from DM anni- affecting our previous results and the LOFAR results is, of hilation in the ambient magnetic field, corresponding to course, the large distances to dSph galaxies. each of the fits. In each case, Murchison Widefield Array (MWA) data constrain the magnetic field - diffusion co- efficient plane for ! Cen. It is of particular significance 2. Observations of Omega Centauri (! Cen) that some knowledge of the magnetic field is used here A case has been recently made by Ref. [9] and Ref. [10] to constrain the diffusion coefficient, which is difficult to that ! Cen, historically classified as the largest globular estimate. arXiv:2005.11962v2 [astro-ph.HE] 17 Jul 2020 cluster associated with our Galaxy, is the captured and Previously we have reported the results of the first low stripped core of a dSph galaxy and has a significant DM radio frequency search for the synchrotron emission sig- component to its mass. These characteristics potentially nal expected from some DM annihilation models [1]. Such make ! Cen a suitable object for studying DM annihilation radio signals arise from electrons and positrons, produced models, since it is only 5.4 kpc from the Earth. in DM annihilation cascades, undergoing cycloidal motion Ref. [9] and Ref. [10] analyze the Fermi-LAT data in the ambient magnetic field. We targeted 14 DM-rich on γ-ray emission from the direction of ! Cen and claim dwarf spheroidal (dSph) galaxies using observations from consistency of the signal with DM annihilation. Ref. [9] claims the best fit to be mDM = 31±4 GeV and a velocity- 3 −1 ∗Corresponding author averaged annihilation cross-section of log10[hσvi (cm s )] = 0:6 ¯ Email address: [email protected] (Arpan Kar) −28:2±1:2, with bb as the principal annihilation channel. Preprint submitted to Elsevier July 20, 2020 The fit by Ref. [10], on the other hand, favours mDM = 0:69 2 −2 −1 9:1±0:62 GeV and log10[hσviJ (GeV cm s )] = −5:5 ± 0:09 47.1 0:03 for the qq¯ channel, or mDM = 4:3±0:08 GeV and 0.2 2 −2 −1 + − log10[hσviJ (GeV cm s )] = −4:34 ± 0:03) for a µ µ 47.2 0.1 channel. These best-fit values assume that the γ-ray signal 47.3 from ! Cen is arising solely due to DM annihilation. Since 47.4 our emphasis is on the corresponding radio synchrotron 0.0 47.5 signals, we have used this at face value. Jy/beam Dec (degrees) No obvious populations of conventional high energy as- 47.6 0.1 trophysical objects are known to be associated with ! Cen 47.7 to readily explain the γ-ray emission, although a more 47.8 0.2 comprehensive examination of possible high energy pho- 13.47 13.46 13.45 13.44 13.43 ton sources is clearly required. For example, Ref. [11] RA (hours) find 30 objects in ! Cen with X-ray luminosities and col- ors consistent with a millisecond pulsar interpretation and Figure 1: Difference image for ! Cen using MWA and TGSS ADR1 data. The blue circle denotes the optical half-light diameter for ! suggest that these objects could be the source of the γ-ray Cen. emission seen with Fermi-LAT. In order to predict synchrotron emission due to DM annihilation, the ambient magnetic field strength is an im- trophysical parameters in Omega Centauri. While models portant parameter. The determination of magnetic field differing from those in Ref. [9] and Ref. [10] can certainly strength in dSph galaxies is not straightforward. However, be there, our approach can be used to predict and analyse measurements of the magnetic field in globular clusters are the concomitantly altered radio synchrotron flux for each possible through the observation of pulsars, in particular of them. Thus the adherence to particular models used in via measurement of their dispersion measure and rotation the recent literature enables us to lay out a general prin- measure. For example, Ref. [12] find evidence for mag- ciple. netic fields possibly as high as 200 µG in the globular cluster 47 Tucanae. While a similar analysis is not cur- 3. Radio data analysis rently possible for ! Cen (no pulsars are known in ! Cen), approximate estimates of its magnetic field strength are Following Ref. [1], we produce an image for ! Cen possible by virtue of its location within our Galaxy. that represents only extended, diffuse radio emission at 200 Using the best-fit DM annihilation models, suggested MHz, via the difference between an MWA image (sensitive by Ref. [9] and Ref. [10] noted above (obtained by using to diffuse emission) and an image from the GMRT TGSS the best-fit J-factor, as explained later in this paper), the ADR1 (sensitive to compact emission). ! Cen is located proximity of ! Cen, and the prospect of independently es- close to the powerful radio galaxy Centaurus A (Cen A) timating the magnetic field strength within ! Cen (as out- and the MWA GLEAM data utilised in Ref. [1] cannot be lined below), we re-visit the techniques used in our previ- utilised here, the reason being the difficulties of producing ous work to compute the corresponding radio synchrotron high quality images in the vicinity of extremely bright and annihilation signals at low frequencies. The reader is re- complex objects such as Cen A. Thus, for ! Cen, we utilise ferred to Ref. [1] for the details of our observational ap- MWA images that were produced specifically for Cen A, proach, a summary of the relevant literature, and our re- by [13]. We note that the GMRT data for ! Cen are not sults for dSph galaxies. Finally, we compare our radio data affected by Cen A in the same way that the MWA data are, with the theoretical expectations. due to the fact that the GMRT field of view is far smaller As has been mentioned above, our emphasis is on the than the MWA field of view, meaning that the structure of correlation between gamma-ray and radio signals from a Cen A that challenges MWA imaging is greatly attenuated nearby globular cluster like Omega Centauri. The new by the primary beam response of the GMRT. step that we take in this study is the calculation of the ra- The difference image resulting from our analysis of the dio flux for the same dark matter profile(s), based on which MWA data from Ref. [13] and the GMRT TGSS ADR1 gamma-ray data have been interpreted, and its compari- data of [3] is shown in Figure 1. son with the MWA data. Thus [9] and [10] serve as our While the images of the Cen A region by [13] are of reference models for gamma-ray emission from dark mat- a very high quality, imaging artefacts at a low level are ter annihilation in Omega Centauri. Going beyond them still apparent in Figure 1. These artefacts are in the form in a study like this would have taken us to territories where of radial stripes that originate near the peak intensity re- the aforementioned correlation would be impossible.