arXiv subjects

Ciriaco Goddi

Publications and source records attributed to Ciriaco Goddi.

At least 55 records · Page 3Linked to original sources

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.

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arXiv2021

Polarimetric properties of Event Horizon Telescope targets from ALMA

We present the results from a full polarization study carried out with ALMA during the first VLBI campaign, which was conducted in Apr 2017 in the $λ$3mm and $λ$1.3mm bands, in concert with the Global mm-VLBI Array (GMVA) and the Event Horizon Telescope (EHT), respectively. We determine the polarization and Faraday properties of all VLBI targets, including Sgr A*, M87, and a dozen radio-loud AGN. We detect high linear polarization fractions (2-15%) and large rotation measures (RM $>10^{3.3}-10^{5.5}$ rad m$^{-2}$). For Sgr A* we report a mean RM of $(-4.2\pm0.3) \times10^5$ rad m$^{-2}$ at 1.3 mm, consistent with measurements over the past decade, and, for the first time, an RM of $(-2.1\pm0.1) \times10^5$ rad m$^{-2}$ at 3 mm, suggesting that about half of the Faraday rotation at 1.3 mm may occur between the 3 mm photosphere and the 1.3 mm source. We also report the first unambiguous measurement of RM toward the M87 nucleus at mm wavelengths, which undergoes significant changes in magnitude and sign reversals on a one year time-scale, spanning the range from -1.2 to 0.3 $\times\,10^5$ rad m$^{-2}$ at 3 mm and -4.1 to 1.5 $\times\,10^5$ rad m$^{-2}$ at 1.3 mm. Given this time variability, we argue that, unlike the case of Sgr A*, the RM in M87 does not provide an accurate estimate of the mass accretion rate onto the black hole. We put forward a two-component model, comprised of a variable compact region and a static extended region, that can simultaneously explain the polarimetric properties observed by both the EHT and ALMA. These measurements provide critical constraints for the calibration, analysis, and interpretation of simultaneously obtained VLBI data with the EHT and GMVA.

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arXiv2021

An 86-GHz search for Pulsars in the Galactic Center with the Atacama Large Millimeter/submillimeter Array

We report on the first pulsar and transient survey of the Galactic Center (GC) with the Atacama Large Millimeter/submillimeter Array (ALMA). The observations were conducted during the Global Millimeter VLBI Array campaign in 2017 and 2018. We carry out searches using timeseries of both total intensity and other polarization components in the form of Stokes parameters. We incorporate acceleration and its derivative in the pulsar search, and also search in segments of the entire observation to compensate for potential orbital motion of the pulsar. While no new pulsar is found, our observations yield the polarization profile of the GC magnetar PSR J1745-2900 at mm-wavelength for the first time, which turns out to be nearly 100 % linearly polarized. Additionally, we estimate the survey sensitivity placed by both system and red noise, and evaluate its capability of finding pulsars in orbital motion with either Sgr A* or a binary companion. We show that the survey is sensitive to only the most luminous pulsars in the known population, and future observations with ALMA in Band-1 will deliver significantly deeper survey sensitivity on the GC pulsar population.

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arXiv2020

Multi-directional mass-accretion and collimated outflows in W51

We observed the W51 high-mass star-forming complex with ALMA's longest-baseline configurations, achieving an angular resolution of $\sim$20 milliarcseconds, corresponding to a linear resolution of $\sim$100 au at $D_{\mathrm{W51}}=5.4$ kpc. The observed region contains three high-mass protostars in which the dust continuum emission at 1.3 mm is optically-thick up to a radius $\lesssim$1000 au and has brightness temperatures $\gtrsim$200 K. The high luminosity ($\gtrsim10^4$ L$_{\odot}$) in the absence of free-free emission suggests the presence of massive stars ($M\gtrsim20$ M$_{\odot}$) at the earliest stages of their formation. Our continuum images reveal remarkably complex and filamentary structures arising from compact cores. Molecular emission shows no clear signs of rotation nor infall on scales from 150 to 2000 au: we do not detect disks. The central sources drive young ($\sim$100 years), fast ($\sim 100$ km s$^{-1}$), powerful ($\dot{M}>10^{-4}$ M$_{\odot} \ yr^{-1}$), collimated outflows. These outflows provide indirect evidence of accretion disks on scales $r\lesssim$100--500 au (depending on the object). The active outflows are connected to fossil flows that have different orientations on larger spatial scales, implying that the orientations of these small disks change over time. These results together support a variant of an accretion model for high-mass star formation in which massive protostars do not form a large, stable Keplerian disk during their early stages, but instead they accrete material from multiple massive flows with different angular momentum vectors. This scenario therefore contrasts with the simplified classic paradigm of a stable disk+jet system, which is the standard model for low-mass star formation, and provides an experimental confirmation of a multi-directional and unsteady accretion model for massive star formation.

