Flux Density Of Sgr Research Articles

Monitoring the Size and Flux Density of Sgr A* during the Active State in 2019 with East Asian VLBI Network

In this work, we studied the Galactic Center supermassive black hole (SMBH), Sagittarius A* (Sgr A*), with the KVN and VERA Array (KaVA)/East Asian VLBI Network (EAVN) monitoring observations. Especially in 13 May 2019, Sgr A* experienced an unprecedented bright near infra-red (NIR) flare; so, we find a possible counterpart at 43 GHz (7 mm). As a result, a large temporal variation of the flux density at the level ∼15.4%, with the highest flux density of 2.04 Jy, is found on 11 May 2019. Interestingly, the intrinsic sizes are also variable, and the area and major-axis size show marginal correlation with flux density with ≳2 σ. Thus, we interpret that the emission region at 43 GHz follows the larger-when-brighter relation in 2019. The possible origins are discussed with an emergence of a weak jet/outflow component and the position angle change of the rotation axis of the accretion disk in time.

Monitoring the Galactic Centre with the Australia Telescope Compact Array

The supermassive black hole, Sagittarius A* (Sgr A*), at the centre of the Milky Way undergoes regular flaring activity which is thought to arise from the innermost region of the accretion flow. We performed the monitoring observations of the Galactic Centre to study the flux-density variations at 3mm using the Australia Telescope Compact Array (ATCA) between 2010 and 2014. We obtain the light curves of Sgr A* by subtracting the contributions from the extended emission around it, and the elevation and time dependent gains of the telescope. We perform structure function analysis and the Bayesian blocks representation to detect flare events. The observations detect six instances of significant variability in the flux density of Sgr A* in three observations, with variations between 0.5 to 1.0 Jy, which last for 1.5 $-$ 3 hours. We use the adiabatically expanding plasmon model to explain the short time-scale variations in the flux density. We derive the physical quantities of the modelled flare emission, such as the source expansion speed $v_{\mathrm{exp}}$, source sizes, spectral indices, and the turnover frequency. These parameters imply that the expanding source components are either confined to the immediate vicinity of Sgr A* by contributing to the corona or the disc, or have a bulk motion greater than $v_{\mathrm{exp}}$. No exceptional flux density variation on short flare time-scales was observed during the approach and the flyby of the dusty S-cluster object (DSO/G2). This is consistent with its compactness and the absence of a large bow shock.

22 GHz MONITORING OF SGR A*DURING THE G2 PERI-CENTER PASSING WITH THE JAPANESE VLBI NETWORK

We have been performing daily VLBI monitoring of the flux density of Sagittarius (Sgr) <TEX>$A^{\ast}$</TEX> at 22 GHz from February 2013 to August 2014 using a sub-array of the Japanese VLBI Network (JVN). The purpose of this monitoring is to explore the flux density variability at daily time resolution for a period longer than one year with the G2 cloud approaching. The flux density of Sgr <TEX>$A^{\ast}$</TEX> is basically stable during the observational period, though there are some small variations. The average and scattering range are consistent with the previously observed values. We have observed no strong flare of Sgr <TEX>$A^{\ast}$</TEX> although it is near the expected peri-center passing.

Simultaneous Multi-Wavelength Spectrum of Sgr A* and Long Wavelength Cutoff at λ > 30 cm

We reviewed the multiple-wavelength spectrum of Sgr A*. The recent continuum spectrum was determined using data obtained from the Very Large Array (VLA), the Giant Metrewave Radio Telescope (GMRT), the Submillimeter Array (SMA) and the Keck II 10m telescope on the same date (2003 June 17). The nearly simultaneous spectrum (S∝ λα) covers a broad wavelength (A) range from 90 cm to 3.8 micron. From about 2 cm to 1 mm, the source shows a rising spectrum with a spectral index of α = 0.43 ± 0.04. However, the IR, measurements suggested that the flux density of Sgr A* must drastically drop towards the shorter wavelengths and the turnover likely occurs in the submillimeter regime. The spectrum at wavelengths between 47 and 3.6 cm is flat with α = 0.11 ± 0.03. The flux density of Sgr A* is 0.22±0.06 Jy at 90 cm on this date in 2003. Compared with measurement at 47 cm, the flux density at 90 cm has decreased by a factor of two (or ∼ 4σ), suggesting a cutoff at wavelength longer than 47 cm. The fits using free-free absorption opacity to the spectrum suggest a cutoff wavelength at ∼ 100 cm. The cutoff wavelength appears to be three times longer than that of ∼ 30 cm determined by Davies, Walsh, & Booth [1] three decades ago based on observations in 1974 and 1975. A model interpreting the variation of the long wavelength cutoff using stellar winds (An et al. [2]) is summarized in this paper.

