arXiv subjects

F. Yusef-Zadeh

Publications and source records attributed to F. Yusef-Zadeh.

At least 19 recordsLinked to original sources

arXiv2026

Observational Tests of Sgr A*'s State of the Accretion Flow

General relativistic magneto-hydrodynamic (GRMHD) simulations of the possibly magnetically arrested disk (MAD) of the accretion flow of Sgr A* predict short and long time variability, lumpiness in the flow, strong magnetic fields, reconnection driving flux eruptions and a jet aligned with its spin angular momentum. We carried out simultaneous multi-wavelength observations with the objective of placing some constraints on these predictions. Our analysis supports most of the predictions of the MAD picture of the flow. It is not clear if there is evidence for a jet, but this could be the result of confusion in the complex region surrounding Sgr A*. Also, the geometry of the magnetic fields, which accelerate particles to produce X-ray emission via inverse Compton Scattering (ICS), appears to be consistent with a hybrid state where the accretion flow is represented by MAD-like magnetic field strength and with Standard and Normal Evolution-like (SANE-like) magnetic field geometry.

astro-ph.GA↗

arXiv2025

Simultaneous JWST, NuSTAR, and VLA Monitoring of Sgr A*: A Unified Picture of the Variable IR, X-ray and Radio Emission

Flux variability is a fundamental channel of information from Sgr A* because of its direct probe of processes occurring within an accretion disk under strong gravity. We present simultaneous IR, X-ray and radio observations of Sgr A* on 2024 Apr 05 using JWST, NuSTAR, and VLA. We report the detection of a strong X-ray flare with a luminosity of $\sim5.2x10^{35}$ erg/s coincident with a bright near-IR flare, and a brightening in radio about an hour later. We investigate the candidate physical mechanisms for the X-ray flare emission and conclude that this can best be explained by inverse Compton scattering of near-IR flare radiation. We propose a dynamic scenario analogous to a coronal mass ejection in which a magnetic flux rope is ejected from Sgr A*'s inner accretion flow with a current sheet extending down from the rope to the bulk of the accretion flow. Reconnection within the sheet produces oppositely directed flows of accelerated particles moving upwards towards the rope and downwards towards the accretion flow. Infrared radiation from the approaching energetic electrons is enhanced by beaming and up-scattered by thermal electrons in the accretion flow to produce the strong X-ray flare. Meanwhile, the relativistic electrons moving in the opposite direction away from the disk experience weaker magnetic fields so radiate at longer wavelengths by feeding into the magnetic flux tube and adiabatically cooled during its subsequent expansion. This physical picture attempts to unify the origin of the variable emission from Sgr A* at IR, X-ray and radio/submm wavelengths.

astro-ph.HE↗

arXiv2025

A Catalog of Filaments in the Central Molecular Zone

We present a catalog of 1.28 GHz radio filaments observed by MeerKAT over the innermost 200 pc of the Galaxy (roughly $\pm 1.5\deg$), which includes the central molecular zone (CMZ). The catalog is generated by repurposing software developed for the automated detection of filaments in solar coronal loops. There are two parts to the catalog. The first part, the main catalog, provides a point-by-point listing of locations and basic observational properties along each detected filament. The second part is a summary catalog which provides a listing of mean, median, or total values of various properties for each filament. Tabulated quantities include position, length, curvature, brightness, and spectral index. The catalogs contain a heterogeneous mix of filamentary structures, including nonthermal radio filaments (NRFs), and parts of supernova remnants (SNRs) and thermally emitting regions (e.g. H II regions). We discuss criteria for selecting useful subsamples of filaments from the catalogs, and some of the details encountered in examining filaments or selections of filaments from the catalogs.

