Nonthermal Filaments Research Articles

Discovery of a non-thermal X-ray filament in the Galactic Centre

We report the discovery of an X-ray filament, XMMJ174540–2904.5, in the Galactic Centre region. Images from Chandra and XMM-Newton show the X-ray source is extended and coincides with a non-thermal radio structure of somewhat larger extent. The X-ray spectrum is clearly not thermal in nature, and is well approximated as a heavily absorbed power-law continuum with a photon index ≈2. Combining the radio and X-ray spectra, we concluded that the emission in both wavebands probably originates in the synchrotron process. We discuss some possible origins for this peculiar non-thermal structure.

TheChandraDetection of Galactic Center X-Ray Features G359.89-0.08 and G359.54+0.18

We report the detection of two elongated X-ray features, G359.89-0.08 and G359.54+0.18, in the Galactic center (GC) region using the Chandra X-Ray Observatory. G359.89-0.08 is an elongated X-ray feature located ~2' in projection south of the center of the Galaxy, Sgr A*. This X-ray feature source is partially coincident with a slightly curved (wisplike) nonthermal radio source. The X-ray spectrum of G359.89-0.08 can best be characterized as nonthermal, with a photon index of 2. The morphological and spectral characteristics of the X-ray and radio emission associated with G359.89-0.08 are best interpreted as the synchrotron emission from a ram pressure–confined pulsar wind nebula. G359.54+0.18 is one of the most prominent radio nonthermal filaments (NTFs) in the GC region, located ~30' in projection from Sgr A*. A narrow (~10'') filament of X-ray emission appears to arise from one of the two strands that make up the radio NTF. Although the photon statistics are poor for this source, the X-ray emission is also likely to be nonthermal in nature. Several models for the production of X-ray emission in G359.54+0.18 are discussed.

Small‐Scale Structure of the SN 1006 Shock withChandraObservations

The northeast shell of SN 1006 is the most probable acceleration site of high-energy electrons (up to ~100 TeV) with the Fermi acceleration mechanism at the shock front. We resolved nonthermal filaments from thermal emission in the shell with the excellent spatial resolution of Chandra. The thermal component is extended over ~100'' (about 1 pc at 1.8 kpc distance) in width, consistent with the shock width derived from the Sedov solution. The spectrum is fitted with a thin thermal plasma of kT = 0.24 keV in nonequilibrium ionization, typical for a young supernova remnant. The nonthermal filaments are likely thin sheets with scale widths of ~4'' (0.04 pc) and ~20'' (0.2 pc) upstream and downstream, respectively. The spectra of the filaments are fitted with a power-law function of index 2.1-2.3, with no significant variation from position to position. In a standard diffusive shock acceleration model, the extremely small scale length in the upstream region requires the magnetic field nearly perpendicular to the shock normal. The injection efficiency (η) from thermal to nonthermal electrons around the shock front is estimated to be ~1 × 10-3 under the assumption that the magnetic field in the upstream region is 10 μG. In the filaments, the energy densities of the magnetic field and nonthermal electrons are similar to each other, and both are slightly smaller than that of thermal electrons. These results suggest that the acceleration occurs in more compact regions with larger efficiency than suggested by previous studies.

Constraints on distances to Galactic Centre non-thermal filaments from HI absorption

We have studied HI absorption towards three non-thermal filaments (NTFs) Sgr C, G359.54+0.18 and G359.79+0.17 using the Giant Metrewave Radio Telescope (GMRT). Our study, for the first time, constrains the distance of theSgr C NTF and the HII region seen associated with the NTF in the sky plane, to within a few hundred parsecs from the Galactic Centre (GC). A molecular cloud with a velocity of -100 km s - 1 appears to be associated with the central part of the Sgr C NTF. Our study also indicates that the Sgr C HII region is relatively farther away than the NTF along our line of sight, and thereby provides evidence against any possible interaction between the two objects. The NTF G359.54+0.18 shows weak HI absorption (4 σ detection) at a velocity of -140 km s - 1 , which is the velocity of a known dense molecular cloud seen towards the NTF. This cloud is expected to be located within ∼200 pc of the GC and thereby provides a lower limit to the distance. The upper limit to the distance of this NTF from the Sun is 10.5 kpc. The distance to the NTF G359.79+0.17 is between 5.1 and 10.5 kpc from the Sun.

