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Teraelectronvolt gamma-ray emission near globular cluster Terzan 5 as a probe of cosmic ray transport

Nature Astronomy (2024)Cite this article

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Abstract

The propagation directions of cosmic rays travelling through interstellar space are repeatedly scattered by fluctuating interstellar magnetic fields. The nature of this scattering is a major unsolved problem in astrophysics, one that has resisted solution largely due to a lack of direct observational constraints on the scattering rate. Here we show that very high-energy γ-ray emission from the globular cluster Terzan 5, which has unexpectedly been found to be displaced from the cluster, presents a direct probe of this process. We show that this displacement is naturally explained by cosmic rays accelerated in the bow shock around the cluster, which then propagate a finite distance before scattering processes re-orient enough of them towards Earth to produce a detectable γ-ray signal. The angular distance between the cluster and the signal places tight constraints on the scattering rate, which we show are consistent with a model in which scattering is primarily due to excitation of magnetic waves by the cosmic rays themselves. The analysis method we develop here will make it possible to use sources with similarly displaced non-thermal X-ray and tera-electronvolt γ-ray signals as direct probes of cosmic ray scattering across a range of Galactic environments.

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Fig. 1: The measured γ-ray spectrum of Terzan 5 together with model fits.
Fig. 2: Schematic illustration of the mechanism underlying the displaced tera-electronvolt γ-ray signal of Terzan 5.
Fig. 3: Sample steady-state joint distributions of CR position and pitch angle.
Fig. 4: Comparison between observed and model-predicted angular distribution of emission.

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Data availability

The simulation data, interpolation tables and MCMC chains are not online due to their size (~200 GB) but are available upon request from the corresponding author.

Code availability

The CR transport simulations in this paper were carried out with CRIPTIC18, which is freely available from https://bitbucket.org/krumholz/criptic/. The interpolation tables were calculated with CUBATURE50, which is available from https://github.com/stevengj/cubature. The MCMC analysis used EMCEE51, which is available from https://github.com/dfm/emcee. The XSPEC software that we used to calculate the X-ray absorption is available from https://heasarc.gsfc.nasa.gov/xanadu/xspec/. The input files used for the CRIPTIC simulations, along with source code to carry out all the analysis steps presented above, are available from https://bitbucket.org/krumholz/terzan5. All the software used in this project is available under an open source licence.

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Acknowledgements

We acknowledge support from the Australian Government through the Australian Research Council (Award No. DP230101055 to M.R.K. and R.M.C.), from the National Computational Infrastructure, which is supported by the Australian Government (Award No. jh2 to M.R.K.), from the State Agency for Research of the Spanish Ministry of Science and Innovation (Grant Nos. PID2019-105510GB-C31/AEI/10.13039/501100011033, PID2019-104114RB-C33/AEI/10.13039/501100011033 and PID2022-138172NB-C43/AEI/10.13039/501100011033/ERDF/EU to P.B.), the Departament de Recerca i Universitats of Generalitat de Catalunya (Grant No. 2021SGR00679 to P.B.) and from a Unit of Excellence María de Maeztu 2020–2023 award to the Institute of Cosmos Sciences (Award No. CEX2019-000918-M to P.B.). Part of this work was completed at the Kavli Institute for Theoretical Physics. That work is supported in part by the National Science Foundation (Grant No. PHY-2309135 to M.R.K.). We thank P. Blasi, M. Baring, M. Barkov, H. Baumgardt, G. Bicknell, M. Donahue, E. di Teodoro, M. Filopovic, O. Macias, D. Mackey, E. Moulin, M. McKenzie, C. O’Hare, B. Olmi, B. Reville, G. Rowell, A. Shalchi and D. Song for useful communications. R.M.C. thanks J. Hinton for alerting him to the peculiar tera-electronvolt phenomenology of Terzan 5.