astro-ph.GA↗

arXiv2025

Long-Term Astrometric Monitoring of the Galactic Center Magnetar PSR J1745--2900

We present new astrometric observations of the Galactic Center magnetar, PSR J1745-2900, with the Very Long Baseline Array (VLBA). Combined with previously published measurements in 10 epochs that spanned 477 days, the complete data set consists of 25 epochs and 41 independent measurements that span 1984 days. These data constrain the proper motion to an accuracy of $\lesssim 2\%$ and set an upper limit on the absolute value of the magnetar's acceleration of $\lesssim (0.4, 0.2)\, {\rm mas\,y^{-2}}$ in the two celestial coordinates, consistent with the maximum value of $\sim 0.03\,{\rm mas\,y^{-2}}$ expected for an orbit around Sgr A*. Future measurements have the potential to detect the acceleration of PSR J1745-2900 due to Sgr A* should PSR J1745-2900 re-brighten. We consider several potential sources of systematic variations in the astrometric residuals after fitting for standard parameters, including refractive wander, changes in the structure of Sgr A*, and the presence of an unseen binary companion. While a stellar companion model can be fit to the astrometric data, pulse period measurements are inconsistent with that model. No changes in the apparent image size of the magnetar were detected over the duration of these observations, indicating a lack of change in the properties of the line-of-sight scattering during this period. We also show that the upper limit to the mean core shift of Sgr A* is consistent with expectations for a compact jet or symmetric accretion flow.

astro-ph.HE↗

arXiv2025

Non-stop Variability of Sgr A* using JWST at 2.1 and 4.8 micron Wavelengths: Evidence for Distinct Populations of Faint and Bright Variable Emission

We present first results of JWST Cycle 1 and 2 observations of Sgr A* using NIRCam taken simultaneously at 2.1 and 4.8 micron for a total of ~48 hours over seven different epochs in 2023 and 2024. We find correlated variability at 2.1 and 4.8 micron in all epochs, continual short-time scale (a few seconds) variability and epoch-to-epoch variable emission implying long-term ( ~days to months) variability of Sgr A*. A highlight of this analysis is the evidence for sub-minute, horizon-scale time variability of Sgr A*, probing inner accretion disk size scales. The power spectra of the light curves in each observing epoch also indicate long-term variable emission. With continuous observations, JWST data suggest that the flux of Sgr A* is fluctuating constantly. The flux density correlation exhibits a distinct break in the slope at ~3 mJy at 2.1 micron. The analysis indicates two different processes contributing to the variability of Sgr A*. Brighter emission trends towards shallower spectral indices than the fainter emission. Cross correlation of the light curves indicates for the first time, a time delay of 3 - 40 sec in the 4.8 micron variability with respect to 2.1 micron. This phase shift leads to loops in plots of flux density vs spectral index as the emission rises and falls. Modeling suggests that the synchrotron emission from the evolving, age-stratified electron population reproduces the shape of the observed light curves with a direct estimate of the magnetic field strengths in the range between 40-90 G, and upper cutoff energy, E_c, between 420 and 720 MeV.

astro-ph.GA↗

arXiv2024

Radio study of the Arc and the Sgr A complex near the Galactic center

Radio continuum and radio recombination line observations of the inner degree of the galactic center reveal a rich collection of thermal and nonthermal radio structures: (a) A network of linear filaments that are oriented perpendicular to the galactic plane constitute the major portion of the radio Arc at l~ 0.2°. These filaments have nonthermal characteristics, show polarized emission at 6, 3 and 2 cm, are organized over a 100 pc scale, and have a flat spectrum. (b) A number of thread-like filaments are situated asymmetrical with respect to the galactic plane and appear to be isolated unlike the linear filaments which are grouped together. The polarization and spectra of these so called "threads" are not yet known. (c) A network of arched filamentary structures that is disorganized in its appearance constitutes the curved portion of the Arc. Radio recombination line emission from these filaments indicates a thermal character for the emission. (d) A very steep-spectrum ridge of emission is seen to emerge from Sgr A. (e) The relative location of Sgr A East, West, a cluster of HII regions and the 50 km/s molecular cloud are discussed. Comparisons of the low and high-frequency maps show clearly that Sgr A East lies behind Sgr A West.

