arXiv subjects

Michael Bremer

Publications and source records attributed to Michael Bremer.

32 records · Page 2Linked to original sources

arXiv2022

A very luminous jet from the disruption of a star by a massive black hole

Tidal disruption events (TDEs) are bursts of electromagnetic energy released when supermassive black holes (SMBHs) at the centers of galaxies violently disrupt a star that passes too close. TDEs provide a new window to study accretion onto SMBHs; in some rare cases, this accretion leads to launching of a relativistic jet, but the necessary conditions are not fully understood. The best studied jetted TDE to date is Swift J1644+57, which was discovered in gamma-rays, but was too obscured by dust to be seen at optical wavelengths. Here we report the optical discovery of AT2022cmc, a rapidly fading source at cosmological distance (redshift z=1.19325) whose unique lightcurve transitioned into a luminous plateau within days. Observations of a bright counterpart at other wavelengths, including X-rays, sub-millimeter, and radio, supports the interpretation of AT2022cmc as a jetted TDE containing a synchrotron "afterglow", likely launched by a SMBH with spin $a \gtrsim 0.3$. Using 4 years of Zwicky Transient Facility (ZTF) survey data, we calculate a rate of $0.02 ^{+ 0.04 }_{- 0.01 }$ Gpc$^{-3}$ yr$^{-1}$ for on-axis jetted TDEs based on the luminous, fast-fading red component, thus providing a measurement complementary to the rates derived from X-ray and radio observations. Correcting for the beaming angle effects, this rate confirms that about 1% of TDEs have relativistic jets. Optical surveys can use AT2022cmc as a prototype to unveil a population of jetted TDEs.

astro-ph.HE↗

arXiv2022

Luminous Millimeter, Radio, and X-ray Emission from ZTF20acigmel (AT2020xnd)

Observations of the extragalactic ($z=0.0141$) transient AT2018cow established a new class of energetic explosions shocking a dense medium, which produce luminous emission at millimeter and sub-millimeter wavelengths. Here we present detailed millimeter- through centimeter-wave observations of a similar transient, ZTF20acigmel (AT2020xnd) at $z=0.2433$. Using observations from the NOrthern Extended Millimeter Array and the Very Large Array, we model the unusual millimeter and radio emission from AT2020xnd under several different assumptions, and ultimately favor synchrotron radiation from a thermal electron population (relativistic Maxwellian). The thermal-electron model implies a fast but sub-relativistic ($v\approx0.3c$) shock and a high ambient density ($n_e\approx4\times10^{3}$cm$^{-3}$ at $Δt\approx40$ days). The X-ray luminosity of $L_X\approx10^{43}$ erg sec$^{-1}$ exceeds simple predictions from the radio and UVOIR luminosity and likely has a separate physical origin, such as a central engine. Using the fact that month-long luminous ($L_ν\approx 2\times10^{30}$ erg sec$^{-1}$ Hz$^{-1}$ at 100 GHz) millimeter emission appears to be a generic feature of transients with fast ($t_{1/2}\approx3$ days) and luminous ($M_\mathrm{peak}\approx -21 $mag) optical light curves, we estimate the rate at which transients like AT2018cow and AT2020xnd will be detected by future wide-field millimeter transient surveys such as CMB-S4, and conclude that energetic explosions in dense environments may represent a significant population of extragalactic transients in the 100 GHz sky.

astro-ph.HE↗

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.