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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

VLBI20-30: a scientific roadmap for the next decade -- The future of the European VLBI Network

This white paper describes the science case for Very Long Baseline Interferometry (VLBI) and provides suggestions towards upgrade paths for the European VLBI Network (EVN). The EVN is a distributed long-baseline radio interferometric array, that operates at the very forefront of astronomical research. Recent results, together with the new science possibilities outlined in this vision document, demonstrate the EVN's potential to generate new and exciting results that will transform our view of the cosmos. Together with e-MERLIN, the EVN provides a range of baseline lengths that permit unique studies of faint radio sources to be made over a wide range of spatial scales. The science cases are reviewed in six chapters that cover the following broad areas: cosmology, galaxy formation and evolution, innermost regions of active galactic nuclei, explosive phenomena and transients, stars and stellar masers in the Milky Way, celestial reference frames and space applications. The document concludes with identifying the synergies with other radio, as well as multi-band/multi-messenger instruments, and provide the recommendations for future improvements. The appendices briefly describe other radio VLBI arrays, the technological framework for EVN developments, and a selection of spectral lines of astrophysical interest below 100 GHz. The document includes a glossary for non-specialists, and a list of acronyms at the end.

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arXiv2020

Detection of pulses from the Vela pulsar at millimeter wavelengths with phased ALMA

We report on the first detection of pulsed radio emission from a radio pulsar with the ALMA telescope. The detection was made in the Band-3 frequency range (85-101 GHz) using ALMA in the phased-array mode developed for VLBI observations. A software pipeline has been implemented to enable a regular pulsar observing mode in the future. We describe the pipeline and demonstrate the capability of ALMA to perform pulsar timing and searching. We also measure the flux density and polarization properties of the Vela pulsar (PSR J0835$-$4510) at mm-wavelengths, providing the first polarimetric study of any ordinary pulsar at frequencies above 32 GHz. Finally, we discuss the lessons learned from the Vela observations for future pulsar studies with ALMA, particularly for searches near the supermassive black hole in the Galactic Center, and the potential of using pulsars for polarization calibration of ALMA.

astro-ph.GA↗

arXiv2020

Protostellar Outflows at the EarliesT Stages (POETS). IV. Statistical properties of the 22 GHz H2O masers

We wish to perform a statistical study of the location and motion of individual 22 GHz water maser cloudlets, characterized by sizes that are within a few au, with respect to the radio thermal emission from young stellar objects (YSO). For this purpose, we have been carrying out the Protostellar Outflows at the EarliesT Stages (POETS) survey of a sample (38) of high-mass YSOs. The water maser positions and three-dimensional (3D) velocities were determined through Very Long Baseline Array observations with accuracies of a few milliarcsec (mas) and a few km/s, respectively. The position of the ionized core of the protostellar wind, marking the YSO, was determined through sensitive continuum Jansky Very Large Array observations with a typical error of 20 mas. The statistic of the separation of the water masers from the radio continuum shows that 84% of the masers are found within 1000 au from the YSO and 45% of them are within 200 au. Therefore, we can conclude that the 22 GHz water masers are a reliable proxy for the YSO position. The distribution of maser luminosity is strongly peaked towards low values, indicating that about half of the maser population is still undetected with the current Very Long Baseline Interferometry detection thresholds of 50-100 mJy/beam. Next-generation, sensitive radio interferometers will exploit these weak masers for an improved sampling of the velocity and magnetic fields around the YSOs. The average direction of the water maser proper motions provides a statistically-significant estimate for the orientation of the jet emitted by the YSO: 55% of the maser proper motions are directed on the sky within an angle of 30 deg from the jet axis. Finally, we show that our measurements of 3D maser velocities statistically support models in which water maser emission arises from planar shocks with propagation direction close to the plane of the sky.

