arXiv subjects

Silke Britzen

Publications and source records attributed to Silke Britzen.

At least 37 records · Page 2Linked to original sources

arXiv2023

A ring-like accretion structure in M87 connecting its black hole and jet

The nearby radio galaxy M87 is a prime target for studying black hole accretion and jet formation^{1,2}. Event Horizon Telescope observations of M87 in 2017, at a wavelength of 1.3 mm, revealed a ring-like structure, which was interpreted as gravitationally lensed emission around a central black hole^3. Here we report images of M87 obtained in 2018, at a wavelength of 3.5 mm, showing that the compact radio core is spatially resolved. High-resolution imaging shows a ring-like structure of 8.4_{-1.1}^{+0.5} Schwarzschild radii in diameter, approximately 50% larger than that seen at 1.3 mm. The outer edge at 3.5 mm is also larger than that at 1.3 mm. This larger and thicker ring indicates a substantial contribution from the accretion flow with absorption effects in addition to the gravitationally lensed ring-like emission. The images show that the edge-brightened jet connects to the accretion flow of the black hole. Close to the black hole, the emission profile of the jet-launching region is wider than the expected profile of a black-hole-driven jet, suggesting the possible presence of a wind associated with the accretion flow.

astro-ph.HE↗

arXiv2022

Multimessenger Picture of J1048+7143

We draw the multimessenger picture of J1048+7143, a flat-spectrum radio quasar known to show quasi-periodic oscillations in the $γ$-ray regime. We generate the adaptively-binned Fermi Large Area Telescope light curve of this source above 168 MeV to find three major $γ$-ray flares of the source, such that all three flares consist of two-two sharp sub-flares. Based on radio interferometric imaging data taken with the Very Large Array, we find that the kpc-scale jet is directed towards west, while our analysis of $8.6$-GHz very long baseline interferometry data, mostly taken with the Very Long Baseline Array, revealed signatures of two pc-scale jets, one pointing towards east, one pointing towards south. We suggest that the misalignment of the kpc- and pc-scale jets is a revealing signature of jet precession. We also analyze the $5$-GHz total flux density curve of J1048+7143 taken with the Nanshan(Ur) and RATAN-600 single dish radio telescopes and find two complete radio flares, slightly lagging behind the $γ$-ray flares. We model the timing of $γ$-ray flares as signature of the spin-orbit precession in a supermassive black hole binary, and find that the binary could merge in the next $\sim 60-80$ years. We show that both the Pulsar Timing Arrays and the planned Laser Interferometer Space Antenna lack sensitivity and frequency coverage to detect the hypothetical supermassive black hole binary in J1048$+$7143. We argue that the identification of sources similar to J1048+7143 plays a key role to reveal periodic high-energy sources in the distant Universe.

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arXiv2022

The Photon Ring in M87*

We report measurements of the gravitationally lensed secondary image -- the first in an infinite series of so-called "photon rings" -- around the supermassive black hole M87* via simultaneous modeling and imaging of the 2017 Event Horizon Telescope (EHT) observations. The inferred ring size remains constant across the seven days of the 2017 EHT observing campaign and is consistent with theoretical expectations, providing clear evidence that such measurements probe spacetime and a striking confirmation of the models underlying the first set of EHT results. The residual diffuse emission evolves on timescales comparable to one week. We are able to detect with high significance a southwestern extension consistent with that expected from the base of a jet that is rapidly rotating in the clockwise direction. This result adds further support to the identification of the jet in M87* with a black hole spin-driven outflow, launched via the Blandford-Znajek process. We present three revised estimates for the mass of M87* based on identifying the modeled thin ring component with the bright ringlike features seen in simulated images, one of which is only weakly sensitive to the astrophysics of the emission region. All three estimates agree with each other and previously reported values. Our strongest mass constraint combines information from both the ring and the diffuse emission region, which together imply a mass-to-distance ratio of $4.20^{+0.12}_{-0.06}~μ{\rm as}$ and a corresponding black hole mass of $(7.13\pm0.39)\times10^9M_\odot$, where the error on the latter is now dominated by the systematic uncertainty arising from the uncertain distance to M87*.

