Photometric Constraints on Intermediate-mass Black Holes in the Galactic Centre

TL;DR

This work tackles whether an intermediate-mass black hole may reside in the GC near IRS 13E by using JWST/MIRI time-series photometry to constrain short-timescale infrared variability expected from accreting IMBHs. By modeling emission with radiatively inefficient accretion flows (RIAFs) and anchoring to Bondi accretion scalings, the authors translate nondetections of variability into stringent limits on $M_{ m BH}$ and $\dot m$. They rule out IMBHs with $M_{ m BH}\gtrsim 10^{3}\,M_\\odot$ accreting at $\dot m\gtrsim 10^{-6}$ near IRS 13E and extend these constraints to the central $6\arcsec\times6\arcsec$ GC field (with $M_{ m BH}\gtrsim 2\times10^{3}\,M_\odot$ ruled out at similar $\dot m$). The results demonstrate the viability of photometric variability measurements to constrain accreting black holes in GC-like environments and provide guidance for future ELT/SKA follow-up efforts.

Abstract

JWST/MIRI observations can place photometric limits on the presence of an intermediate-mass black hole (IMBH) near the Galactic Centre. The stellar complex IRS 13E, a co-moving conglomerate of young and massive stars, is a prime location to study because it has been speculated to be bound by an IMBH. Assuming a standard radiatively inefficient accretion flow (RIAF) and a minimum fractional variability of 10% of intrinsic luminosity, the wavelength of peak emission in the spectral energy distribution for an IMBH would lie in the mid-infrared ($\sim$ 5-25 $μ$m), and variability would be detectable in MIRI time-series observations. Monitoring fails to detect such variable emission (other than from Sgr A*) in and around the IRS 13E complex, and upper limits on a putative IMBH's intrinsic variability on timescales of minutes to about 1 hour are $\lesssim$1 mJy at 12 $μ$m and $\lesssim$2 mJy at 19 $μ$m. These translate to luminosities $\lesssim 25 \times 10^{32}$ erg/s. The resulting limits on the IMBH mass and accretion rate rule out any IMBH with mass $\gtrsim 10^3$ M$_\odot$ accreting at $\gtrsim 10^{-6}$ times Eddington rate at the location of IRS 13E. Further, the observations rule out an IMBH anywhere in the central 6" $\times$ 6" region that is more massive than $\approx$ 2 $\times 10^3$ M$_\odot$ and accreting at $\gtrsim 10^{-6}$ of the Eddington rate. Assuming Bondi accretion scaled to typical RIAF-accretion efficiencies, albeit somewhat uncertain, also allows us to rule out IMBHs moving with typical velocities of about 200 km/s and masses $\gtrsim 2 \times 10^3$ M$_\odot$. These methods showcase the effectiveness of photometric variability measurements in constraining the presence of accreting black holes in Galactic centre-like environments.

Figures (4)

• Figure 1: JWST/MIRI images of channels 3 (${\sim} 12.3~\mu$m, left) and 4 (${\sim} 19.5~\mu$m, right) overlaid on the VISIR 8.6 $\mu$m image Dinh2024. IRS 13E is visible as a diffuse and moderately bright object marked by pink boxes in the lower right corner of each MIRI image. Regions are marked A--F are reference regions ordered in decreasing brightness. The box sizes indicate the $3\times 3$ pixel apertures used for extracting fluxes. The channel 4 pixels are nearly a factor of two larger than the the channel 3 pixels, and the aperture centres are not exactly aligned, but the pink boxes still enclose the brightest region of IRS 13E. The large orange square is $6"\times6"$ in size and indicates the approximate MIRI FoV.
• Figure 2: Sample light curve of IRS 13E. Coloured points show residual flux densities for channels 3 and 4 as indicated in the legend. The residuals were calculated by subtracting the median flux density of 2859 and 5394 mJy for the two channels, respectively and removing a linear drift. The light curve is from April 6 exposure 2, the same epoch where Sgr A* showed a mid-infrared flare Fellenberg_2025. Unlike the Sgr A* light curve, there is no correlation between the two channels.
• Figure 3: Likelihoods of $M_{\rm{BH}}$,$\dot{m}$ combinations imposed by the variability upper limits. The upper panel is for IRS 13E, and the lower one is for all regions in the field of view. Both are smoothed using a Gaussian kernel of width $0.02 \times$ the span of each parameter. Likelihoods are shown according to the respective colour bars, and the dashed line in each panel (white top panel, red bottom panel) shows the $\mathcal{L}=0.5$ contour for IRS 13E. In the upper panel, the green cross marks the proposed black-hole mass and accretion rate of Peissker_2024, and the yellow diamond shows the accretion rate derived by Labaj_2025arXiv from MHD simulation of stellar winds from six WR stars in the IRS 13E cluster. The stellar mass is the value used by both authors. Allowed mass ranges from previous studies are marked by pairs of dashed lines: green, purple, and blue for Schoedel2005Fritz2010Tsuboi2017, respectively. The nominal Bondi accretion regime is indicated by a solid pink line, and the range of reasonable parameters is between the solid red lines. In the lower panel, solid lines show contours of $\mathcal{L}=0.5$ for the six representative regions (A-–F), and background shading shows the combined likelihood.
• Figure 4: Galactic Centre IMBH parameter ranges excluded by observations. IMBH mass is shown horizontally and projected distance from Sgr A* vertically, and shaded areas indicate excluded ranges. This figure is based on Figure D2 of GRAVITYCollaboration_schwarzschild, and our new exclusions based on Bondi RIAF accretion are shown in orange. The differing zones for IMBH velocities of 10, 200, and 600 km s$^{-1}$ are marked. Other constraints are from Naoz2020, Will2023, Reid2004Reid2020, Gillessen2009, Hansen2003, and Yu2003. The grey area at bottom is excluded by the gravitational-wave inspiral timescale GRAVITYCollaboration_schwarzschild.