astro-ph.SR↗

arXiv2019

Observations of Orion Source I Disk and Outflow Interface

We imaged the continuum and molecular line emission from Orion Source I (SrcI) with up to 30 mas (12 AU) resolution at 43, 99, 223, and 340 GHz in an attempt to probe the structure and chemistry of the circumstellar disk and bipolar outflow associated with this high mass protostar. The continuum spectral index ranges from $\sim$2 along the midplane of the disk to $\sim$3 along the edges, consistent with dust that is optically thick in the midplane but becomes optically thin at the periphery. Salt (NaCl) emission is visible where the dust is optically thin; it provides a unique tracer of the velocity field within the disk. All other molecules that we have mapped - H$_2$O, AlO, SiO, SiS, SO, and SO$_2$ - appear to originate primarily in the bipolar outflow. The base of the outflow is corotating with the disk. SiS shows a filamentary structure that is most prominent along the edges of the outflow. The molecular distributions suggest that Si and Al released from dust grains in the disk react with oxygen derived from H$_2$O to form SiO and AlO, and with SO and SO$_2$ to form SiS.

astro-ph.GA↗

arXiv2019

First M87 Event Horizon Telescope Results and the Role of ALMA

In April 2019, the Event Horizon Telescope (EHT) collaboration revealed the first image of the candidate super-massive black hole (SMBH) at the centre of the giant elliptical galaxy Messier 87 (M87). This event-horizon-scale image shows a ring of glowing plasma with a dark patch at the centre, which is interpreted as the shadow of the black hole. This breakthrough result, which represents a powerful confirmation of Einstein's theory of gravity, or general relativity, was made possible by assembling a global network of radio telescopes operating at millimetre wavelengths that for the first time included the Atacama Large Millimeter/ submillimeter Array (ALMA). The addition of ALMA as an anchor station has enabled a giant leap forward by increasing the sensitivity limits of the EHT by an order of magnitude, effectively turning it into an imaging array. The published image demonstrates that it is now possible to directly study the event horizon shadows of SMBHs via electromagnetic radiation, thereby transforming this elusive frontier from a mathematical concept into an astrophysical reality. The expansion of the array over the next few years will include new stations on different continents - and eventually satellites in space. This will provide progressively sharper and higher-fidelity images of SMBH candidates, and potentially even movies of the hot plasma orbiting around SMBHs. These improvements will shed light on the processes of black hole accretion and jet formation on event-horizon scales, thereby enabling more precise tests of general relativity in the truly strong field regime.

astro-ph.HE↗

arXiv2019

First detection of CS masers around a high-mass young stellar object, W51 e2e

We report the discovery of maser emission in the two lowest rotational transitions of CS toward the high-mass protostar W51 e2e with ALMA and the JVLA. The masers from CS J=1-0 and J=2-1 are neither spatially nor spectrally coincident (they are separated by ~150 AU and ~30 km/s), but both appear to come from the base of the blueshifted outflow from this source. These CS masers join a growing list of rarely-detected maser transitions that may trace a unique phase in the formation of high-mass protostars.

astro-ph.GA↗

arXiv2019

Protostellar Outflows at the EarliesT Stages (POETS). III. H2O masers tracing disk-winds and jets near luminous YSOs