astro-ph.HE↗

arXiv2022

Observation of S4716 -- a Star with a 4 yr Orbit around Sgr A*

The ongoing monitoring of the Galactic center and Sgr A*, the central supermassive black hole, produces surprising and unexpected findings. This goes hand in hand with the technical evolution of ground- and space-based telescopes and instruments, but also with the progression of image filter techniques such as the Lucy Richardson algorithm. As we continue to trace the members of the S cluster close to Sgr A* on their expected trajectory around the supermassive black hole, we present the finding of a new stellar source, which we call S4716. The newly found star orbits SgrA* in about 4.0 yr and can be detected with NIRC2 (Keck), OSIRIS (Keck), SINFONI (VLT), NACO (VLT), and GRAVITY (VLTI). With a periapse distance of about 100 au, S4716 shows an equivalent distance toward Sgr A* as S4711. These fast-moving stars undergo a similar dynamical evolution, since S4711-S4716 share comparable orbital properties. We will furthermore draw a connection between the recent finding of a new faint star called S300 and the data presented here. Additionally, we observed a blend-star event with S4716 and another newly identified S star S148 in 2017.

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arXiv2022

Unravelling the Innermost Jet Structure of OJ 287 with the First GMVA+ALMA Observations

We present the first very-long-baseline interferometric (VLBI) observations of the blazar OJ287 carried out jointly with the Global Millimeter VLBI Array (GMVA) and the phased Atacama Large Millimeter/submillimeter Array (ALMA) at 3.5 mm on April 2, 2017. Participation of phased-ALMA not only has improved the GMVA north-south resolution by a factor of ~3, but also has enabled fringe detection with signal-to-noise ratios up to 300 at baselines longer than 2 Gλ. The high sensitivity has motivated us to image the data with the newly developed regularized maximum likelihood imaging methods, revealing the innermost jet structure with unprecedentedly high angular resolution. Our images reveal a compact and twisted jet extending along the northwest direction with two bends within the inner 200 μas that resembles a precessing jet in projection. The component at the southeastern end shows a compact morphology and high brightness temperature, and is identified as the VLBI core. An extended jet feature that lies at ~200 μas northwest of the core shows a conical shape in both total and linearly polarized intensity, and a bimodal distribution of the linear polarization electric vector position angle. We discuss the nature of this feature by comparing our observations with models and simulations of oblique and recollimation shocks with various magnetic field configurations. Our high-fidelity images also enabled us to search for possible jet features from the secondary supermassive black hole (SMBH) and test the SMBH binary hypothesis proposed for this source.

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arXiv2021

Event Horizon Telescope observations of the jet launching and collimation in Centaurus A

Very-long-baseline interferometry (VLBI) observations of active galactic nuclei at millimeter wavelengths have the power to reveal the launching and initial collimation region of extragalactic radio jets, down to $10-100$ gravitational radii ($r_g=GM/c^2$) scales in nearby sources. Centaurus A is the closest radio-loud source to Earth. It bridges the gap in mass and accretion rate between the supermassive black holes (SMBHs) in Messier 87 and our galactic center. A large southern declination of $-43^{\circ}$ has however prevented VLBI imaging of Centaurus A below $λ1$cm thus far. Here, we show the millimeter VLBI image of the source, which we obtained with the Event Horizon Telescope at $228$GHz. Compared to previous observations, we image Centaurus A's jet at a tenfold higher frequency and sixteen times sharper resolution and thereby probe sub-lightday structures. We reveal a highly-collimated, asymmetrically edge-brightened jet as well as the fainter counterjet. We find that Centaurus A's source structure resembles the jet in Messier 87 on ${\sim}500r_g$ scales remarkably well. Furthermore, we identify the location of Centaurus A's SMBH with respect to its resolved jet core at $λ1.3$mm and conclude that the source's event horizon shadow should be visible at THz frequencies. This location further supports the universal scale invariance of black holes over a wide range of masses.

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arXiv2021

The Variability of the Black-Hole Image in M87 at the Dynamical Time Scale

The black-hole images obtained with the Event Horizon Telescope (EHT) are expected to be variable at the dynamical timescale near their horizons. For the black hole at the center of the M87 galaxy, this timescale (5-61 days) is comparable to the 6-day extent of the 2017 EHT observations. Closure phases along baseline triangles are robust interferometric observables that are sensitive to the expected structural changes of the images but are free of station-based atmospheric and instrumental errors. We explored the day-to-day variability in closure phase measurements on all six linearly independent non-trivial baseline triangles that can be formed from the 2017 observations. We showed that three triangles exhibit very low day-to-day variability, with a dispersion of $\sim3-5^\circ$. The only triangles that exhibit substantially higher variability ($\sim90-180^\circ$) are the ones with baselines that cross visibility amplitude minima on the $u-v$ plane, as expected from theoretical modeling. We used two sets of General Relativistic magnetohydrodynamic simulations to explore the dependence of the predicted variability on various black-hole and accretion-flow parameters. We found that changing the magnetic field configuration, electron temperature model, or black-hole spin has a marginal effect on the model consistency with the observed level of variability. On the other hand, the most discriminating image characteristic of models is the fractional width of the bright ring of emission. Models that best reproduce the observed small level of variability are characterized by thin ring-like images with structures dominated by gravitational lensing effects and thus least affected by turbulence in the accreting plasmas.