astro-ph.GA↗

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

A persistent double nuclear structure in 3C 84

3C 84 (NGC 1275) is the radio source at the center of the Perseus Cluster and exhibits a bright radio jet. We observed the source with the Global Millimeter VLBI Array (GMVA) between 2008 and 2015, with a typical angular resolution of $\sim$50 $μ$as. The observations revealed a consistent double nuclear structure separated by $\sim$770 gravitational radii assuming a Black Hole mass of 3.2 $\times 10^{8}$ $M_{\odot}$. The region is likely too broad and bright to be the true jet base anchored in the accretion disk or Black Hole ergosphere. A cone and parabola were fit to the stacked (time averaged) image of the nuclear region. The data did not strongly prefer either fit, but combined with a jet/counter-jet ratio analysis, an upper limit on the viewing angle to the inner jet region of $\leq$35$^{\circ}$ was found. This provides evidence for a variation of the viewing angle along the jet (and therefore a bent jet) within $\sim$0.5 parsec of the jet launching region. In the case of a conical jet, the apex is located $\sim$2400 gravitational radii upstream of the bright nuclear region and up to $\sim$600 gravitational radii upstream in the parabolic case. We found a possible correlation between the brightness temperature and relative position angle of the double nuclear components, which may indicate rotation within the jet.

astro-ph.GA↗

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↗

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↗

arXiv2019

The magnetic field structure in CTA 102 from high resolution mm-VLBI observations during the flaring state in 2016-2017

Investigating the magnetic field structure in the innermost regions of relativistic jets is fundamental to shed light on the crucial physical processes giving rise to the jet formation, as well as to its extraordinary radiation output up to gamma-ray energies. We study the magnetic field structure of the quasar CTA 102 with 3 and 7 mm-VLBI polarimetric observations, reaching an unprecedented resolution (~50 microarcsec). We also investigate the variability and physical process occurring in the source during the observing period which coincides with a very active state of the source till high-energies. The Faraday rotation analysis between 3 and 7mm shows a gradient in rotation measure with a maximum value of ~6X10^4 rad/m^2 and intrinsic electric vector position angles (EVPAs) oriented around the centroid of the core, suggesting the presence of large-scale helical magnetic fields. Such a magnetic field structure is also visible in 7 mm images when a new superluminal component is crossing the core region. The 7mm EVPAs orientation is different when the component is exiting the core or crossing a stationary feature at ~0.1 mas. The interaction between the superluminal component and a recollimation shock at ~0.1 mas could have triggered the multi-wavelengths flares. The variability Doppler factor associated with such interaction is large enough to explain the high energy emission, as we infer from the analysis of gamma-ray and X-ray data, and it is in agreement with the Doppler factor obtained to explain the extraordinary optical flare by Raiteri et al.(2017).

astro-ph.HE↗

arXiv2018

Global millimeter VLBI array survey of ultracompact extragalactic radio sources at 86 GHz

(abridged) Very long baseline interferometry (VLBI) observations at 86$\,$GHz (wavelength, $λ= 3\,$mm) reach a resolution of about 50 $μ$as, probing the collimation and acceleration regions of relativistic outflows in active galactic nuclei. To extend the statistical studies of compact extragalactic jets, a large global 86 GHz VLBI survey of 162 radio sources was conducted in 2010-2011 using the Global Millimeter VLBI Array. The survey data attained a typical baseline sensitivity of 0.1 Jy and a typical image sensitivity of 5 mJy/beam, providing successful detections and images for all of the survey targets. For 138 objects, the survey provides the first ever VLBI images made at 86 GHz. Gaussian model fitting of the visibility data was applied to represent the structure of the sources. The Gaussian model-fit-based estimates of brightness temperature ($T_\mathrm{b}$) at the jet base (core) and in moving regions (jet components) downstream from the core were compared to the estimates of $T_\mathrm{b}$ limits made directly from the visibility data, demonstrating a good agreement between the two methods. The apparent brightness temperature estimates for the jet cores in our sample range from $2.5 \times 10^{9}\,$K to $ 1.3\times 10^{12}\,$K. A population model with a single intrinsic value of brightness temperature, $T_\mathrm{0}$, is applied to reproduce the observed $T_\mathrm{b}$ distribution. It yields $T_\mathrm{0} = (3.77^{+0.10}_{-0.14}) \times 10^{11}\,$K for the jet cores, implying that the inverse Compton losses dominate the emission. In the jet components, $T_\mathrm{0} =(1.42^{+0.16}_{-0.19})\times 10^{11}\,$K is found, slightly higher than the equipartition limit of $\sim5\times 10^{10}\,$K expected for these jet regions. For objects with sufficient structural detail detected, the adiabatic energy losses dominate the observed changes of $T_\mathrm{b}$ along the jet.