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Authors and Affiliations

  1. Research School of Astronomy and Astrophysics, Australian National University, Canberra, Australian Capital Territory, Australia

    Mark R. Krumholz & Roland M. Crocker

  2. ARC Centre of Excellence for Astronomy in Three Dimensions (ASTRO-3D), Canberra, Australian Capital Territory, Australia

    Mark R. Krumholz

  3. International Centre for Radio Astronomy Research – Curtin University, Bentley, Western Australia, Australia

    Arash Bahramian

  4. Departament de Física Quàntica i Astrofísica, Institut de Ciències del Cosmos, Universitat de Barcelona, Barcelona, Spain

    Pol Bordas

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Contributions

R.M.C. initiated the project and conceived the theoretical interpretation of the displaced tera-electronvolt source. M.R.K. conducted the numerical modelling. The text was written by M.R.K. and R.M.C. All authors were involved in the interpretation of the results, and all reviewed the manuscript.

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Correspondence to Mark R. Krumholz or Roland M. Crocker.

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Extended data

Extended Data Fig. 1 Geometry of Terzan 5 relative to the Sun and the Milky Way.

We show Terzan 5’s position (blue circle) and velocity (blue arrow) in a coordinate system where the Galactic Centre (black cross) lies at the origin and the Sun lies at (X, Y, Z) = ( − 8.22, 0, 0.208) kpc, in the direction indicated by the orange arrow. The top panel shows a top-down view of the Galactic Plane, and the bottom panel shows a side-on view. In both panels the dashed blue line shows the distance from the Galactic Centre to Terzan 5. The shaded green band in the top panel indicates the rough position of the Galactic Bar; we take the Bar thickness to be ≈ 1 kpc and the orientation to be 30∘ from the Sun-Galactic Centre line in the direction of Galactic rotation52. The black arrow indicates our 50th percentile value for the direction of Terzan 5’s magnetotail as inferred from our 50th percentile value for μobs together with the observed sky position of the TeV emission centroid7; shaded grey arcs around this arrow show the 1σ and 2σ uncertainty intervals. We take the position, proper motion, and radial velocity of Terzan 5 from ref. 12 and we transform all quantities from sky coordinates to Galactocentric coordinates using the astropy Galactocentric coordinate package version 4.0 (refs. 53,54,55).

Extended Data Fig. 2 Corner plot showing full posterior PDF determined by MCMC.

Panels along the diagonal show histograms of the marginal posterior PDF for each quantity, scaled to a maximum of unity. All other panels show the joint posterior PDF of two quantities; in these panels, colours show the 2D PDF scaled to a maximum of unity, and black contours enclose the marginal 95% confidence interval on each pair of quantities. Points outside the contours show individual randomly-selected MCMC samples that fall outside the 95% confidence interval. The quantities shown, and their units (omitted on the axis labels for reasons of space), are from left to right: log pitch angle scattering rate Kμ[s−1], cosine of angle between magnetic field and line of sight μobs, log CR momentum for which loss and isotropisation times are equal peq[GeV/c], log CR momentum at which the injection distribution cuts off pcut[GeV/c], injection spectral index kp, cosine of the maximum injection angle μ0, log magnetic field strength B [μG], log total CR kinetic luminosity L [erg s−1], and log streaming instability growth rate Γ0[s−1]. In the lower left panel, the grey band shows the relation Kμ = Γ0 with a factor of 3 spread.

Extended Data Fig. 3 Model-predicted synchrotron spectrum.

The blue line shows the median synchrotron spectrum as a function of photon energy Eγ predicted by our best-fitting model, and the shaded blue bands around it show the 68% and 95% confidence intervals. The quantity shown includes the effects of interstellar absorption between Terzan 5 and the Sun, assuming a hydrogen column NH = 2 × 1022 cm−2 (ref. 28); the sharp features visible below 1 keV correspond to absorption edges. The black dashed line is an approximate limit corresponding to \({L}_{X}=4\pi {d}_{Ter5}^{\,2}{E}_{\gamma }^{\,2}(d{\Phi }_{\gamma }/d{E}_{\gamma })=2\times 1{0}^{33}{{{\rm{erg}}}}\,{s}^{-1}\), the X-ray luminosity estimated by ref. 28.

Extended Data Table 1 Marginal posteriors for model parameters
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Krumholz, M.R., Crocker, R.M., Bahramian, A. et al. Teraelectronvolt gamma-ray emission near globular cluster Terzan 5 as a probe of cosmic ray transport. Nat Astron (2024). https://doi.org/10.1038/s41550-024-02337-1

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