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Register a new userConsistency of the Infrared Variability of Sgr A* over 22 years
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We report new infrared measurements of the supermassive black hole at the Galactic Center, Sgr A*, over a decade that was previously inaccessible at these wavelengths. This enables a variability study that addresses variability timescales that are ten times longer than earlier published studies. Sgr A* was initially detected in the near-infrared with adaptive optics observations in 2002. While earlier data exists in form of speckle imaging (1995 - 2005), Sgr A* was not detected in the initial analysis. Here, we improved our speckle holography analysis techniques. This has improved the sensitivity of the resulting speckle images by up to a factor of three. Sgr A* is now detectable in the majority of epochs covering 7 years. The brightness of Sgr A* in the speckle data has an average observed K magnitude of 16.0, which corresponds to a dereddened flux density of $3.4$ mJy. Furthermore, the flat power spectral density (PSD) of Sgr A* between $sim$80 days and 7 years shows its uncorrelation in time beyond the proposed single power-law break of $sim$245 minutes. We report that the brightness and its variability is consistent over 22 years. This analysis is based on simulations using Witzel et al. (2018) model to characterize infrared variability from 2006 to 2016. Finally, we note that the 2001 periapse of the extended, dusty object G1 had no apparent effect on the near-infrared emission from accretion flow onto Sgr A*. The result is consistent with G1 being a self-gravitating object rather than a disrupting gas cloud.
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The electromagnetic counterpart to the Galactic center supermassive black hole, Sgr A*, has been observed in the near-infrared for over 20 years and is known to be highly variable. We report new Keck Telescope observations showing that Sgr A* reached much brighter flux levels in 2019 than ever measured at near-infrared wavelengths. In the K$^prime$ band, Sgr A* reached flux levels of $sim6$ mJy, twice the level of the previously observed peak flux from $>13,000$ measurements over 130 nights with the VLT and Keck Telescopes. We also observe a factor of 75 change in flux over a 2-hour time span with no obvious color changes between 1.6 $mu$m and 2.1 $mu$m. The distribution of flux variations observed this year is also significantly different than the historical distribution. Using the most comprehensive statistical model published, the probability of a single night exhibiting peak flux levels observed this year, given historical Keck observations, is less than $0.3%$. The probability to observe the flux levels similar to all 4 nights of data in 2019 is less than $0.05%$. This increase in brightness and variability may indicate a period of heightened activity from Sgr A* or a change in its accretion state. It may also indicate that the current model is not sufficient to model Sgr A* at high flux levels and should be updated. Potential physical origins of Sgr A*s unprecedented brightness may be from changes in the accretion-flow as a result of the star S0-2s closest passage to the black hole in 2018 or from a delayed reaction to the approach of the dusty object G2 in 2014. Additional multi-wavelength observations will be necessary to both monitor Sgr A* for potential state changes and to constrain the physical processes responsible for its current variability.
We performed the observation of the flux densities of SgrA* at 90 and 102GHz in order to detect the time lag between these frequencies using the Nobeyama Millimeter Array, which was previously reported at lower frequencies. We detected a radio flare during the observation period on 6 April 2005 and calculated the z-transformed discrete correlation function between the light curves. The time lag between these frequencies was not detected. If the expanding plasma model which explains the time lag at lower frequencies is valid, the light curve at 90GHz would be delayed with respect to the one at 102GHz. This result suggests that the plasma blobs ejected near the Galactic Center black hole may be widely diverse especially in optical thickness. Another possibility is that the major portion of the flux above 100GHz does not originate from the blobs.
The IceCube instrument detected a high-energy cosmic neutrino event on 2017 September 22 (IceCube_170922A, IceCube Collaboration 2018), which the electromagnetic follow-up campaigns associated with the flaring $gamma$-ray blazar TXS 0506$+$056 (e.g., Padovani et al., 2018). We investigated the mid-infrared variability of the source by using the available single exposure data of the WISE satellite at $3.4$ and $4.6mu$m. TXS 0506$+$056 experienced a $sim 30$% brightening in both of these bands a few days prior to the neutrino event. Additional intraday infrared variability can be detected in 2010. Similar behaviour seen previously in $gamma$-ray bright radio-loud AGN has been explained by their jet emission (e.g., Jiang et al. 2012).
We report the first detection of the Galactic Centre massive black hole, Sgr~A*, in the far infrared. Our measurements were obtained with PACS on board the emph{Herschel} satellite at $100~mathrm{mu m}$ and $160~mathrm{mu m}$. While the warm dust in the Galactic Centre is too bright to allow for a direct detection of Sgr~A*, we measure a significant and simultaneous variation of its flux of $Delta F_{ uwidehat{=}160 ~mathrm{mu m}} = (0.27pm0.06)~mathrm{Jy}$ and $Delta F_{ uwidehat{=}100 ~mathrm{mu m}}= (0.16pm0.10)~mathrm{Jy}$ during one observation. The significance level of the $160 ~mathrm{mu m}$ band variability is $4.5sigma$ and the corresponding $100 ~mathrm{mu m}$ band variability is significant at $1.6sigma$. We find no example of an equally significant false positive detection. Conservatively assuming a variability of $25%$ in the FIR, we can provide upper limits to the flux. Comparing the latter with theoretical models we find that 1D RIAF models have difficulties explaining the observed faintness. However, the upper limits are consistent with modern ALMA and VLA observations. Our upper limits provide further evidence for a spectral peak at $sim 10^{12} ~ mathrm{Hz}$ and constrain the number density of $gamma sim 100$ electrons in the accretion disk and or outflow.
We performed the observation of the flux densities of Sgr A* at 90 and 102 GHz on 6 April 2005 using the Nobeyama Millimeter Array in order to detect the time lag between these frequencies. We constructed light curves covering a few hour with 1 min bin, and the Intra-Day Variability, which had a rising phase and intensity peak, of Sgr A* is clearly seen at both frequencies. We calculated the z-transformed discrete correlation function between the light curves of Sgr A* at 90 and 102 GHz. The derived time lag of the flares at these frequencies was approximately zero, contrary to our expectations based on the previously reported time lag at lower frequencies. If the radio flares of Sgr A* are explained by the expanding plasma model, the light curve at 90 GHz would be delayed with respect to the one at 102 GHz. However, we could not find such a delay with statistical significance in our data.
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