astro-ph.GA↗

arXiv2024

G359.13142-0.20005: A steep spectrum radio pulsar candidate with an X-ray counterpart running into the Galactic Center Snake (G359.1-0.2)

The Snake is a remarkable Galactic center radio filament with a morphology characterized by two kinks along its $\sim 20'$ extent. The major and minor kinks are located where the filament is most distorted from a linear magnetized structure running perpendicular to the Galactic plane. We present {\em Chandra}, VLA, and MeerKAT data and report the detection of an X-ray and radio source at the location of the major kink. High-resolution radio images of the major kink reveal a compact source with a steep spectrum with spectral index alpha ~ -2.7 surrounded by extended emission. The radio luminosity and steep spectrum of the compact source are consistent with a pulsar. We also show flattening of the spectrum and enhanced synchrotron emissivity away from the position of the major kink along the Snake, which suggests injection of relativistic particles along the Snake. We argue that the major kink is created by a fast-moving (~500-1000 km/s), object punching into the Snake, distorting its magnetic structure, and producing X-ray emission. X-ray emission pinpoints an active acceleration site where the interaction is taking place. A secondary kink is argued to be induced by the impact of the high-velocity object producing the major kink.

astro-ph.HE↗

arXiv2024

The interaction of a large-scale nuclear wind with the high velocity HII region G0.17+0.15

We investigate the nature of a Galactic center source, G0.17+0.15, lying along the northern extension of the Radio Arc near l~0.2deg. G0.17+0.15 is an HII region located toward the eastern edge of the radio bubble, embedded within the highly polarized Galactic center eastern Lobe where a number of radio filaments appear to cross through the HII region. We report the detection of hydrogen and helium recombination lines with a radial velocity exceeding 140 km/s based on GBT and VLA observations. The morphology of G0.17+0.15, aided by kinematics, and spectral index characteristics, suggests the presence of an external pressure dragging and shredding the ionized gas. We argue that this ionized cloud is interacting with a bundle of radio filaments and is entrained by the ram pressure of the radio bubble, which itself is thought to be produced by cosmic-ray driven outflows at the Galactic center. In this interpretation, the gas streamers on the western side of G0.17+0.15 are stripped, accelerated from 0 to deltav~35 km/s, over a time scale roughly 8x10^4 years, implying that ablating ram pressure is ~700 eV cm-3, comparable to the ~10^3 eV cm-3 cosmic-ray driven wind pressure in the Galactic center region.

astro-ph.GA↗

arXiv2023

Detection of large-scale synchrotron radiation from the molecular envelope of the Sgr B cloud complex at the Galactic center

We present highly sensitive measurements taken with MeerKAT at 1280 MHz as well as archival GBT, MWA and VLA images at 333, 88 and 74 MHz. We report the detection of synchrotron radio emission from the infrared dark cloud (IRDC) associated with the halo of the Sgr B complex on a scale of ~60 pc. A strong spatial correlation between low-frequency radio continuum emission and dense molecular gas, combined with spectral index measurements, indicates enhanced synchrotron emission by cosmic-ray electrons. Correlation of the FeI 6.4 keV Kalpha line and synchrotron emission provides compelling evidence that the low energy cosmic-ray electrons are responsible for producing the Kalpha line emission. The observed synchrotron emission within the halo of the Sgr B cloud complex has mean spectral index alpha -1+/-1 gives the magnetic field strength ~100 muG for cloud densities nH = 10^4-10^5 cm-3 and estimate cosmic-ray ionization rates between 10^-13 and 10^-14 s^-1. Furthermore, the energy spectrum of primary cosmic-ray electrons is constrained to be E^-3 +/-1 for typical energies of few hundred MeV. The extrapolation of this spectrum to higher energies is consistent with X-ray and gamma-ray emission detected from this cloud. These measurements have important implications on the role that high cosmic-ray electron fluxes at the Galactic center play in production of radio synchrotron emission, the FeI Kalpha line emission at 6.4 keV and ~GeV gamma-ray emission throughout the central molecular zone (CMZ).

