Anton Zensus

Polarized NIR and X-ray flares from Sagittarius A

Context. Stellar dynamics indicate the presence of a supermassive 3−4 × 10 6 Mblack hole at the Galactic Center. It is associated with the variable radio, near-infrared, and X-ray source Sagittarius A* (SgrA*). Aims. The goal is the investigation and understanding of the physical processes responsible for the variable emission from SgrA*. Methods. 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). Results. We find that for the July 2005 flare the variable and polarized NIR emission of SgrA* occurred synchronous with a moder- ately bright flare event in the X-ray domain with an excess 2−8 keV luminosity of about 8 × 10 33 erg/s. We find no time lag between the flare events in the two wavelength bands with a lower limit of ≤10 min. 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. Conclusions. We confirm that highly variable and NIR polarized flare emission is non-thermal and that there exists a class of syn- chronous NIR/X-ray flares. We find that the flaring state can be explained via the synchrotron self-Compton (SSC) process involving up-scattered X-rays from the 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 by spots on relativistic orbits around the central supermassive black hole (SMBH). The profile variations for the May 2007 flare can be interpreted as a variation of the spot structure due to differential rotation within the disk.

230 GHz VLBI Observations of M87: Event-horizon-scale Structure during an Enhanced Very-high-energy γ--Ray State in 2012

We report on 230 GHz (1.3 mm) VLBI observations of M87 with the Event Horizon Telescope using antennas on Mauna Kea in Hawaii, Mt. Graham in Arizona and Cedar Flat in California. For the first time, we have acquired 230 GHz VLBI interferometric phase information on M87 through measurement of closure phase on the triangle of long baselines. Most of the measured closure phases are consistent with 0 ◦ as expected by physically-motivated models for 230 GHz structure such as jet models and accretion disk models. The brightness temperature of the event-horizon-scale structure is � 1 × 10 10 K derived from the compact flux density of � 1 Jy and the angular size of � 40 µas � 5.5 Rs, which is broadly consistent with the peak brightness of the radio cores at 1-86 GHz located within � 10 2 Rs. Our observations occurred in the middle of an enhancement in very-high-energy (VHE) -ray flux, presumably originating in the vicinity of the central black hole. Our measurements, combined with results of multi-wavelength observations, favor a scenario in which the VHE region has an extended size of �20-60 Rs. Subject headings: galaxies: active —galaxies: individual (M87) —galaxies: jets —radio continuum: galaxies —techniques: high angular resolution —techniques: interferometric

Modeling mm- to X-ray flare emission from Sagittarius A*

Context. We report on new modeling results based on the mm- to X-ray emission of the SgrA* counterpart associated with the massive∼4×10 6 M⊙ black hole at the Galactic Center. Aims. We investigate the physical processes responsible for the variable emission from SgrA*. Methods. Our modeling is based on simultaneous observations carried out on 07 July, 2004, using the NACO adaptive optics (AO) instrument at the European Southern Observatory’s Very Lar ge Telescope ⋆ and the ACIS-I instrument aboard the Chandra X-ray Observatory as well as the Submillimeter Array SMA ⋆⋆ on Mauna Kea, Hawaii, and the Very Large Array ⋆⋆⋆ in New Mexico. Results. The observations revealed several flare events in all wavele ngth domains. Here we show that the flare emission can be described with a combination of a synchrotron self-Compton (SSC) model followed by an adiabatic expansion of the source components. The SSC emission at NIR and X-ray wavelengths involves up-scattered sub-millimeter photons from a compact source component. At the start of the flare, spectra of these components peak at fre quencies between several 100 GHz and 2 THz. The adiabatic expansion then accounts for the variable emission observed at sub-mm/mm wavelengths. The derived physical quantities that describe the flare emission give a blob expansion speed of vexp∼ 0.005c, magnetic field of B around 60 G or less and spectral indic es ofα=0.8 to 1.4, corresponding to a particle spectral index p∼2.6 to 3.8. Conclusions. A combined SSC and adiabatic expansion model 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 model parameters suggest that the adiabatic expansion takes place in source components that have a bulk motion larger than vexp or the expanding material contributes to a corona or disk, confined to the immediate surroundings of SgrA*.