astro-ph.HE↗

arXiv2021

Constraints on black-hole charges with the 2017 EHT observations of M87*

Our understanding of strong gravity near supermassive compact objects has recently improved thanks to the measurements made by the Event Horizon Telescope (EHT). We use here the M87* shadow size to infer constraints on the physical charges of a large variety of nonrotating or rotating black holes. For example, we show that the quality of the measurements is already sufficient to rule out that M87* is a highly charged dilaton black hole. Similarly, when considering black holes with two physical and independent charges, we are able to exclude considerable regions of the space of parameters for the doubly-charged dilaton and the Sen black holes.

gr-qc↗

arXiv2021

The Polarized Image of a Synchrotron Emitting Ring of Gas Orbiting a Black Hole

Synchrotron radiation from hot gas near a black hole results in a polarized image. The image polarization is determined by effects including the orientation of the magnetic field in the emitting region, relativistic motion of the gas, strong gravitational lensing by the black hole, and parallel transport in the curved spacetime. We explore these effects using a simple model of an axisymmetric, equatorial accretion disk around a Schwarzschild black hole. By using an approximate expression for the null geodesics derived by Beloborodov (2002) and conservation of the Walker-Penrose constant, we provide analytic estimates for the image polarization. We test this model using currently favored general relativistic magnetohydrodynamic simulations of M87*, using ring parameters given by the simulations. For a subset of these with modest Faraday effects, we show that the ring model broadly reproduces the polarimetric image morphology. Our model also predicts the polarization evolution for compact flaring regions, such as those observed from Sgr A* with GRAVITY. With suitably chosen parameters, our simple model can reproduce the EVPA pattern and relative polarized intensity in Event Horizon Telescope images of M87*. Under the physically motivated assumption that the magnetic field trails the fluid velocity, this comparison is consistent with the clockwise rotation inferred from total intensity images.

astro-ph.HE↗

arXiv2021

A ring accelerator? Unusual jet dynamics in the IceCube candidate PKS 1502+106

On 2019/07/30.86853 UT, IceCube detected a high-energy astrophysical neutrino candidate. The Flat Spectrum Radio Quasar PKS 1502+106 is located within the 50 percent uncertainty region of the event. Our analysis of 15 GHz Very Long Baseline Array (VLBA) and astrometric 8 GHz VLBA data, in a time span prior and after the IceCube event, reveals evidence for a radio ring structure which develops with time. Several arc-structures evolve perpendicular to the jet ridge line. We find evidence for precession of a curved jet based on kinematic modelling and a periodicity analysis. An outflowing broad line region (BLR) based on the C IV line emission (Sloan Digital Sky Survey, SDSS) is found. We attribute the atypical ring to an interaction of the precessing jet with the outflowing material. We discuss our findings in the context of a spine-sheath scenario where the ring reveals the sheath and its interaction with the surroundings (narrow line region, NLR, clouds). We find that the radio emission is correlated with the $γ$-ray emission, with radio lagging the $γ$-rays. Based on the $γ$-ray variability timescale, we constrain the $γ$-ray emission zone to the BLR (30-200 $r_{\rm g}$) and within the jet launching region. We discuss that the outflowing BLR provides the external radiation field for $γ$-ray production via external Compton scattering. The neutrino is most likely produced by proton-proton interaction in the blazar zone (beyond the BLR), enabled by episodic encounters of the jet with dense clouds, i.e. some molecular cloud in the NLR.

astro-ph.HE↗

arXiv2020

Gravitational Test Beyond the First Post-Newtonian Order with the Shadow of the M87 Black Hole

The 2017 Event Horizon Telescope (EHT) observations of the central source in M87 have led to the first measurement of the size of a black-hole shadow. This observation offers a new and clean gravitational test of the black-hole metric in the strong-field regime. We show analytically that spacetimes that deviate from the Kerr metric but satisfy weak-field tests can lead to large deviations in the predicted black-hole shadows that are inconsistent with even the current EHT measurements. We use numerical calculations of regular, parametric, non-Kerr metrics to identify the common characteristic among these different parametrizations that control the predicted shadow size. We show that the shadow-size measurements place significant constraints on deviation parameters that control the second post-Newtonian and higher orders of each metric and are, therefore, inaccessible to weak-field tests. The new constraints are complementary to those imposed by observations of gravitational waves from stellar-mass sources.

