Research Interests
I am an X-ray astronomer with research interests spanning the areas of astroparticle physics, astronomy and cosmology.
My research focusses on using the X-ray spectrum of Active Galactic Nuclei (AGN) to probe light Axion-Like Particles (ALPs) and the angular momentum (or spin) of Supermassive Black Holes (SMBHs). I have broad interests in Dark Matter (DM), astrophysical magnetic fields and turbulence.
I am generally interested in addressing the following research questions:
What is the nature of DM?
How can we use astrophysics to test theories extending Beyond the Standard Model of Particle Physics?
How can we use the observed distribution of SMBH spins to inform models of SMBH growth and evolution over cosmic time?
My research work involves investigating how our current knowledge on ALPs and SMBH spins from X-ray observations will improve with future telescopes.
My research has highlighted the broad axion science case foreseen with Athena, the next-generation European flagship X-ray Observatory, launching in 2036, as well as that foreseen with the Probe-Class STROBE-X Observatory being proposed to NASA for a 2032 launch. I joined the STROBE-X international collaboration as a working group member in May 2023.
Probing light ALPs with X-ray observations of AGNs
ALPs are generic extensions of the original axion that was proposed in the 1970s. The ‘QCD’ axion is the leading solution to “clean up” one of the ongoing problems (or tensions) in the Standard Model of Particle Physics - which explains why the ‘QCD’ axion was named after a cleaning detergent.
My research concerns the study of light ALPs, whose existence is predicted by string theory and Ultra-Light DM frameworks where ALPs constitute part of the ubiquitous DM in the Universe but whose nature we are yet to unveil. A compelling way in which light ALPs (of masses < 1e-12 electronvolts) could be detected -or equivalently, constrained on the basis of non-detection- is by determining how strongly they interact with light (or electromagnetic forces). This interaction manifests itself in the presence of magnetic fields. It follows that galaxy clusters are excellent probes of light ALPs, as they are permeated by magnetised gas. My research is based on the idea that the X-ray spectrum of AGNs hosted by rich galaxy clusters is distorted by light converting into axions along the cluster line-of-sight.
My collaborators and I have set the most stringent upper bounds on the coupling strength of light ALPs to electromagnetism based on the non-detection of such spectral distortions using observations of an extraordinary AGN (H1821+643) taken by the Chandra X-ray Observatory in 2001. We have also led a cutting-edge analysis which assesses the prospects of improving on these current best limits with upcoming X-ray observatories, in particular, the European flagship Athena mission which will be launching in 2036.
Left: Most stringent upper bounds on the coupling of light ALPs to electromagnetism based on X-ray observations of cluster-hosted AGNs.
The current best limits extracted from Chandra observations of H1821+643 are shown as Sisk-Reynes et al., (2022a).
Projected bounds from Athena observations of the cluster-hosted AGN NGC1275 are shown as Sisk-Reynes et al. (2023) both with and without an assessment of systematic uncertainties.
Projected bounds from a future ALP DM laboratory-based experiment called ADBC are shown.
Right: Upper bounds on the coupling of ALPs to electromagnetism based on astrophysical (green) and laboratory-based (red) searches over a wide range of ALP masses. The validity of these limits extend down to axion masses <1e-14 eV (not shown). Our current best limits on light ALPs (which hold regardless of whether ALPs make up for the observed DM abundance) appear with an asterisk as ‘Chandra H1821’. Projected limits from the next-generation Athena/X-IFU micro calorimeter (pre-redefinition) also appear with an asterisk as ‘Athena/conservative’ and as ‘Athena/optimal’ depending on whether systematic uncertainties are accounted for or not.
My published and accepted works on ALPs are:
4. “Current and Future constraints on Very-Light Axion-Like Particles from X-ray observations of cluster-hosted Active Galaxies.”
Sisk-Reynes, J., Reynolds, C and Matthews, J.
Accepted for publication at Memorie della SAIt in April 2023 (arXiv: 2304.08513).
3. “Physics Beyond the Standard Model with Future X-ray Observatories: Projected Constraints on Very-Light Axion-Like Particles with Athena and AXIS.”