Simultaneous Multiwavelength Observations of Sagittarius A*

We observed Sgr A* using the Very Large Array (VLA) and the Giant Metrewave Radio Telescope (GMRT) at multiple centimeter and millimeter wavelengths on 2003 June 17. The measured flux densities of Sgr A*, together with those obtained from the Submillimeter Array (SMA) and the Keck II 10 m telescope on the same date, are used to construct a simultaneous spectrum of Sgr A* from 90 cm to 3.8 mu m. The simultaneous spectrum shows a spectral break at about 3.6 cm, a possible signature of synchrotron self-absorption of the strong radio outburst that occurred near epoch 2003 July 17. At 90 cm, the flux density of Sgr A* is 0.22 +/- 0.06 Jy, suggesting a sharp decrease in flux density at wavelengths longer than 47 cm. The spectrum at long cm wavelengths appears to be consistent with free-free absorption by a screen of ionized gas with a cutoff similar to 100 cm. This cutoff wavelength appears to be three times longer than that of similar to 30 cm suggested by Davies, Walsh, & Booth based on observations in 1974 and 1975. Our analysis suggests that the flux densities of Sgr A* at wavelengths longer than 30 cm could be attenuated and modulated by stellar winds from massive stars close to Sgr A*.

First simultaneous NIR/X-ray detection of a flare from Sgr A*

We report on the first simultaneous near-infrared/X-ray detection of the Sgr A* counterpart associated with the massive 3-4 × 10 6 Mblack hole at the center of the Milky Way. The observations have been carried out using the NACO adaptive optics (AO) instrument at the European Southern Observatory's Very Large Telescopeand the ACIS-I instrument aboard the Chandra X-ray Observatory. We also report on quasi-simultaneous observations at a wavelength of 3.4 mm using the Berkeley-Illinois-Maryland Association (BIMA) array. A flare was detected in the X-domain with an excess 2-8 keV luminosity of about 6 × 10 33 erg/s. A fading flare of Sgr A* with >2 times the interim-quiescent flux was also detected at the beginning of the NIR observations, that overlapped with the fading part of the X-ray flare. Compared to 8-9 h before the NIR/X-ray flare we detected a marginally significant increase in the millimeter flux density of Sgr A* during measurements about 7-9 h afterwards. We find that the flaring state can be conveniently explained with a synchrotron self-Compton model involving up-scattered sub-millimeter photons from a compact source component, possibly with modest bulk relativistic motion. The size of that component is assumed to be of the order of a few times the Schwarzschild radius. The overall spectral indices αNIR/X−ray (S ν ∝ ν −α ) of both states are quite comparable with a value of ∼1.3. Since the interim-quiescent X-ray emission is spatially extended, the spectral index for the interim-quiescent state is probably only a lower limit for the compact source Sgr A*. A conservative estimate of the upper limit of the time lag between the ends of the NIR and X-ray flare is of the order of 15 min.

Monitoring Sagittarius A* in the MIR with the VLT

The ISAAC and NAOS/CONICA systems on the ESO VLT UT1 and UT4 telescopes have been used to monitor the MIR L and M band (3.3 μm and 4.6 μm) flux density at the position of Sgr A* and to obtain new information on the stellar populations within the central cluster. The monitoring program was carried out simultaneously to Chandra observations and resulted in upper limits on the flux density of Sgr A* during a newly detected out-burst of the X-ray source at the position of Sgr A*.

Intrinsic Radio Variability of Sgr A*

We review and summarize the results on the variability of Sgr A* based on the recent monitoring programs with the SMA at 1.3 and 0.87 mm, and the VLA at 2, 1.3 and 0.7 cm. We discuss the flares at 1.3 mm and a cross-correlation of the SMA flux density at 1.3 mm with the VLA data at 1.3 cm. We also present a preliminary result on the double quasi-periodic oscillation (DQPO) in flux density of Sgr A* based on our analysis of radio light curves observed with the VLA at 1.3 cm.