The goal of the Protostellar Outflows at the EarliesT Stages (POETS) survey is to image the disk-outflow interface on scales of 10-100 au in a statistically significant sample (36) of luminous young stellar objects (YSO), targeting both the molecular and ionized components of the outflows. The outflow kinematics is studied at milliarcsecond scales through VLBI observations of the 22 GHz water masers. We employed the JVLA at 6, 13, and 22 GHz in the A- and B-Array configurations to determine the spatial structure and the spectral index of the radio continuum emission. In about half of the targets, the water masers observed at separation <= 1000 au from the YSOs trace either or both of these kinematic structures: 1) a spatially elongated distribution oriented at close angle with the direction of collimation of the maser proper motions (PM), and 2) a linear LSR velocity (Vlsr) gradient across the YSO position. The kinematic structure (1) is readily interpreted in terms of a protostellar jet, as confirmed in some targets via the comparison with independent observations of the YSO jets, in thermal (continuum and line) emissions, reported in the literature. The kinematic structure (2) is interpreted in terms of a disk-wind (DW) seen almost edge-on on the basis of several pieces of evidence: first, it is invariably directed perpendicular to the YSO jet; second, it agrees in orientation and polarity with the Vlsr gradient in thermal emissions (when reported in the literature) identifying the YSO disk at scales of <= 1000~au; third, the PMs of the masers delineating the Vlsr gradients hint at flow motions at a speed of 10-20 km/s directed at large angles with the disk midplane. In the remaining targets, the maser PMs are not collimated but rather tend to align along two almost perpendicular directions, and could originate in DW-jet systems slightly inclined (<= 30 deg) with respect to edge-on.

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arXiv2019

rPICARD: A CASA-based Calibration Pipeline for VLBI Data. Calibration and imaging of 7 mm VLBA observations of the AGN jet in M87

(Abridged) The CASA software suite, can now reduce very long baseline interferometry (VLBI) data with the recent addition of a fringe fitter. Here, we present the Radboud PIpeline for the Calibration of high Angular Resolution Data (rPICARD), which is an open-source VLBI calibration and imaging pipeline built on top of the CASA framework. The pipeline is capable of reducing data from different VLBI arrays. It can be run non-interactively after only a few non-default input parameters are set and delivers high-quality calibrated data. CPU scalability based on a message-passing interface (MPI) implementation ensures that large bandwidth data from future arrays can be processed within reasonable computing times. Phase calibration is done with a Schwab-Cotton fringe fit algorithm. For the calibration of residual atmospheric effects, optimal solution intervals are determined based on the signal-to-noise ratio (S/N) of the data for each scan. Different solution intervals can be set for different antennas in the same scan to increase the number of detections in the low S/N regime. These novel techniques allow rPICARD to calibrate data from different arrays, including high-frequency and low-sensitivity arrays. The amplitude calibration is based on standard telescope metadata, and a robust algorithm can solve for atmospheric opacity attenuation in the high-frequency regime. Standard CASA tasks are used for CLEAN imaging and self-calibration. In this work we demonstrate the capabilities of rPICARD by calibrating and imaging 7 mm VLBA data of the central radio source in the M87 galaxy. The reconstructed jet image reveals a complex collimation profile and edge-brightened structure. A potential counter-jet is detected that has 10 % of the brightness of the approaching jet. This constrains jet speeds close to the radio core to about half the speed of light for small inclination angles.

astro-ph.IM↗

arXiv2019

Calibration of ALMA as a phased array: ALMA observations during the 2017 VLBI campaign

We present a detailed description of the special procedures for calibration and quality assurance of Atacama Large Millimeter/submillimeter Array (ALMA) observations in Very Long Baseline Interferometry (VLBI) mode. These procedures are required to turn the phased ALMA array into a fully calibrated VLBI station. As an illustration of these methodologies, we present full-polarization observations carried out with ALMA as a phased array at 3mm (Band 3) and 1.3mm (Band 6) as part of Cycle-4. These are the first VLBI science observations conducted with ALMA and were obtained during a 2017 VLBI campaign in concert with other telescopes worldwide as part of the Global mm-VLBI Array (GMVA, April 1-3) and the Event Horizon Telescope (EHT, April 5-11) in ALMA Bands 3 and 6, respectively.

astro-ph.IM↗

arXiv2019

Orion Source I's disk is salty

We report the detection of NaCl, KCl, and their $^{37}$Cl and $^{41}$K isotopologues toward the disk around Orion SrcI. About 60 transitions of these molecules were identified. This is the first detection of these molecules in the interstellar medium not associated with the ejecta of evolved stars. It is also the first ever detection of the vibrationally excited states of these lines in the ISM above v = 1, with firm detections up to v = 6. The salt emission traces the region just above the continuum disk, possibly forming the base of the outflow. The emission from the vibrationally excited transitions is inconsistent with a single temperature, implying the lines are not in LTE. We examine several possible explanations of the observed high excitation lines, concluding that the vibrational states are most likely to be radiatively excited via rovibrational transitions in the 25-35 μm (NaCl) and 35-45 μm (KCl) range. We suggest that the molecules are produced by destruction of dust particles. Because these molecules are so rare, they are potentially unique tools for identifying high-mass protostellar disks and measuring the radiation environment around accreting young stars.

