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Register a new userPolarized NIR and X-ray Flares from SgrA*
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Stellar dynamics indicate the presence of a super massive 3-4x10^6 Msun solm black hole at the Galactic Center. It is associated with the variable radio, near-infrared, and X-ray counterpart Sagittarius A* (SgrA*). The goal is the investigation and understanding of the physical processes responsible for the variable emission from SgrA*. 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 find that for the July 2005 flare the variable and polarized NIR emission of SgrA* occurred synchronous with a moderately bright flare event in the X-ray domain with an excess 2 - 8 keV luminosity of about 8x10^33erg/s. We find no time lag between the flare events in the two wavelength bands with a lower limit of less than 10 minutes. The May 2007 flare shows the highest sub-flare to flare contrast observed until now. It provides evidence for a variation in the profile of consecutive sub-flares. We confirm that highly variable and NIR polarized flare emission is non-thermal and that there exists a class of synchronous NIR/X-ray flares. We find that the flaring state can be explained via the synchrotron self-Compton (SSC) process involving up-scattered sub-millimeter photons from a compact source component. The observations can be interpreted in a model involving a temporary disk with a short jet. In the disk component the flux density variations can be explained due to spots on relativistic orbits around the central super massive black hole (SMBH). The profile variations for the May 2007 flare are interpreted as a variation of the spot structure due to differential rotation within the disk.
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We report on the first simultaneous near-infrared/X-ray detection of the Sgr A* counterpart which is associated with the massive black 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 Observatorys Very Large Telescope and 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$times10^{33}$ 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 hours 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 hours 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 $alpha_{NIR/X-ray}$ ($S_{ u}$$propto$$ u^{-alpha}$) of both states are quite comparable with a value of $sim$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 minutes.
We report on new modeling results based on the mm- to X-ray emission of the SgrA* counterpart associated with the massive black hole at the Galactic Center. Our modeling is based on simultaneous observations carried out on 07 July, 2004, using the ESO NACO adaptive optics instrument and the ACIS-I instrument aboard the Chandra X-ray Observatory as well as the SMA and the VLA. The observations revealed several flare events in all wavelength domains. Here we show that a combined synchrotron self-Compton (SSC) model followed by an adiabatic expansion of the source components can fully account for the observed flare flux densities and delay times covering the spectral range from the X-ray to the mm-radio domain. The derived physical quantities that describe the flare emission give a blob expansion speed of v{exp}=0.005c, magnetic field of < 60G and spectral indices of 0.8 to 1.4. The derived model parameters suggest that the adiabatic expansion takes place in source components that have a bulk motion larger than v{exp} or the expanding material contributes to a corona or disk, confined to the immediate surroundings of SgrA*.
We report on a successful, simultaneous observation and modeling of the sub-millimeter to near-infrared flare emission of the Sgr A* counterpart associated with the super-massive black hole at the Galactic center. Our modeling is based on simultaneous observations that have been carried out on 03 June, 2008 using the NACO adaptive optics (AO) instrument at the ESO VLT and the LABOCA bolometer at the APEX telescope. Inspection and modeling of the light curves show that the sub-mm follows the NIR emission with a delay of 1.5+/-0.5 hours. We explain the flare emission delay by an adiabatic expansion of the source components.
The detection of a high-energy neutrino from the flaring blazar TXS 0506+056 and the subsequent discovery of a neutrino excess from the same direction have strengthened the hypothesis that blazars are cosmic neutrino sources. The lack, however, of $gamma$-ray flaring activity during the latter period challenges the standard scenario of correlated $gamma$-ray and high-energy neutrino emission in blazars. We propose instead that TeV-PeV neutrinos are produced in coincidence with X-ray flares that are powered by proton synchrotron radiation. In this case, neutrinos are produced by photomeson interactions of protons with their own synchrotron radiation, while MeV to GeV $gamma$-rays are the result of synchrotron-dominated electromagnetic cascades developed in the source. Using a time-dependent approach, we find that this pure hadronic flaring hypothesis has several interesting consequences. The X-ray flux is a good proxy for the all-flavor neutrino flux, while certain neutrino-rich X-ray flares may be dark in GeV-TeV $gamma$-rays. Lastly, hadronic X-ray flares are accompanied by an equally bright MeV component that is detectable by proposed missions like e-ASTROGAM and AMEGO. We then applied this scenario to the extreme blazar 3HSP J095507.9+355101 that has been associated with IceCube-200107A while undergoing an X-ray flare. We showed that the number of muon and antimuon neutrinos above 100 TeV during hadronic flares can be up to $sim3-10$ times higher than the expected number in standard leptohadronic models. Still, frequent hadronic flaring activity is necessary for explaining the detected neutrino event IceCube-200107A.
[truncated] In Spring 2007, we observed SgrA* with XMM with a total exposure of ~230ks. We have performed timing and spectral analysis of the new X-ray flares detected during this campaign. To study the range of flare spectral properties, in a consistent manner, we have also reprocessed, using the same analysis procedure and the latest calibration, archived XMM data of previously reported rapid flares. The dust scattering was taken into account during the spectral fitting. We also used Chandra archived observations of the quiescent state of SgrA* for comparison. On April 4, 2007, we observed for the first time within a time interval of ~1/2 day, an enhanced incidence rate of X-ray flaring, with a bright flare followed by three flares of more moderate amplitude. The former event represents the second brightest X-ray flare from Sgr A* on record. This new bright flare exhibits similar light-curve shape (nearly symmetrical), duration (~3ks) and spectral characteristics to the very bright flare observed in October 3, 2002. The measured spectral parameters of the new bright flare, assuming an absorbed power law model taken into account dust scattering effect, are N_H=12.3(+2.1,-1.8)e22 cm-2 and Gamma~2.3+/-0.3 calculated at the 90% c.l. The spectral parameter fits of the sum of the three following moderate flares, while lower, are compatible within the error bars with those of the bright flares. The column density found, for a power-law, during the flares is at least two times higher than the value expected from the (dust) visual extinction toward SgrA* (AV~25 mag). However, our fitting of the SgrA* quiescent spectra obtained with Chandra shows that an excess of column density is already present during the non-flaring phase. The two brightest X-ray flares observed so far from SgrA* exhibited similar soft spectra.
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