Modeling the Galactic Center Nonthermal Filaments as Magnetized Wakes

We simulate the Galactic center nonthermal filaments as magnetized wakes formed dynamically from amplification of a weak (tens of ?G) global magnetic field through the interaction of molecular clouds with a Galactic center wind. One of the key issues in this cometary model is the stability of the filament against dynamical disruption. Here we show two-dimensional MHD simulations for interstellar conditions that are appropriate for the Galactic center. The structures eventually disrupt through a shear-driven nonlinear instability but maintain coherence for lengths up to 100 times their width as observed. The final instability, which destroys the filament through shredding and plasmoid formation, grows quickly in space (and time) and leads to an abrupt end to the structure, in accord with observations. As a by-product, the simulation shows that emission should peak well downstream from the cloud-wind interaction site.

The Galactic Center at 327 MHz

Figure 1 presents a wide-field, high dynamic-range, 327 MHz VLA2 image of the Galactic center (GC). This image was constructed from archival VLA data using new 3-D image restoration techniques which resolve the problem of non-coplanar baselines encountered at long wavelengths. In a recent paper (LaRosa et al. 2000) we presented a catalog of over a hundred sources from this image, 23 extended sources and 78 small-diameter sources. The catalog contains flux densities, positions, sizes, and, where possible, a 20/90 cm spectral index. We also present subimages of all the extended sources. We refer the reader to LaRosa et al. (2000) for the details. In this note we will concentrate on observations of the nonthermal filaments and briefly describe a new model for their formation.The origin and evolution of the nonthermal filaments (NTFs) observed in the GC is an outstanding problem. All of the 7 classified NTFs are visible on Figure 1: Four of these are labeled threads, the other three are the “Snake,” the “Pelican,” and the Sgr C filament. The wide-field imaging at 327 MHz lead to the discovery of the “Pelican” (Lang et al. 1999). This filament has the distinction of being the farthest NTF in projection from Sgr A and the only NTF that is parallel to the Galactic plane. One critical issue for understanding the activity and overall structure of the GC is whether these filamentary sources trace a pervasive, large-scale magnetic field or are local independent structures (e.g., Yusef-Zadeh 1989; Morris 1994, 1996; Uchida & Gusten 1995; Yusef-Zadeh, Wardle & Parastaran 1997; Shore & LaRosa 1999; Lang et al. 1999; Lang, Morris & Echevarria 1999; LaRosa et al. 2000).

The ionized gas in the Galactic Center Radio Arc

We present Infrared Space Observatory (ISO) observations of fine structure lines toward the Radio Arc region, close to Sgr A, in the Galactic center region (GCR). The ionization at large scale is dominated by the clusters of massive stars in this region: the Quintuplet and the Arches cluster. Any other possible mechanism as interaction with the magnetic fields revealed by the Non-Thermal Filaments (NTFs) must play a minor role. The radiation in this area is relatively hard ($T_\mathrm{eff}\sim 33-35$ kK) but diluted. It reaches large distances due to the inhomogeneity of the medium.

A New System of Parallel Isolated Nonthermal Filaments near the Galactic Center: Evidence for a Local Magnetic Field Gradient

We report the discovery of a system of isolated nonthermal filaments approximately 0.5 deg. northwest (75 pc in projection) of Sgr A. Unlike other isolated nonthermal filaments which show subfilamentation, braiding of subfilaments, and flaring at their ends, these filaments are simple linear structures and more closely resemble the parallel bundled filaments in the Galactic center radio arc. However, the most unusual feature of these filaments is that the 20/90 cm spectral index uniformly decreases as a function of length, in contrast to all other nonthermal filaments in the Galactic center. This spectral gradient may not be due to simple particle aging but could be explained by a curved electron energy spectrum embedded in a diverging magnetic field. If so, the scale of the magnetic gradient is not consistent with a large scale magnetic field centered on Sgr A* suggesting that this filament system is tracing a local magnetic field.

A Molecular Cloud and an Expanding Cavity Adjacent to the Nonthermal Filaments of the Galactic Center Radio Arc

Abstract Large-scale CO images taken with the $2 \times 2$ focal-plane array receiver on the 45 m telescope of Nobeyama Radio Observatory reveal parsec-scale structure of molecular gas around the Galactic center radio arc. The LVG analysis shows that the gas kinetic temperature is considerably enhanced ($T_\mathrm{k} \geq 70 \,\mathrm{K}$) in CO 0.13–0.13, a cloud which seems to be in contact with the nonthermal filaments of the Galactic center radio arc. This lends support for an interaction between CO 0.13–0.13 and the nonthermal filaments. The interaction with the strong magnetic field in and around the nonthermal filaments may heat up the molecular gas in CO 0.13–0.13 through shock dissipation and/or magnetic viscosity. We also find an expanding cavity adjacent to the nonthermal filaments. The shell has a radius of $\sim 5 \,\mathrm{pc}$ and an expansion velocity of $25 \,\mathrm{km} \,\mathrm{s}^{-1}$. This expanding cavity might have been formed by several hundred supernovae or a single hypernova within $3 \times 10^5 \,\mathrm{yr}$. Magnetic tubes in this region could have been swept up by the expanding cavity, and compressed by interactions with CO 0.13–0.13. An interaction between the cloud and magnetic field may accelerate electrons to relativistic energies through shock acceleration or magnetic field reconnection.

