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Register a new userSimultaneous Multi-Wavelength Observations of Sgr A* during 2007 April 1-11
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We report the detection of variable emission from Sgr A* in almost all wavelength bands (i.e. centimeter, millimeter, submillimeter, near-IR and X-rays) during a multi-wavelength observing campaign. Three new moderate flares are detected simultaneously in both near-IR and X-ray bands. The ratio of X-ray to near-IR flux in the flares is consistent with inverse Compton scattering of near-IR photons by submillimeter emitting relativistic particles which follow scaling relations obtained from size measurements of Sgr A*. We also find that the flare statistics in near-IR wavelengths is consistent with the probability of flare emission being inversely proportional to the flux. At millimeter wavelengths, the presence of flare emission at 43 GHz (7mm) using VLBA with milli-arcsecond spatial resolution indicates the first direct evidence that hourly time scale flares are localized within the inner 30$times$70 Schwarzschild radii of Sgr A*. We also show several cross correlation plots between near-IR, millimeter and submillimeter light curves that collectively demonstrate the presence of time delays between the peaks of emission up to three hours. The evidence for time delays at millimeter and submillimeter wavelengths are consistent with the source of emission being optically thick initially followed by a transition to an optically thin regime. In particular, there is an intriguing correlation between the optically thin near-IR and X-ray flare and optically thick radio flare at 43 GHz that occurred on 2007 April 4. This would be the first evidence of a radio flare emission at 43 GHz delayed with respect to the near-IR and X-ray flare emission.
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Suzaku observations of the blazar OJ 287 were performed in 2007 April 10--13 and November 7--9. They correspond to a quiescent and a flaring state, respectively. The X-ray spectra can be well described with single power-law models in both exposures. The derived X-ray photon index and the flux density at 1 keV were found to be Gamma = 1.65 +- 0.02 and S_{1 keV} = 215 +- 5 nJy, in the quiescent state. In the flaring state, the source exhibited a harder X-ray spectrum (Gamma = 1.50 +- 0.01) with a nearly doubled X-ray flux density S_{1 keV} = 404^{+6}_{-5} nJy. Moreover, significant hard X-ray signals were detected up to ~ 27 keV. In cooperation with the Suzaku, simultaneous radio, optical, and very-high-energy gamma-ray observations were performed with the Nobeyama Millimeter Array, the KANATA telescope, and the MAGIC telescope, respectively. The radio and optical fluxes in the flaring state (3.04 +- 0.46 Jy and 8.93 +- 0.05 mJy at 86.75 Hz and in the V-band, respectively) were found to be higher by a factor of 2--3 than those in the quiescent state (1.73 +- 0.26 Jy and 3.03 +- 0.01 mJy at 86.75 Hz and in the V-band, respectively). No notable gamma-ray events were detected in either observation. The spectral energy distribution indicated that the X-ray spectrum was dominated by inverse Compton radiation in both observations, while synchrotron radiation exhibited a spectral cutoff around the optical frequency. Furthermore, no significant difference in the synchrotron cutoff frequency was found between the quiescent and flaring states. According to a simple synchrotron self-Compton model, the change of the spectral energy distribution is due to an increase in the energy density of electrons with small changes of both the magnetic field strength and the maximum Lorentz factor of electrons.
Aims. We report on simultaneous observations and modeling of mid-infrared (MIR), near-infrared (NIR), and submillimeter (submm) emission of the source Sgr A* associated with the supermassive black hole at the center of our Galaxy. Our goal was to monitor the activity of Sgr A* at different wavelengths in order to constrain the emitting processes and gain insight into the nature of the close environment of Sgr A*. Methods. We used the MIR instrument VISIR in the BURST imaging mode, the adaptive optics assisted NIR camera NACO, and the sub-mm antenna APEX to monitor Sgr A* over several nights in July 2007. Results. The observations reveal remarkable variability in the NIR and sub-mm during the five nights of observation. No source was detected in the MIR, but we derived the lowest upper limit for a flare at 8.59 microns (22.4 mJy with A_8.59mu = 1.6+/- 0.5). This observational constraint makes us discard the observed NIR emission as coming from a thermal component emitting at sub-mm frequencies. Moreover, comparison of the sub-mm and NIR variability shows that the highest NIR fluxes (flares) are coincident with the lowest sub-mm levels of our five-night campaign involving three flares. We explain this behavior by a loss of electrons to the system and/or by a decrease in the magnetic field, as might conceivably occur in scenarios involving fast outflows and/or magnetic reconnection.