gr-qc↗

arXiv2020

Distribution of Radio Spectral Slopes of Galaxies in Optical Diagnostic Diagrams

For about 500 intermediate-redshift sources ($0.04<z<0.4$), whose radio flux densities at $1.4\,{\rm GHz}$ are larger than 10 mJy, we performed additional observations at 4.85 GHz and 10.45 GHz using 100-m Effelsberg telescope. Our radio-optical galaxies are located preferentially in the composite and AGN spectral classes in the narrow line optical diagnostic diagrams (ODD). In the analysis, we focused on the distribution of radio spectral indices of radio synchrotron power-law profiles, $S_ν\propto ν^{+α}$, in the ODDs. Using different analysis techniques, both observationally motivated and machine-learning based, we found three distinct groups--clusters in the radio loudness, [OIII]/H$β$ ratio, and spectral index volume: (1) sources with steep radio spectral index, large radio loudness and large [OIII]/H$β$ ratio; (2) sources with flat radio spectral index, intermediate radio loudness and lower [OIII]/H$β$ ratio; (3) sources with inverted radio spectral index, low radio loudness and low [OIII]/H$β$. The groups (1), (2), and (3) are located along the Seyfert-LINER spectral classes towards lower ionization ratios in the ODDs and hence can represent different activity cycles/accretion modes of AGNs, which could be in some cases associated with different merger stages.

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arXiv2020

Effect of Electromagnetic Interaction on Galactic Center Flare Components

Recently, near-infrared GRAVITY@ESO observations at $2.2\,μ{\rm m}$ have announced the detection of three bright "flares" in the vicinity of the Galactic center supermassive black hole (SMBH) that exhibited orbital motion at a distance of about $6 - 11$ gravitational radii from an $\sim 4\times 10^6\, M_{\odot}$ black hole. There are indications of the presence of a large-scale, organized component of the magnetic field at the Galactic center. Electromagnetic effects on the flare dynamics were previously not taken into account despite the relativistic motion of a plasma in magnetic field leading to the charge separation and nonnegligible net charge density in the plasma. Applying various approaches, we find the net charge number density of the flare components of the order of $10^{-3} - 10^{-4}$ cm$^{-3}$, while the particles' total number density is of the order of $10^{6} - 10^{8}$ cm$^{-3}$. However, even such a tiny excess of charged particles in the quasi-neutral plasma can significantly affect the dynamics of flare components, which can then lead to the degeneracy in the measurements of spin of the SMBH. Analyzing the dynamics of recent flares in the case of the rapidly rotating black hole, we also constrain the inclination angle between the magnetic field and spin axis to $α< 50^{\circ}$, as for larger angles, the motion of the hot spot is strongly chaotic.

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arXiv2019

Radio spectral index distribution of SDSS-FIRST sources across optical diagnostic diagrams

A detailed understanding of how the activity of a galactic nucleus regulates the growth of its host is still missing. To understand the activity and the types of accretion of supermassive black holes in different hosts, it is essential to study radio-optical properties of a large sample of extragalactic sources. In particular, we aim at studying the radio spectral index trends across the optical emission line diagnostic diagrams to search for potential (anti)correlations. To this goal, we combine flux densities from the radio FIRST survey at $1.4\,{\rm GHz}$ (with the flux density range $10\,{\rm mJy} \leq F_{1.4} \leq 1000\,{\rm mJy}$) for 209 SDSS sources at intermediate redshift $(0.04\leq z \leq 0.4)$ with the Effelsberg radiotelescope measurements at $4.85\,{\rm GHz}$ and $10.45\,{\rm GHz}$. The information about the optical emission-line ratios is obtained from the SDSS-DR7 catalogue. Using the Effelsberg data, we were able to infer the two-point radio spectral index distributions for star-forming galaxies, composite galaxies (with a combined contribution to the line emission from the star-formation and AGN activity), Seyferts, and low ionization narrow emission region (LINER) galaxies. While studying the distribution of steep, flat, and inverted sources across optical diagnostic diagrams, we found three distinct classes of radio emitters for our sample: (i) sources with steep radio index, high ionization ratio and high radio loudness, (ii) sources with flat radio index, lower ionization ratio and intermediate radio loudness, (iii) sources with inverted radio index, low ionization ratio and low radio loudness. The classes (i), (ii), (iii) cluster mainly along the transition from Seyfert to LINER sources in the BPT diagram. We interpret these groups as a result of the recurrent nuclear-jet activity.