Plasmoid Formation and Acceleration in the Solar Streamer Belt

The dynamical behavior of a configuration consisting of a plane fluid wake flowing in a current sheet embedded in a plasma sheet that is denser than its surroundings is discussed. This configuration is a useful model for a number of structures of astrophysical interest, such as solar coronal streamers, cometary tails, the Earth's magnetotail and Galactic center nonthermal filaments. In this paper, the results are applied to the study of the formation and initial motion of the plasma density enhancements observed by the Large-Angle Spectrometric Coronagraph (LASCO) instrument onboard the Solar and Heliospheric Observatory (SOHO) spacecraft. It is found that beyond the helmet cusp of a coronal streamer, the magnetized wake configuration is resistively unstable, that a traveling magnetic island develops at the center of the streamer, and that density enhancements occur within the magnetic islands. As the massive magnetic island travels outward, both its speed and width increase. The island passively traces the acceleration of the inner part of the wake. The values of the acceleration and density contrasts are in good agreement with LASCO observations.

Foreword

The non-thermal filaments in the Galactic centre constitute one of the great mysteries of this region of the Galaxy. We summarise the observational data on these filaments and critically review the various theories which currently outnumber the observed filaments. We summarise our theory for the longest of these filaments, the Snake, and discuss the relevance of this model for the other filaments in the Galactic centre region. The physics involved in our model for the Snake involves much of the physics that has dominated the career of Professor Don Melrose. In particular, the diffusion of relativistic electrons in the Snake is determined from the theory of resonant scattering by Alfven waves.

Filaments in the Galactic Centre—with Special Reference to the ‘Snake’

AbstractThe non-thermal filaments in the Galactic centre constitute one of the great mysteries of this region of the Galaxy. We summarise the observational data on these filaments and critically review the various theories which currently outnumber the observed filaments. We summarise our theory for the longest of these filaments, the Snake, and discuss the relevance of this model for the other filaments in the Galactic centre region. The physics involved in our model for the Snake involves much of the physics that has dominated the career of Professor Don Melrose. In particular, the diffusion of relativistic electrons in the Snake is determined from the theory of resonant scattering by Alfvén waves.

A Wide-Field 90 Centimeter VLA Image of the Galactic Center Region

We present a wide-field, high dynamic range, high-resolution, long-wavelength (λ = 90 cm) VLA image of the Galactic center region. The image is centered on Sgr A, covers an area of 4° × 5° with an angular resolution of 43'', and has a rms sensitivity of ≈5 mJy beam-1. The image was constructed from archival (1989 and 1991) VLA data of Pedlar et al. and Anantharamaiah et al. using new three-dimensional image restoration techniques. These three-dimensional imaging techniques resolve the problem of non-coplanar baselines encountered at long wavelengths and yield distortion-free imaging of far-field sources with improved sensitivity. At λ = 90 cm the VLA is sensitive to both thermal and nonthermal emission and the resulting image gives an unprecedented contextual perspective of the large-scale radio structure in this unique and complicated region. We have catalogued over a hundred sources from this image and present for each source its 90 cm flux density, position, and size. For many of the small- diameter sources, we also derive the 20/90 cm spectral index. The spectral index as a function of length along several of the isolated nonthermal filaments has been estimated and found to be constant. We have found six new small-diameter sources, as well as several extended regions of emission, which are clearly distinct sources that have not been previously identified at higher frequencies. These data are presented as a first epoch of VLA observations that can be used to search for source variability in conjunction with a second epoch of observations that were recently initiated.