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arXiv2019

A 10-$M_{\odot}$ YSO with a Keplerian disk and a nonthermal radio jet

We previously observed the star-forming region G16.59$-$0.05 through interferometric observations of both thermal and maser lines, and identified a high-mass young stellar object (YSO) which is surrounded by an accretion disk and drives a nonthermal radio jet. We performed high-angular-resolution (beam FWHM ~0.15") 1.2-mm continuum and line observations towards G16.59$-$0.05 with the Atacama Large Millimeter Array (ALMA). The main dust clump, with size ~10$^4$ au, is resolved into four relatively compact (diameter ~2000 au) millimeter (mm) sources. The source harboring the high-mass YSO is the most prominent in molecular emission. By fitting the emission profiles of several unblended and optically thin transitions of CH$_3$OCH$_3$ and CH$_3$OH, we derived gas temperatures inside the mm-sources in the range 42--131 K, and calculated masses of 1--5 $M_{\odot}$. A well-defined Local Standard of Rest velocity (Vlsr) gradient is detected in most of the high-density molecular tracers at the position of the high-mass YSO, pinpointed by compact 22-GHz free-free emission. This gradient is oriented along a direction forming a large (~70 degree) angle with the radio jet, traced by elongated 13-GHz continuum emission. The butterfly-like shapes of the P-V plots and the linear pattern of the emission peaks of the molecular lines at high velocity confirm that this Vlsr gradient is due to rotation of the gas in the disk surrounding the high-mass YSO. The disk radius is ~500 au, and the Vlsr distribution along the major axis of the disk is well reproduced by a Keplerian profile around a central mass of 10$\pm$2 $M_{\odot}$. The position of the YSO is offset by >~ 0.1" from the axis of the radio jet and the dust emission peak. To explain this displacement we argue that the high-mass YSO could have moved from the center of the parental mm source owing to dynamical interaction with one or more companions.

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arXiv2018

Micro-arcsecond structure of Sagittarius A* revealed by high-sensitivity 86 GHz VLBI observations

The compact radio source Sagittarius~A$^*$ (Sgr~A$^*$)in the Galactic Center is the primary supermassive black hole candidate. General relativistic magnetohydrodynamical (GRMHD) simulations of the accretion flow around Sgr\,A$^*$ predict the presence of sub-structure at observing wavelengths of $\sim 3$\,mm and below (frequencies of 86\,GHz and above). For very long baseline interferometry (VLBI) observations of Sgr\,A$^*$ at this frequency the blurring effect of interstellar scattering becomes subdominant, and arrays such as the High Sensitivity Array (HSA) and the global mm-VLBI Array (GMVA) are now capable of resolving potential sub-structure in the source. Such investigations improve our understanding of the emission geometry of the mm-wave emission of Sgr\,A$^*$, which is crucial for constraining theoretical models and for providing a background to interpret 1\,mm VLBI data from the Event Horizon Telescope (EHT). We performed high-sensitivity very long baseline interferometry (VLBI) observations of Sgr\,A$^*$ at 3\,mm using the Very Long Baseline Array (VLBA) and the Large Millimeter Telescope (LMT) in Mexico on two consecutive days in May 2015, with the second epoch including the Green Bank Telescope (GBT). We find an overall source geometry that matches previous findings very closely, showing a deviation in fitted model parameters less than 3\% over a time scale of weeks and suggesting a highly stable global source geometry over time. The reported sub-structure in the 3\,mm emission of Sgr\,A$^*$ is consistent with theoretical expectations of refractive noise on long baselines. However, comparing our findings with recent results from 1\,mm and 7\,mm VLBI observations, which also show evidence for east-west asymmetry, an intrinsic origin cannot be excluded. Confirmation of persistent intrinsic substructure will require further VLBI observations spread out over multiple epochs.

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