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Register a new userUnprecedented variability of Sgr A* in NIR
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Tuan Do
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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.
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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.
To search for optical variability on a wide range of timescales, we have carried out photometric monitoring of two flat spectrum radio quasars, 3C 454.3 and 3C 279, plus one BL Lac, S5 0716+714, all of which have been exhibiting remarkably high activity and pronounced variability at all wavelengths. CCD magnitudes in B, V, R and I pass-bands were determined for $sim$ 7000 new optical observations from 114 nights made during 2011 - 2014, with an average length of $sim$ 4 h each, at seven optical telescopes: four in Bulgaria, one in Greece, and two in India. We measured multiband optical flux and colour variations on diverse timescales. Discrete correlation functions were computed among B, V, R, and I observations, to search for any time delays. We found weak correlations in some cases with no significant time lags. The structure function method was used to estimate any characteristic time-scales of variability. We also investigated the spectral energy distribution of the three blazars using B, V, R, I, J and K pass-band data. We found that the sources almost always follows a bluer-when-brighter trend. We discuss possible physical causes of the observed spectral variability.
We present multi-wavelength spectral and temporal variability analysis of PKS 0027-426 using optical griz observations from DES (Dark Energy Survey) between 2013-2018 and VOILETTE (VEILS Optical Light curves of Extragalactic TransienT Events) between 2018-2019 and near infrared (NIR) JKs observations from VEILS (VISTAExtragalactic Infrared Legacy Survey) between 2017-2019. Multiple methods of cross-correlation of each combination of light curve provides measurements of possible lags between optical-optical, optical-NIR, and NIR-NIR emission, for each observation season and for the entire observational period. Inter-band time lag measurements consistently suggest either simultaneous emission or delays between emission regions on timescales smaller than the cadences of observations. The colour-magnitude relation between each combination of filters was also studied to determine the spectral behaviour of PKS 0027-426. Our results demonstrate complex colour behaviour that changes between bluer when brighter (BWB), stable when brighter (SWB) and redder when brighter (RWB) trends over different timescales and using different combinations of optical filters. Additional analysis of the optical spectra is performed to provide further understanding of this complex spectral behaviour.
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.
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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