High‐Resolution Infrared Imaging and Spectroscopy of the Pistol Nebula: Evidence for Ejection

We present new NICMOS/HST infrared images and CGS4/UKIRT Br-alpha (4.05 um) spectroscopy of the Pistol Star and its associated nebula, finding strong evidence to support the hypothesis that the Pistol Nebula was ejected from the Pistol Star. The Pa-alpha NICMOS image shows that the nebula completely surrounds the Pistol Star, although the line intensity is much stronger on its northern and western edges. The Br-alpha spectra show the classical ring-like signature of quasi-spherical expansion, with weak blueshifted emission (V_max approx -60 km/s) and strong redshifted emission (V_max approx +10 km/s), where the velocities are with respect to the velocity of the Pistol Star; further, the redshifted emission appears to be flattened in the position-velocity diagram. These data suggest that the nebula was ejected from the star several thousand years ago, with a velocity between the current terminal velocity of the stellar wind (95 km/s) and the present expansion velocity of gas in the outer shell of the nebula (60 km/s). The Pa-alpha image reveals several emission-line stars in the region, including two newly-identified emission-line stars north of the Pistol Star with spectral types earlier than WC8 (T_eff > 50,000 K). The presence of these stars, the morphology of the Pa-alpha emission, and the velocity field in the gas suggest that the side of the nebula furthest from us is approaching, and being ionized by, the hot stars of the Quintuplet, and that the highest velocity redshifted gas has been decelerated by winds from the Quintuplet stars. We also discuss the possibility that the nebular gas might be magnetically confined by the ambient magnetic field delineated by the nearby nonthermal filaments.

The Galactic Center Isolated Nonthermal Filaments as Analogs of Cometary Plasma Tails

We propose a model for the origin of the isolated nonthermal filaments observed at the Galactic center based on an analogy to cometary plasma tails. We invoke the interaction between a large-scale magnetized galactic wind and embedded molecular clouds. As the advected wind magnetic field encounters a dense molecular cloud, it is impeded and drapes around the cloud, ultimately forming a current sheet in the wake. This draped field is further stretched by the wind flow into a long, thin filament the aspect ratio of which is determined by the balance between the dynamical wind and amplified magnetic field pressures. The key feature of this cometary model is that the filaments are dynamic configurations, and not static structures. As such, they are local amplifications of an otherwise weak field and not directly connected to any static global field. The derived field strengths for the wind and wake are consistent with observational estimates. Finally, the observed synchrotron emission is naturally explained by the acceleration of electrons to high energy by plasma and MHD turbulence generated in the cloud wake.

A Radio Polarimetric Study of the Galactic Center Threads

The Very Large Array has been used to carry out a multifrequency, polarimetric study of the nonthermal filaments (NTFs), G0.08+0.15, and G359.96+0.09, also known as the Northern and Southern Threads (Morris & Yusef-Zadeh). These linear structures have been observed at λ = 20, 6, 3.6, and 2 cm, with high enough spatial resolution to be resolved for the first time at 6 and 3.6 cm. The 20 cm image reveals a wealth of new detail in the radio sources lying within the inner 60 pc of the Galaxy. The Southern Thread has a prominent split along its length, similar to splitting at the ends of previously studied NTFs. In addition to the prominent Northern and Southern Threads, there are several elongated features that resemble NTFs but that extend for only 5-7 pc. With resolutions as fine as 2'', the 3.6 and 6 cm images reveal a high degree of continuity and little substructure internal to the filament. However, the width of the Northern Thread varies along its length between ~4'' and ~12'' (0.15-0.5 pc) and becomes markedly diffuse at its northwest extremity. The spectral index of the Northern Thread has been determined over a broad range of frequencies. Its flux density falls with frequency, (α = -0.5) between 90 and 6 cm (where Sν ∝ να) and becomes much steeper (α = -2.0) between 6 and 2 cm. The spectral index does not vary significantly along the length of the Northern Thread, which implies either that the diffusion timescale for the emitting electrons is less than their synchrotron lifetime or that the emitting electrons are reaccelerated continuously at multiple positions along the filament. Because of the lack of spectral index variation, we have not located the source of relativistic electrons. Polarization observations at 6 and 3.6 cm confirm the nonthermal nature of the emission from the Northern Thread. The fractional polarization in the Northern Thread reaches 70% in some regions, although the polarized emission is patchy. Large rotation measures (RM ≳ 2000 rad m-2) have been observed with irregular variations across the filament; typical values of RM are 100-2000 rad m-2. The lack of any apparent pattern in the distribution of RM suggests that the Faraday rotating medium is not physically associated with the Northern Thread since the filament itself is so highly ordered. The intrinsic magnetic field in the Northern Thread is predominantly aligned along its long axis. The data on the Southern Thread were less conclusive: the Southern Thread was not detected in total intensity at 6 or 3.6 cm, while the polarized emission at 3.6 cm arising from the Southern Thread is evident, with a bifurcated structure similar to that detected in total 20 cm intensity.