We present and analyze ALMA submillimeter observations from a multi-wavelength campaign of Sgr A* during 18 July 2019. In addition to the submillimeter, we utilize concurrent mid-IR (Spitzer) and X-ray (Chandra) observations. The submillimeter emission lags $delta t=21.48^{+3.44}_{-3.57}$ minutes behind the mid-IR data. The entire submillimeter flare was not observed, raising the possibility that the time delay is a consequence of incomplete sampling of the light curve. The decay of the submillimeter emission is not consistent with synchrotron cooling. Therefore, we analyze these data adopting an adiabatically expanding synchrotron source that is initially optically thick or thin in the submillimeter, yielding time-delayed or synchronous flaring with the IR, respectively. The time-delayed model is consistent with a plasma blob of radius $0.8~R_{text{S}}$ (Schwarzschild radius), electron power-law index $p=3.5$ ($N(E)propto E^{-p}$), equipartition magnetic field of $B_{text{eq}}approx90$ Gauss, and expansion velocity $v_{text{exp}}approx0.004c$. The simultaneous emission is fit by a plasma blob of radius $2~R_{text{S}}$, $p=2.5$, $B_{text{eq}}approx27$ Gauss, and $v_{text{exp}}approx0.014c$. Since the submillimeter time delay is not completely unambiguous, we cannot definitely conclude which model better represents the data. This observation presents the best evidence for a unified flaring mechanism between submillimeter and X-ray wavelengths and places significant constraints on the source size and magnetic field strength. We show that concurrent observations at lower frequencies would be able to determine if the flaring emission is initially optically thick or thin in the submillimeter.
We report on recent near-infrared (NIR) and X-ray observations of Sagittarius A* (Sgr A*), the electromagnetic manifestation of the ~4x10^6 solar masses super-massive black hole (SMBH) at the Galactic Center. The goal of these coordinated multi-wavelength observations is to investigate the variable emission from Sgr A* in order to obtain a better understanding of the underlying physical processes in the accretion flow/outflow. The observations have been carried out using the NACO adaptive optics (AO) instrument at the European Southern Observatorys Very Large Telescope (July 2005, May 2007) and the ACIS-I instrument aboard the Chandra X-ray Observatory (July 2005). We report on a polarized NIR flare synchronous to a 8x1033 erg/s X-ray flare in July 2005, and a further flare in May 2007 that shows the highest sub-flare to flare contrast observed until now. The observations can be interpreted in the framework of a model involving a temporary disk with a short jet. In the disk component flux density variations can be explained due to hot spots on relativistic orbits around the central SMBH. The variations of the sub-structures of the May 2007 flare are interpreted as a variation of the hot spot structure due to differential rotation within the disk.
In order to study short timescale optical variability of $gamma$-ray blazar S5 0716+714, quasi-simultaneous spectroscopic and multi-band photometric observations were performed from 2018 November to 2019 March with the 2.4 m optical telescope located at Lijiang Observatory of Yunnan Observatories. The observed spectra are well fitted with a power-law $F_{lambda}=Alambda ^{-alpha}$ (spectral index $alpha >0$). Correlations found between $dot{alpha}$, $dot{A}$, $dot{A}/A$, $dot{F_{rm{lambda}}}$, and $dot{F_{rm{lambda}}}/F_{rm{lambda}}$ are consistent with the trend of bluer-when-brighter (BWB). textbf{The same case is for colors, magnitudes, color variation rates, and magnitude variation rates of photometric observations.} The variations of $alpha$ lead those of $F_{rm{lambda}}$. Also, the color variations lead the magnitude variations. The observational data are mostly distributed in the I(+,+) and III(-,-) quadrants of coordinate system. Both of spectroscopic and photometric observations show BWB behaviors in S5 0716+714. The observed BWB may be explained by the shock-jet model, and its appearance may depend on the relative position of the observational frequency ranges with respect to the synchrotron peak frequencies, e.g., at the left of the peak frequencies. textbf{Fractional variability amplitudes are $F_{rm{var}}sim 40%$ for both of spectroscopic and photometric observations. Variations of $alpha$ indicate variations of relativistic electron distribution producing the optical spectra. }
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