astro-ph.HE↗

arXiv2023

The Population of the Galactic Center Filaments: Position Angle Distribution Reveal a Degree-scale Collimated Outflow from Sgr A* along the Galactic Plane

We have examined the distribution of the position angle (PA) of the Galactic center filaments with lengths $L > 66''$ and $ < 66''$ as well as their length distribution as a function of PA. We find bimodal PA distributions of the filaments, long and short populations of radio filaments. Our PA study shows the evidence for a distinct population of short filaments with PA close to the Galactic plane. Mainly thermal short radio filaments ($<66''$) have PAs concentrated close to the Galactic plane within $60^\circ < \rm PA <120^\circ$. Remarkably, the short filament PAs are radial with respect to the Galactic center at $l <0^\circ$, and extend in the direction toward Sgr A*. On a smaller scale, the prominent Sgr E HII complex G358.7-0.0 provides a vivid example of the nearly radial distribution of short filaments. The bimodal PA distribution suggests different origin for two distinct filament populations. We argue that alignment of the short filament population results from the ram pressure of a degree-scale outflow from Sgr A* that exceeds the internal filament pressure, and aligns them along the Galactic plane. The ram pressure is estimated to be 2$\times10^6\,$ cm$^{-3}\,$ K at a distance of 300pc, requiring biconical mass outflow rate $10^{-4}$ \msol\, yr$^{-1}$ with an opening angle of $\sim40^\circ$. This outflow aligns not only the magnetized filaments along the Galactic plane but also accelerates thermal material associated with embedded or partially embedded clouds. This places an estimate of $\sim$6 Myr as the age of the outflow.

astro-ph.GA↗

arXiv2022

Populations of Magnetized Filaments in the Intracluster Medium and the Galactic Center

Magnetized radio filaments are found in abundance in the inner few hundred pc of our Galaxy. Progress in understanding this population of filaments has been slow, in part due to a lack of detection elsewhere in the Galaxy or in external galaxies. Recent highly sensitive radio continuum observations of radio galaxies in galaxy clusters have revealed remarkable isolated filamentary structures in the ICM that are linked to radio jets, tails and lobes. The origin of this class of filaments is not understood either. Here, we argue that the underlying physical mechanisms responsible for the creation of the two populations are the same because of their similarity in morphology, spacing between the filaments, aspect ratio, magnetic energy densities to thermal pressure of the medium, and that both populations have undergone synchrotron aging. These similarities provide an opportunity to investigate the physical processes in the ISM and ICM for the first time. We consider that the origin of the filaments in both the GC and ICM is a result of the interaction of a large-scale wind with clouds, or the filaments arise through the stretching and collection of field lines by turbulence in weakly magnetized medium. We examine these ideas toward four radio galaxy filaments associated with four radio galaxies IC 40B, IC 4496, J1333-3141, ESO137-006 and argue that much can be understood in the future by comparing these two populations of filaments.

astro-ph.GA↗

arXiv2022

The population of Galactic centre filaments III: candidate radio and stellar sources

Recent MeerKAT radio continuum observations of the Galactic center at 20 cm show a large population of nonthermal radio filaments (NRFs) in the inner few hundred pc of the Galaxy. We have selected a sample of 57 radios ources, mainly compact objects, in the MeerKAT mosaic image that appear to be associated with NRFs. The selected sources are about 4 times the number of radio point sources associated with filaments than would be expected by random chance. Furthermore, an apparent correlation between bright IR stars and NRFs is inferred from their similar latitude distributions, suggesting that they both co-exist within the same region. To examine if compact radio sources are related to compact IR sources, we have used archival 2MASS, and {\em Spitzer} data to make spectral energy distribution of individual stellar sources coincident or close to radio sources. We provide a catalogue of radio and IR sources for future detailed observations to investigate a potential 3-way physical association between NRFs, compact radio and IR stellar sources. This association is suggested by models in which NRFs are cometary tails produced by the interaction of a large-scale nuclear outflow with stellar wind bubbles in the Galactic center.

