New Evidence for a Flux-independent Spectral Index of Sgr A* in the Near-infrared

Abstract

In this work, we measure the spectral index of Sagittarius A* (Sgr A*) between the $H$ (1.6 $\mu$m) and $K^{\prime }$ (2.2 $\mu$m) broadband filters in the near-infrared (NIR), sampling over a factor $\sim 40$ in brightness, the largest range probed to date by a factor $\sim 3$. Sgr A*-NIR is highly variable, and studying the spectral index $\alpha$ (with $F_{\nu }\propto {\nu }^{\alpha }$) is essential to determine the underlying emission mechanism. For example, variations in $\alpha$ with flux may arise from shifts in the synchrotron cutoff frequency, changes in the distribution of electrons, or multiple concurrent emission mechanisms. We investigate potential variations of ${\alpha }_{H-K^{\prime }}$ with flux by analyzing 7 epochs (2005 to 2022) of Keck Observatory imaging observations from the Galactic Center Orbits Initiative (GCOI). We remove the flux contribution of known sources confused with Sgr A*-NIR, which can significantly impact color at faint flux levels. We interpolate between the interleaved $H$ and $K^{\prime }$ observations using Multi-Output Gaussian Processes. We introduce a flexible empirical model to quantify $\alpha$ variations and probe different scenarios. The observations are best fit by an ${\alpha }_{H-K^{\prime }}=-0.50\pm 0.08_{\mathrm{stat}}\pm 0.17_{\mathrm{sys}}$ that is constant from $\sim 1$ mJy to $\sim 40$ mJy (dereddened 2 $\mu$m flux). We find no evidence for a flux-dependence of Sgr A*'s intrinsic spectral index. In particular, we rule out a model explaining NIR variability purely by shifts in the synchrotron cutoff frequency. We also constrain the presence of redder, quiescent emission from the black hole, concluding that the dereddened 2 $\mu$m flux contribution must be $\le 0.3$ mJy at 95% confidence level.

Accepted for publication in The Astrophysical Journal, 42 pages, 18 figures

References in corpus (25)

• The Astropy Project: Sustaining and Growing a Community-oriented Open-source Project and the Latest Major Release (v5.0) of the Core Package
• First Sagittarius A* Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole in the Center of the Milky Way
• The Two Young Star Disks in the Central Parsec of the Galaxy: Properties, Dynamics and Formation
• First Sagittarius A* Event Horizon Telescope Results. V. Testing Astrophysical Models of the Galactic Center Black Hole
• Magnetic Reconnection and Hot Spot Formation in Black Hole Accretion Disks
• Polarimetry of near-infrared flares from Sagittarius A*
• Interstellar Extinction Law toward the Galactic Center II: V, J, H, and Ks Bands
• A Constant Spectral Index for Sagittarius A* During Infrared/X-ray Intensity Variations
• X-ray hiccups from SgrA* observed by XMM-Newton. The second brightest flare and three moderate flares caught in half a day
• Flares in the Galactic center I: orbiting flux tubes in Magnetically Arrested Black Hole Accretion Disks
• A polarised infrared flare from Sagittarius A* and the signatures of orbiting plasma hotspots
• Sgr A* near-infrared flares from reconnection events in a magnetically arrested disc
• Time Dependent Models of Flares from Sagittarius A*
• Rapid Variability of Sgr A* across the Electromagnetic Spectrum
• Constraining particle acceleration in Sgr A* with simultaneous GRAVITY, Spitzer, NuSTAR and Chandra observations
• The Flux Distribution of Sgr A*
• General relativistic MHD simulations of non-thermal flaring in Sagittarius A*
• Exploring plasma evolution during Sagittarius A* flares
• GALACTICNUCLEUS: A high-angular-resolution JHKs imaging survey of the Galactic centre III. Evidence for wavelength dependence of the extinction curve in the near-infrared
• On the Comparison of AGN with GRMHD Simulations: I. Sgr A*
• The Galactic Center: An Improved Astrometric Reference Frame for Stellar Orbits around the Supermassive Black Hole
• Diffraction Limited Imaging Spectroscopy of the SgrA* Region using OSIRIS, a new Keck Instrument
• Multi-wavelength Variability of Sagittarius A* in July 2019
• The Relationship Between Simulated Sub-Millimeter and Near-Infrared Images of Sagittarius A* from a Magnetically Arrested Black Hole Accretion Flow
• S0-2 star, G1- and G2-objects and flaring activity of the Milky Way's Galactic Center black hole in 2019

Cited by in corpus (2)

• Tidal phenomena in the Galactic Center: The curious case of X7
• Variability of Sagittarius A* at 3 GHz on minute-scale with MeerKAT