The flux distribution of Sgr A*

E Wiezorrek,G Ponti,Y Clénet,K Perraut,A Eckart,S D Von Fellenberg,S Scheithauer,M Habibi,G Zins,M Horrobin,T Paumard,G Rodriguez Coira,M Bauböck,M Nowak,A Jiménez-Rosales,F Widmann,F Vincent,X Haubois,I Waisberg,E Sturm,P T De Zeeuw,F Eisenhauer,L Jochum,A Amorim,S Gillessen,E Wieprecht,O Straub,L J Tacconi,R Genzel,V Lapeyrère,S Yazici,S Hippler,O Pfuhl,W Brandner,L Jocou,J Shangguan,S Lacour,G Perrin,P Kervella,J Woillez,J.-B Le Bouquin,R Abuter,N M Förster Schreiber,Y Dallilar,A Kaufer,F Gao,C Straubmeier,Thomas Ott ,Paulo J V Garcia ,Jason Dexter ,Pierre Léna ,Julia Stadler ,É Gendron ,Henri Bonnet ,Vítor Cardoso ,J B Berger ,Thomas Henning

doi:10.1051/0004-6361/202037717

Abstract

The Galactic center black hole Sagittarius A* is a variable near-infrared (NIR) source that exhibits bright flux excursions called flares. When flux from Sgr A* is detected, the light curve has been shown to exhibit red noise characteristics and the distribution of flux densities is non-linear, non-Gaussian, and skewed to higher flux densities. However, the low-flux density turnover of the flux distribution is below the sensitivity of current single-aperture telescopes. For this reason, the median NIR flux has only been inferred indirectly from model fitting, but it has not been directly measured. In order to explore the lowest flux ranges, to measure the median flux density, and to test if the previously proposed flux distributions fit the data, we use the unprecedented resolution of the GRAVITY instrument at the VLTI. We obtain light curves using interferometric model fitting and coherent flux measurements. Our light curves are unconfused, overcoming the confusion limit of previous photometric studies. We analyze the light curves using standard statistical methods and obtain the flux distribution. We find that the flux distribution of Sgr A* turns over at a median flux density of (1.1 ± 0.3) mJy. We measure the percentiles of the flux distribution and use them to constrain the NIR K-band spectral energy distribution. Furthermore, we find that the flux distribution is intrinsically right-skewed to higher flux density in log space. Flux densities below 0.1 mJy are hardly ever observed. In consequence, a single powerlaw or lognormal distribution does not suffice to describe the observed flux distribution in its entirety. However, if one takes into account a power law component at high flux densities, a lognormal distribution can describe the lower end of the observed flux distribution. We confirm the rms–flux relation for Sgr A* and find it to be linear for all flux densities in our observation. We conclude that Sgr A* has two states: the bulk of the emission is generated in a lognormal process with a well-defined median flux density and this quiescent emission is supplemented by sporadic flares that create the observed power law extension of the flux distribution.

Highlights

The supermassive black hole at the Galactic center, Sagittarius A* (Sgr A*) is associated with a variable radio/(sub-)mm source, a variable near-infrared (NIR) source, and a continuum source in the X-ray coupled with occasional strong X-ray flares (Genzel et al 2010).The NIR counterpart of Sgr A* is highly variable and not always detected in photometry of ground-based telescopes and space observatories
Follow-up studies of the flux distribution have found that a power law tail is not necessary and, instead, a single power law distribution or lognormal distribution suffices to describe the observed distribution of flux densities when temporal correlations are taken into account (Witzel et al 2012, 2018)
We build on our previous work on the flux distribution, extending the measurements beyond the detection limit of single-telescope observations using interferometric model fitting and coherent flux measurements

Summary

Introduction

The supermassive black hole at the Galactic center, Sagittarius A* (Sgr A*) is associated with a variable radio/(sub-)mm source, a variable near-infrared (NIR) source, and a continuum source in the X-ray coupled with occasional strong X-ray flares (Genzel et al 2010).The NIR counterpart of Sgr A* is highly variable and not always detected in photometry of ground-based telescopes and space observatories. The NIR flux distribution has been modeled with a multi-component distribution function, where the fainter flux levels, if detected, are described by a lognormal distribution and the brighter so-called flare states follow a power law tail (Dodds-Eden et al 2009). Follow-up studies of the flux distribution have found that a power law tail is not necessary and, instead, a single power law distribution or lognormal distribution suffices to describe the observed distribution of flux densities when temporal correlations are taken into account (Witzel et al 2012, 2018). By comparing the inferred spectral slope from parallel observations in the NIR K and M band, Witzel et al (2018) favor a lognormal distribution for both bands

Results

Discussion

Conclusion