Near infrared flares of Sagittarius A* - Importance of near infrared polarimetry

Context. We report on the results of new simulations of near-infrared (NIR) observations of the Sagittarius A* (Sgr A*) counterpart associated with the super-massive black hole at the Galactic Center. Aims. Our goal is to investigate and understand the physical processes behind the variability associated with the NIR flaring emission from Sgr A * . Methods. The observations have been carried out using the NACO adaptive optics (AO) instrument at the European Southern Observatorys Very Large Telescope and CIAO NIR camera on the Subaru telescope (13 june 2004; 30 july 2005; 1 june 2006; 15 may 2007; 17 may 2007 and 28 may 2008). We used a model of synchrotron emission from relativistic electrons in the inner parts of an accretion disk. The relativistic simulations have been carried out using the Karas-Yaqoob (KY) ray-tracing code. Results. We probe the existence of a correlation between the modulations of the observed flux density light curves and changes in polarimetric data. Furthermore, we confirm that the same correlation is also predicted by the hot spot model. Correlations between intensity and polarimetric parameters of the observed light curves as well as a comparison of predicted and observed light curve features through a pattern recognition algorithm result in the detection of a signature of orbiting matter under the influence of strong gravity. This pattern is detected statistically significant against randomly polarized red noise. Expected results from future observations of VLT interferometry like GRAVITY experiment are also discussed. Conclusions. The observed correlations between flux modulations and changes in linear polarization degree and angle can be a sign that the NIR flares have properties that are not expected from purely random red-noise. We find that the geometric shape of the emission region plays a major role in the predictions of the model. From fully relativistic simulations of a spiral shape emitting region, we conclude that the observed swings of the polarization angle during NIR flares support the idea of compact orbiting spots instead of extended patterns. The effects of gravitational shearing, fast synchrotron cooling of the components and confusion from a variable accretion disk have been taken into account. Simulated centroids of NIR images lead us to the conclusion that a clear observation of the position wander of the center of NIR images with future infrared interferometers will prove the existence of orbiting hot spots in the vicinity of our Galactic super-massive black hole.

Giant pulses with nanosecond time resolution detected from the Crab pulsar at 8.5 and 15.1 GHz

Aims. We present a study of shapes, spectra, and polarization properties of giant pulses (GPs) from the Crab pulsar at the very high frequencies of 8.5 and 15.1 GHz. Studies at 15.1 GHz are performed for the first time. We probe GP emission at high frequencies and examine its intrinsic spectral and polarization properties with high time and spectral resolution. The use of high radio frequencies also alleviates the effects of pulse broadening caused by interstellar scattering, which masks the intrinsic properties of GPs at low frequencies. Methods. Observations were conducted with the 100-m radio telescope in Effelsberg in Oct.‐Nov. 2007 at the frequencies of 8.5 and 15.1 GHz as part of an extensive campaign of multi-station multi-frequency observations of the Crab pulsar. A selection of the strongest pulses was recorded with a new data acquisition system, based on a fast digital oscilloscope, providing nanosecond time resolution in two polarizations with a bandwidth of about 500 MHz. In total, 29 and 85 GPs at longitudes of the main pulse and interpulse were recorded at 8.5 and 15.1 GHz during 10 and 17 h of observing time respectively. We analyzed the pulse shapes, polarisation and dynamic spectra of GPs as well as the cross-correlations between their LHC and RHC signals. Results. No events were detected outside the main pulse and interpulse windows. The GP properties were found to be very different for GPs emitted at longitudes of the main pulse and the interpulse. Cross-correlations of the LHC and RHC signals show regular patterns in the frequency domain for the main pulse, which are missing for the interpulse GPs. We consider the consequences of applying the rotating vector model to explain the apparent smooth variation in the position angle of linear polarization for main pulse GPs. We also introduce a new scenario of GP generation as a direct consequence of the polar cap discharge. Conclusions. We find further evidence of strong nano-shot discharges in the magnetosphere of the Crab pulsar. The repetitive frequency spectrum seen in GPs at the main pulse phase is interpreted as a diffraction pattern of regular structures in the emission region. The interpulse GPs however have a spectrum that resembles that of amplitude modulated noise. Propagation effects may be the cause of the differences.

SPECTRAL ANALYSIS OF THE ACCRETION FLOW IN NGC 1052 WITH SUZAKU

We present an analysis of the 101 ks, 2007 Suzaku spectrum of the low ionization nuclear emission region galaxy NGC 1052. The 0.5-10 keV continuum is well modeled by a power-law continuum modified by Galactic and intrinsic absorption, and it exhibits a soft, thermal emission component below 1 keV. Both a narrow core and a broader component of Fe Kα emission centered at 6.4 keV are robustly detected. While the narrow line is consistent with an origin in material distant from the black hole, the broad line is best fit empirically by a model that describes fluorescent emission from the inner accretion disk around a rapidly rotating black hole. We find no evidence in this observation for Comptonized reflection of the hard X-ray source by the disk above 10 keV, however, which casts doubt on the hypothesis that the broad iron line originates in the inner regions of a standard accretion disk. We explore other possible scenarios for producing this spectral feature and conclude that the high equivalent width (EW ~185 keV) and full-width-half-maximum velocity of the broad iron line (v ≥ 0.37c) necessitate an origin within d ~ 8 r g of the hard X-ray source. Based on the confirmed presence of a strong radio jet in this galaxy nucleus, the broad iron line may be produced in dense plasma near the base of the jet, implying that emission mechanisms in the centralmost portions of active galactic nuclei are more complex than previously thought.