Discovery of a Nonthermal Galactic Center Filament (G358.85+0.47) Parallel to the Galactic Plane

We report the discovery of a new nonthermal filament, G358.85+0.47, the Pelican, located ~225 pc in projection from Sagittarius A and oriented parallel to the Galactic plane. VLA continuum observations at 20 cm reveal that this 7' (17.5 pc) structure bends at its northern extension and is comprised of parallel strands, which are most apparent at its ends. Observations at 6 and 3.6 cm reveal that the Pelican is a synchrotron-emitting source and is strongly linearly polarized over much of its extent. The spectral index of the filament changes from α20/6 = -0.8 to α6/3.6 = -1.5. The rotation measures exhibit a smooth gradient, with values ranging from -1000 to 500 rad m-2. The intrinsic magnetic field is well aligned along the length of the filament. Based on these properties, we classify the Pelican as one of the nonthermal filaments unique to the Galactic center. Since these filaments (most of which are oriented perpendicular to the Galactic plane) are believed to trace the overall magnetic field in the inner Galaxy, the Pelican is the first detection of a component of this field parallel to the plane. The Pelican may thus mark a transition region of the magnetic field orientation in the inner 1 kpc of the Galaxy.

Massive Stars in the Quintuplet Cluster

We present near-infrared photometry and K-band spectra of newly-identified massive stars in the Quintuplet Cluster, one of the three massive clusters projected within 50 pc of the Galactic Center. We find that the cluster contains a variety of massive stars, including more unambiguously identified Wolf-Rayet stars than any cluster in the Galaxy, and over a dozen stars in earlier stages of evolution, i.e., LBV, Ofpe/WN9, and OB supergiants. One newly identified star is the second ``Luminous Blue Variable'' in the cluster, after the ``Pistol Star.'' Given the evolutionary stages of the identified stars, the cluster appears to be about 4 \pm 1 Myr old, assuming coeval formation. The total mass in observed stars is $\sim 10^3 \Msun$, and the implied mass is $\sim 10^4 \Msun$, assuming a lower mass cutoff of 1 \Msun and a Salpeter initial mass function. The implied mass density in stars is at least a few thousand $\Msun pc^{-3}$. The newly-identified stars increase the estimated ionizing flux from this cluster by about an order of magnitude with respect to earlier estimates, to 10^{50.9} photons/s, or roughly what is required to ionize the nearby ``Sickle'' HII region (G0.18 - 0.04). The total luminosity from the massive cluster stars is $\approx 10^{7.5}$ \Lsun, enough to account for the heating of the nearby molecular cloud, M0.20 - 0.033. We propose a picture which integrates most of the major features in this part of the sky, excepting the non-thermal filaments. We compare the cluster to other young massive clusters and globular clusters, finding that it is unique in stellar content and age, except, perhaps, for the young cluster in the central parsec of the Galaxy. In addition, we find that the cluster is comparable to small ``super star clusters.''

4.5. Sub-mm [C I] and CO observations of molecular clouds presumably interacting by the G359.54+0.18 nonthermal filaments

We have investigated the physical properties of molecular clouds which are presumably interacting with the G359.54+0.18 Nonthermal Filaments and an associated H ii region east of the filaments (Staguhn et al., 1996). The sub-mm spectra of 12CO(3-2) were observed with the KOSMA 3 m telescope, while the 490 GHz [C i] 3P1 →3P0 observations were made with the AST/RO 1.7 m sub-mm telescope. Fig. 1 shows channel maps of the integrated CO and [C i] line intensities in the velocity range of the recombination line observed towards the nearby H ii region. This H ii region is traced by the VLA 5 GHz continuum observations which are shown as contours in the central parts of the maps. The G359.54+0.18 Nonthermal Filaments, situated further to the west, appear to be morphologically associated with the H ii region. The [C i] emission of the molecular cloud east of the filaments which is kinematically linked to the H ii region is anti-correlated with the molecular line emission over a large angular extent. It is unlikely that the large morphological differences between [C i] and CO in this region can be explained exclusively by a high abundance of neutral carbon in the surface regions of dense molecular clumps, as is usually the case in PDR regions near the Sun.

4.4. The Sagittarius C region mapped in CS(2→1) and (3→2) by the IRAM 30m telescope

We study the interaction of a dense giant molecular cloud with the HII region Sagittarius C, and a prominent nonthermal filament (NTF). For this purpose, we mapped the CS(2→1) and (3→2) transitions simultaneously with the IRAM 30m radio telescope, using the on-the-fly observing mode, and covering a 20pc×37pc (8′ × 15′) region. The high spatial resolution, 0.7pc (16″) at 147 GHz, allows for the first time to analyze in detail the morphology and kinematics of the molecular material and its relation to the ionized gas.