astro-ph.GA↗

arXiv2022

Statistical Properties of the Population of the Galactic Center Filaments II: The Spacing Between Filaments

We carry out a population study of magnetized radio filaments in the Galactic center using MeerKAT data by focusing on the spacing between the filaments that are grouped. The morphology of a sample of 43 groupings containing 174 magnetized radio filaments are presented. Many grouped filaments show harp-like, fragmented cometary tail-like, or loop-like structures in contrast to many straight filaments running mainly perpendicular to the Galactic plane. There are many striking examples of a single filament splitting into two prongs at a junction, suggestive of a flow of plasma along the filaments. Spatial variations in spectral index, brightness, bending and sharpening along the filaments indicate that they are evolving on a 10^{5-6}-year time scale. The mean spacings between parallel filaments in a given grouping peaks at $\sim16''$. We argue by modeling that the filaments in a grouping all lie on the same plane and that the groupings are isotropically oriented in 3D space. One candidate for the origin of filamentation is interaction with an obstacle, which could be a compact radio source, before a filament splits and bends into multiple filaments. In this picture, the obstacle or sets the length scale of the separation between the filaments. Another possibility is synchrotron cooling instability occurring in cometary tails formed as a result of the interaction of cosmic-ray driven Galactic center outflow with obstacles such as stellar winds. In this picture, the mean spacing and the mean width of the filaments are expected to be a fraction of a parsec, consistent with observed spacing.

astro-ph.HE↗

arXiv2022

The 1.28 GHz MeerKAT Galactic Center Mosaic

The inner $\sim$200 pc region of the Galaxy contains a 4 million M$_{\odot}$ supermassive black hole (SMBH), significant quantities of molecular gas, and star formation and cosmic ray energy densities that are roughly two orders of magnitude higher than the corresponding levels in the Galactic disk. At a distance of only 8.2 kpc, the region presents astronomers with a unique opportunity to study a diverse range of energetic astrophysical phenomena, from stellar objects in extreme environments, to the SMBH and star-formation driven feedback processes that are known to influence the evolution of galaxies as a whole. We present a new survey of the Galactic center conducted with the South African MeerKAT radio telescope. Radio imaging offers a view that is unaffected by the large quantities of dust that obscure the region at other wavelengths, and a scene of striking complexity is revealed. We produce total intensity and spectral index mosaics of the region from 20 pointings (144 hours on-target in total), covering 6.5 square degrees with an angular resolution of 4$"$,at a central frequency of 1.28 GHz. Many new features are revealed for the first time due to a combination of MeerKAT's high sensitivity, exceptional $u,v$-plane coverage, and geographical vantage point. We highlight some initial survey results, including new supernova remnant candidates, many new non-thermal filament complexes, and enhanced views of the Radio Arc Bubble, Sgr A and Sgr B regions. This project is a SARAO public legacy survey, and the image products are made available with this article.

astro-ph.GA↗

arXiv2022

Statistical Properties of the Population of the Galactic Center Filaments: The Spectral Index and Equipartition Magnetic Field