astro-ph.GA↗

arXiv2014

The latest results from the Global mm-VLBI Array

The Global mm-VLBI Array (GMVA) is the highest angular resolution imaging interferometer currently available as a common user facility. It is capable of angular resolutions on the order of 40 microarcseconds. Currently 14 stations in the United States and Europe participate in global 3 mm VLBI observations. The GMVA is used for continuum and spectroscopic imaging, probing the central regions of active galaxies and the origin of jets as these regions are typically not observable at longer wavelengths due to synchrotron self-absorption. In early 2012, fringes were detected to the three stations of the Korean VLBI Network (KVN), opening the possibility of extending the baseline coverage of the VLBI array to the East. In these proceedings, we will present recent images from a monitoring program of gamma-ray blazars using the GMVA, including the sources 3C454.3 and 0235+164, and an update of its current status and abilities.

astro-ph.HE↗

arXiv2011

A Closer Look at the LkCa 15 Protoplanetary Disk

We present 870 micron observations of dust continuum emission from the LkCa 15 protoplanetary disk at high angular resolution (with a characteristic scale of 0.25" = 35 AU), obtained with the IRAM Plateau de Bure interferometer and supplemented by slightly lower resolution observations from the Submillimeter Array. We fit these data with simple morphological models to characterize the spectacular ring-like emission structure of this disk. Our analysis indicates that a small amount of 870 micron dust emission (~5 mJy) originates inside a large (40-50 AU radius) low optical depth cavity. This result can be interpreted either in the context of an abrupt decrease by a factor of ~5 in the radial distribution of millimeter-sized dust grains or as indirect evidence for a gap in the disk, in agreement with previous inferences from the unresolved infrared spectrum and scattered light images. A preliminary model focused on the latter possibility suggests the presence of a low-mass (planetary) companion, having properties commensurate with those inferred from the recent discovery of LkCa 15b.

astro-ph.EP↗

arXiv2008

A Global 86GHz VLBI Survey of Compact Radio Sources

We present results from a large 86GHz global VLBI survey of compact radio sources. The main goal of the survey is to increase by factors of 3--5 the total number of objects accessible for future 3-mm VLBI imaging. The survey observations reach the baseline sensitivity of 0.1Jy and image sensitivity of better than 10 mJy/beam. The total of 127 compact radio sources have been observed. The observations have yielded images for 109 sources, extending the database of the sources imaged at 86GHz with VLBI observation by a factor of 5, and only 6 sources have not been detected. The remaining 12 objects have been detected but could not be imaged due to insufficient closure phase information. Radio galaxies are less compact than quasars and BL Lacs on sub-milliarcsecond scale. Flux densities and sizes of core and jet components of all imaged sources have been estimated using Gaussian model fitting. From these measurements, brightness temperatures have been calculated, taking into account resolution limits of the data. The cores of 70% of the imaged sources are resolved. The core brightness temperatures of the sources peak at $\sim 10^{11}$ K and only 1% have brightness temperatures higher than $10^{12}$ K. Cores of Intraday Variable (IDV) sources are smaller in angular size than non-IDV sources, and so yield higher brightness temperatures.

astro-ph↗

arXiv2002

A CO Survey of Gravitationally Lensed Quasars with the IRAM Interferometer

We present the results of a CO survey of gravitationally lensed quasars, conducted with the Plateau de Bure Interferometer over the last three years. Among the 18 objects surveyed, one was detected in CO line emission, while six were detected in the continuum at 3mm and three in the continuum at 1mm. The low CO detection rate may at least in part be due to uncertainties in the redshifts derived from quasar broad emission lines. The detected CO source, the z=3.2 radio quiet quasar MG0751+2716, is quite strong in the CO(4-3) line and in the millimeter/submillimeter continuum, the latter being emission from cool dust. The integrated CO line flux is 5.96 +- 0.45 Jy.km/s, and the total molecular gas mass is estimated to be in the range M(H_2) = 1.6-3.1 X 10^9 solar masses.

astro-ph↗