Very Long Baseline Interferometry with the SKA

Adding VLBI capability to the SKA arrays will greatly broaden the science of the SKA, and is feasible within the current specifications. SKA-VLBI can be initially implemented by providing phased-array outputs for SKA1-MID and SKA1-SUR and using these extremely sensitive stations with other radio telescopes, and in SKA2 by realising a distributed configuration providing baselines up to thousands of km, merging it with existing VLBI networks. The motivation for and the possible realization of SKA-VLBI is described in this paper.

The LOFAR long baseline snapshot calibrator survey

Aims. An efficient means of locating calibrator sources for International LOFAR is developed and used to determine the average density of usable calibrator sources on the sky for subarcsecond observations at 140 MHz. Methods. We used the multi-beaming capability of LOFAR to conduct a fast and computationally inexpensive survey with the full International LOFAR array. Sources were pre-selected on the basis of 325 MHz arcminute-scale flux density using existing catalogues. By observing 30 different sources in each of the 12 sets of pointings per hour, we were able to inspect 630 sources in two hours to determine if they possess a sufficiently bright compact component to be usable as LOFAR delay calibrators. Results. Over 40% of the observed sources are detected on multiple baselines between international stations and 86 are classified as satisfactory calibrators. We show that a flat low-frequency spectrum (from 74 to 325 MHz) is the best predictor of compactness at 140 MHz. We extrapolate from our sample to show that the density of calibrators on the sky that are sufficiently bright to calibrate dispersive and non-dispersive delays for the International LOFAR using existing methods is 1.0 per square degree. Conclusions. The observed density of satisfactory delay calibrator sources means that observations with International LOFAR should be possible at virtually any point in the sky, provided that a fast and efficient search using the methodology described here is conducted prior to the observation to identify the best calibrator.

Large-scale motion, oscillations and a possible halo on the counter-jet side in 1803+784

We present world-array VLBI observations of the blazar 1803+784 performed on May 29, 1993 at A = 18 cm. A 17-station VLBI array, the phased VLA and the 7-station MERLIN array observed the source simultaneously for 11 h. We present the global VLBI map, the MERLIN map, and combined-array maps at different resolutions giving an overview of the morphology of 1803+784 from the inner 10 mas to ∼2 arcsec. We show that the jet bends by around 90° at a core separation of about 0.5 arcsec towards the South. On larger scales, a Westerbork Synthesis Radio Telescope (WSRT) map made at a wavelength of 6 cm resolves the bridge connecting the core with the secondary component some 45 to the south. Wiggles in the ridge of this emission suggest that, as on the 100-pc scale, the jet may also oscillate on 50-100 kpc scales. In addition to improved imaging of the southern component, the 6 cm map provides evidence for amorphous emission to the north, as well as an extended, halo-like component about the nucleus. We compare the source structure at 6 and 18 cm with the structure at 2 cm in a VLBA image obtained on November 6. 1999. Jet wiggling is clearly seen on all scales between 1 mas and 50 arcsec. We find some indication for apparent superluminal motion between ∼5c and 7c for the 25 mas jet component in model-fit results for five epochs of VLBI observations performed at 6 and 18 cm. In addition, we find some evidence for further so-called stationary components in the jet of this source which will have to be confirmed in future observations spaced more densely in time. We discuss the self-similar source morphology observed from mm to cm wavelengths - namely the wiggling on all observed scales - and the implications of the deviations from self-similarity. This is the second of two papers describing the radio structure of S5 1803+784. The first presented the results of 6 years of VLBI monitoring in the X-band (Britzen et al. 2005, MNRAS, in press).

Binary black holes in nuclei of extragalactic radio sources

If we assume that nuclei of extragalactic radio sources contain binary black hole systems, the two black holes can eject VLBI components, in which case two families of di erent VLBI trajectories will be observed. Another important consequence of a binary black hole system is that the VLBI core is associated with one black hole, and if a VLBI component is ejected by the second black hole, one expects to be able to detect the o set of the origin of the VLBI component ejected by the black hole that is not associated with the VLBI core. The ejection of VLBI components is perturbed by the precession of the accretion disk and the motion of the black holes around the center of gravity of the binary black hole system. We modeled the ejection of the component taking into account the two pertubations and present a method to fit the coordinates of a VLBI component and to deduce the characteristics of the binary black hole system. Specifically, this is the ratio Tp=Tb where Tp is the precession period of the accretion disk and Tb is the orbital period of the binary black hole system, the mass ratio M1=M2, and the radius of the binary black hole system Rbin. From the variations of the coordinates as a function of time of the ejected VLBI component, we estimated the inclination angle io and the bulk Lorentz factor of the modeled component. We applied the method to component S1 of 1823+568 and to component C5 of 3C 279, which presents a large o set of the space origin from the VLBI core. We found that 1823+568 contains a binary black hole system whose size is Rbin 60 as ( as is a microarcsecond) and 3C 279 contains a binary black hole system whose size is Rbin 420 as. We calculated the separation of the two black holes and the coordinates of the second black hole from the VLBI core. This information will be important to link the radio reference-frame system obtained from VLBI observations and the optical reference-frame system obtained from Gaia.