Sisk-Reynes, J., Reynolds, C., Parker, M., Matthews, J. and M. C. David Marsh.
Published as Sisk-Reynes et al. (2023a), ApJ, 591, 1 (Link on NASA/ADS).
2. “How do Magnetic Field Models Affect Astrophysical Limits on Light Axion-like Particles? An X-ray Case Study with NGC 1275.”
Matthews, J., Reynolds, C., M. C. David Marsh, Sisk-Reynes, J. and Rodman, P.
Published as Matthews et al. (2022), ApJ, 530, 1 (Link on NASA/ADS).
1. “New constraints on light Axion-Like Particles using Chandra Transmission Grating Spectroscopy of the powerful cluster-hosted quasar H1821+643.”
Sisk-Reynes, J., Matthews, J., Reynolds, C., Russel, H., Smith, R. and M. C. David Marsh.
Published as Sisk-Reynes et al. (2022a), MNRAS, 510, 1 (Link on NASA/ADS).
Black Hole Spin as an indicator of recent growth
Black holes sustain the strongest gravitational fields in the Universe, distorting the nature of spacetime around them. At a fundamental level, astrophysical black holes are defined by two quantum numbers: (i) angular momentum (which astronomers like to quantify through a dimensionless spin parameter) or rotation and (ii) mass. One of my ongoing efforts is to study the observed distribution of spin over mass scale for SMBHs, that is, the heaviest types of black holes, believed to be hosted by most -if not, all- galaxies like our own Milky Way.
Interestingly, signatures of reflected X-ray emission in AGN spectra can arise due to the reprocessing -or reflection- of light originating from the innermost regions of the ionised accretion disc in AGNs. These signatures are affected by the strong gravitational and relativistic effects imparted by the central SMBH, meaning that they can be examined to study the properties of (i) the central SMBH, including SMBH spin and (ii) the inner accretion disc. This constitutes the basis of X-ray reflection spectroscopy, one of the most reliable observational techniques through which SMBH spin can be estimated.
My collaborators and I found evidence that one of the most massive SMBHs ever constrained with X-ray reflection spectroscopy is spinning at moderate rates, that is, below 84% of the speed of light. Our result, which constitutes one of the few well-defined spin estimates for such a massive object based on X-ray reflection spectroscopy, suggests that the heaviest SMBHs primarily grow through mergers and/or incoherent accretion. Incoherent accretion is expected to progressively spin SMBHs down as matter is accreted through chaotic trajectories. In contrast, coherent accretion, thought to dominate the late-time growth of low-mass SMBHs, is expected to boost the observed SMBH spin to maximal speeds (that is, the speed of light) as matter is fed into the hole in alignment with its direction of rotation.
Left: Composite image of the luminous AGN H1821+643, hosting the massive SMBH whose spin we constrained based on X-ray reflection spectroscopy.
Right: SMBH spin constraints over mass scales from the literature, based on X-ray reflection spectroscopy. The error bars on spin and mass show the 90% and 68% levels, respectively. Dimensionless SMBH spins of 1 and 0 denote maximally-spinning and non-rotating black holes, respectively.
The spin constraint we set for the high-mass SMBH hosted by the AGN H1821+643 is shown as Sisk-Reynes et al., (2022b).
Most spin constraints shown in blue were taken from the Bambi et al. (2021), SSR, 217, 5 spin review.
Spin constraints shown in green were extracted from Mallick et al. (2022), MNRAS, 513, 3 from a high disc-density reflection model.
I am currently writing a first-author paper where I am exploring the use of Bayesian statistics to describe the observed distribution of SMBH spins over mass scales from X-ray reflection. My published work on SMBHs comprises the following paper:
1. “Evidence for a moderate spin from X-ray reflection of the high-mass supermassive black hole in the cluster-hosted quasar H1821+643”.
Sisk-Reynes, J., Reynolds, C., Matthews, J and Smith, R.
Published as Sisk-Reynes et al. (2022b), MNRAS, 514, 2 (Link on NASA/ADS).