We present high-pass filtered continuum images of the inner $3.5^\circ\times2.5^\circ$ of the Galactic center at 20 cm with $6.4''$ resolution. These mosaic images are taken with MeerKAT and reveal a large number of narrow filaments, roughly an order of magnitude increase in their numbers compared to past measurements. For the first time, we carry out population studies of the spectral index and magnetic field of the entire region. The mean spectral indices of the filaments are steeper than supernova remnants (SNRs) (-0.62) with a value of $α\sim-0.83$. The variation in $α$ is much larger than for the SNRs, suggesting that these characteristics have a different origin. A large-scale cosmic-ray driven wind has recently been proposed to explain the origin of filaments and the large-scale 430 pc bipolar radio and X-ray structure. This favors the possibility that the large-scale bipolar radio/X-ray structure is produced by past activity of Sgr A* rather than coordinated burst of supernovae. A trend of steeper indices is also noted with increasing distance from the Galactic plane. This could be explained either by synchrotron cooling or weak shocks accelerating cosmic-ray particles in the context of the cosmic-ray driven wind. The mean magnetic field strengths along the filaments ranges from $\sim100$ to 400 $μ$G depending on the assumed ratio of cosmic-ray protons to electrons. Given that there is a high cosmic ray pressure in the Galactic center, the large equipartition magnetic field implies that the magnetic field is weak in most of the interstellar volume of the Galactic center.

astro-ph.GA↗

arXiv2021

Radio observations of massive stars in the Galactic centre: The Quintuplet cluster

We present high-angular-resolution radio continuum observations of the Quintuplet cluster, one of the most emblematic massive clusters in the Galactic centre. Data were acquired in two epochs and at 6 and 10 GHz with the Karl J. Jansky Very Large Array. With this work, we have quadrupled the number of known radio stars in the cluster. Nineteen of them have spectral indices consistent with thermal emission from ionised stellar winds, five are consistent with colliding wind binaries, two are ambiguous cases, and one was only detected in a single band. Regarding variability, remarkably we find a significantly higher fraction of variable stars in the Quintuplet cluster (approximately 30%) than in the Arches cluster (< 15%), probably due to the older age of the Quintuplet cluster. Our determined stellar wind mass-loss rates are in good agreement with theoretical models. Finally, we show that the radio luminosity function can be used as a tool to constrain the age and the mass function of a cluster.

astro-ph.GA↗

arXiv2021

Radio observations of massive stars in the Galactic centre: The Arches Cluster

We present high-angular-resolution radio observations of the Arches cluster in the Galactic centre, one of the most massive young clusters in the Milky Way. The data were acquired in two epochs and at 6 and 10 GHz with the Karl G. Jansky Very Large Array (JVLA). The rms noise reached is three to four times better than during previous observations and we have almost doubled the number of known radio stars in the cluster. Nine of them have spectral indices consistent with thermal emission from ionised stellar winds, one is a confirmed colliding wind binary (CWB), and two sources are ambiguous cases. Regarding variability, the radio emission appears to be stable on timescales of a few to ten years. Finally, we show that the number of radio stars can be used as a tool for constraining the age and/or mass of a cluster and also its mass function.

astro-ph.SR↗

arXiv2020

G0.173-0.42: an X-ray and radio magnetized filament near the galactic center

The recent detection of an X-ray filament associated with the radio filament G0.173-0.42 adds to four other nonthermal radio filaments with X-ray counterparts, amongst the more than 100 elongated radio structures that have been identified as synchrotron-emitting radio filaments in the inner couple of degrees of the Galactic center. The synchrotron mechanism has also been proposed to explain the emission from X-ray filaments. However, the origin of radio filaments and the acceleration sites of energetic particles to produce synchrotron emission in radio and X-rays remain mysterious. Using MeerKAT, VLA, Chandra, WISE and Spitzer, we present structural details of G0.173-0.42 which consists of multiple radio filaments, one of which has an X-ray counterpart. A faint oblique radio filament crosses the radio and X-ray filaments. Based on the morphology, brightening of radio and X-ray intensities, and radio spectral index variation, we argue that a physical interaction is taking place between two magnetized filaments. We consider that the reconnection of the magnetic field lines at the interaction site leads to the acceleration of particles to GeV energies. We also argue against the synchrotron mechanism for the X-ray emission due to the short $\sim$30 year lifetime of TeV relativistic particles. Instead, we propose that the inverse Compton scattering mechanism is more likely to explain the X-ray emission by upscattering of seed photons emitted from a 10^6 solar luminosity star located at the northern tip of the X-ray filament.

astro-ph.HE↗