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arXiv2019

The diversity of AGN variability: Some highlights and challenges

This article focuses on certain variability and emission characteristics of Active Galactic Nuclei (AGN), especially their radio-loud subset consisting of quasars, BL Lacs and $γ$-ray detected narrow-line Seyfert 1 galaxies, all of which exhibit relativistically beamed jets of nonthermal radiation. Several striking trends and correlations, including some that have received scant attention, drawn from the available comparatively recent literature are highlighted. These can provide very useful inputs to models of AGN and be probed at a deeper level using the optical telescopes recently set up at Devasthal (Nainital).

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arXiv2019

Constraining the charge of the Galactic centre black hole

In this contribution, we summarize our results concerning the observational constraints on the electric charge associated with the Galactic centre black hole - Sgr A*. According to the no-hair theorem, every astrophysical black hole, including supermassive black holes, is characterized by at most three classical, externally observable parameters - mass, spin, and the electric charge. While the mass and the spin have routinely been measured by several methods, the electric charge has usually been neglected, based on the arguments of efficient discharge in astrophysical plasmas. From a theoretical point of view, the black hole can attain charge due to the mass imbalance between protons and electrons in fully ionized plasmas, which yields about $\sim 10^8\,{\rm C}$ for Sgr A*. The second, induction mechanism concerns rotating Kerr black holes embedded in an external magnetic field, which leads to electric field generation due to the twisting of magnetic field lines. This electric field can be associated with the induced Wald charge, for which we calculate the upper limit of $\sim 10^{15}\,{\rm C}$ for Sgr A*. Although the maximum theoretical limit of $\sim 10^{15}\,{\rm C}$ is still 12 orders of magnitude smaller than the extremal charge of Sgr A*, we analyse a few astrophysical consequences of having a black hole with a small charge in the Galactic centre. Two most prominent ones are the effect on the X-ray bremsstrahlung profile and the effect on the position of the innermost stable circular orbit.

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arXiv2018

On the charge of the Galactic centre black hole

The Galactic centre supermassive black hole (SMBH), in sharp contrast with its complex environment, is characterized by only three classical parameters -- mass, spin, and electric charge. Its charge is poorly constrained. It is, however, usually assumed to be zero because of neutralization due to the presence of plasma. We revisit the question of the SMBH charge and put realistic limits on its value, timescales of charging and discharging, and observable consequences of the potential, small charge associated with the Galactic centre black hole. The electric charge due to classical arguments based on the mass difference between protons and electrons is $\lesssim 10^9\,{\rm C}$ and is of a transient nature on the viscous time-scale. However, the rotation of a black hole in magnetic field generates electric field due to the twisting of magnetic field lines. This electric field can be associated with induced charge, for which we estimate an upper limit of $\lesssim 10^{15}\,{\rm C}$. Moreover, this charge is most likely positive due to an expected alignment between the magnetic field and the black-hole spin. Even a small charge of this order significantly shifts the position of the innermost stable circular orbit (ISCO) of charged particles. In addition, we propose a novel observational test based on the presence of the bremsstrahlung surface brightness decrease, which is more sensitive for smaller unshielded electric charges than the black-hole shadow size. Based on this test, the current upper observational limit on the charge of Sgr A* is $\lesssim 3\times 10^{8}\,{\rm C}$.

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arXiv2017

Molecular gas in the immediate vicinity of Sgr A* seen with ALMA

We report serendipitous detections of line emission with ALMA in band 3, 6, and 7 in the central parsec of the Galactic center at an up to now highest resolution (<0.7''). Among the highlights are the very first and highly resolved images of sub-mm molecular emission of CS, H13CO+, HC3N, SiO, SO, C2H, and CH3OH in the immediate vicinity (~1'' in projection) of Sgr A* and in the circumnuclear disk (CND). The central association (CA) of molecular clouds shows three times higher CS/X (X: any other observed molecule) luminosity ratios than the CND suggesting a combination of higher excitation - by a temperature gradient and/or IR-pumping - and abundance enhancement due to UV- and/or X-ray emission. We conclude that the CA is closer to the center than the CND is and could be an infalling clump consisting of denser cloud cores embedded in diffuse gas. Moreover, we identified further regions in and outside the CND that are ideally suited for future studies in the scope of hot/cold core and extreme PDR/XDR chemistry and consequent star formation in the central few parsecs.

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