arXiv subjects

Michel Tagger

Publications and source records attributed to Michel Tagger.

14 recordsLinked to original sources

arXiv2025

Observations of Carbon Radio Recombination Lines with the NenuFAR telescope. I. Cassiopeia A and Cygnus A

Carbon Radio Recombination Lines (CRRLs) at decametre wavelengths trace the diffuse phase of the interstellar medium (ISM) of the Galaxy. Their observation allows to measure physical parameters of this phase. We observed CRRLs with the recently commissioned New Extension in Nan\c{c}ay Upgrading LOFAR (NenuFAR) telescope towards two of the brightest sources at low-frequency (10-85 MHz): Cassiopeia A and Cygnus A (hereafter Cas A and Cyg A respectively), to measure the density n_e and temperature T_e of electrons in line-of-sight clouds. We used NenuFAR's beamforming mode, and we integrated several tens of hours on each source. The nominal spectral resolution was 95.4 Hz. We developed a pipeline to remove radio frequency interference (RFI) contamination and correct the baselines. We then fitted the spectral lines observed in absorption, associated to line-of-sight clouds. Cas A is the brightest source in the sky at low frequencies and represents an appropriate test bench for this new telescope. On this source, we detected 398 C\alpha lines between principal quantum numbers n=426 and n=826. C\alpha lines towards Cyg A were fainter. We stacked the signal by groups of a few tens of lines to improve the quality of our fitting process. On both sources we reached significantly higher S/N and spectral resolution than the most recent detections by the LOw Frequency ARray (LOFAR). The variation of line shape with n provides constraints on the physical properties of the clouds: T_e, n_e, the temperature T_0 of the radiation field, the mean turbulent velocity v_t and the typical size of the cloud. The NenuFAR observations sample a larger space volume than LOFAR's towards the same sources due to the differences in instrumental beamsizes, and the discrepancies highlight the sensitivity of low-frequency CRRLs as probes of the diffuse ISM, paving the way towards large area surveys of CRRLs in our Galaxy.

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arXiv2021

Pilot study and early results of the Cosmic Filaments and Magnetism Survey with Nenufar: the Coma cluster field

NenuFAR, the New Extension in Nancay Upgrading LOFAR, is currently in its early science phase. It is in this context that the Cosmic Filaments and Magnetism Pilot Survey is observing sources with the array as it is still under construction - with 57 (56 core, 1 distant) out of a total planned 102 (96 core, 6 distant) mini-arrays online at the time of observation - to get a first look at the low-frequency sky with NenuFAR. One of its targets is the Coma galaxy cluster: a well-known object, host of the prototype radio halo. It also hosts other features of scientific import, including a radio relic, along with a bridge of emission connecting it with the halo. It is thus a well-studied object. In this paper, we show the first confirmed NenuFAR detection of the radio halo and radio relic of the Coma cluster at 34.4 MHz, with associated intrinsic flux density estimates: we find an integrated flux value of 106.3 +- 3.5 Jy for the radio halo, and 102.0 +- 7.4 Jy for the radio relic. These are upper bound values, as they do not include point-source subtraction. We also give an explanation of the technical difficulties encountered in reducing the data, along with steps taken to resolve them. This will be helpful for other scientific projects which will aim to make use of standalone NenuFAR imaging observations in the future.

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arXiv2021

Observations of shock propagation through turbulent plasma in the solar corona

Eruptive activity in the solar corona can often lead to the propagation of shock waves. In the radio domain the primary signature of such shocks are type II radio bursts, observed in dynamic spectra as bands of emission slowly drifting towards lower frequencies over time. These radio bursts can sometimes have inhomogeneous and fragmented fine structure, but the cause of this fine structure is currently unclear. Here we observe a type II radio burst on 2019-March-20th using the New Extension in Nan\c{c}ay Upgrading LOFAR (NenuFAR), a radio interferometer observing between 10-85 MHz. We show that the distribution of size-scales of density perturbations associated with the type II fine structure follows a power law with a spectral index in the range of $\alpha=-1.7$ to -2.0, which closely matches the value of $-5/3$ expected of fully developed turbulence. We determine this turbulence to be upstream of the shock, in background coronal plasma at a heliocentric distance of $\sim$2 R$_{\odot}$. The observed inertial size-scales of the turbulent density inhomogeneities range from $\sim$62 Mm to $\sim$209 km. This shows that type II fine structure and fragmentation can be due to shock propagation through an inhomogeneous and turbulent coronal plasma, and we discuss the implications of this on electron acceleration in the coronal shock.

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arXiv2007

General Relativistic Flux Modulations from Disk Instabilities in Sagittarius A*

Near-IR and X-ray flares have been detected from the supermassive black hole Sgr A* at the center of our Galaxy with a (quasi)-period of ~17-20 minutes, suggesting an emission region only a few Schwarzschild radii above the event horizon. The latest X-ray flare, detected with XMM-Newton, is notable for its detailed lightcurve, yielding not only the highest quality period thus far, but also important structure reflecting the geometry of the emitting region. Recent MHD simulations of Sgr A*'s disk have demonstrated the growth of a Rossby wave instability, that enhances the accretion rate for several hours, possibly accounting for the observed flares. In this Letter, we carry out ray-tracing calculations in a Schwarzschild metric to determine as accurately as possible the lightcurve produced by general relativistic effects during such a disruption. We find that the Rossby wave induced spiral pattern in the disk is an excellent fit to the data, implying a disk inclination angle of ~77 deg. Note, however, that if this association is correct, the observed period is not due to the underlying Keplerian motion but, rather, to the pattern speed. The favorable comparison between the observed and simulated lightcurves provides important additional evidence that the flares are produced in Sgr A*'s inner disk.

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arXiv2006

Global MHD instabilities: from Low Frequency to High Frequency QPOs, and to Sgr A*

I review recent work that goes beyond our model for the Low-Frequency Quasi-Periodic Oscillation of microquasars, based on the Accretion-Ejection Instability. I show that similar instabilities, which can be viewed as strongly unstable versions of the diskoseismologic modes, provide explanations for both the High-Frequency QPO and for the quasi-periodicity observed durng the flares of Sgr A*, the supermassive black hole at the Galactic Center.

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arXiv2006

Accretion-Ejection Instability, MHD Rossby Wave Instability, diskoseismology, and the high-frequency QPO of microquasars

We present a possible explanation for the high-frequency Quasi-Periodic Oscillations of microquasars by an MHD instability that combines the physics developed, in different contexts, for the Accretion-Ejection Instability, the Rossby-Wave Instability, and the normal modes of diskoseismic models (which rely on the properties of the relativistic rotation curve in the vicinity of the Marginally Stable Orbit). This instability can appear as modes of azimuthal wavenumbers m=2, 3,... that have very similar pattern speeds ω/m, while the m=1 mode, which would appear as the fundamental of this discrete spectrum, is less unstable. This would readily explain the 2:3 (and sometimes higher) frequency ratio observed between these QPO. These instabilites form eigenmodes, i.e. standing wave patterns at a constant frequency in the disk; they are strongly unstable, and thus do not need an external excitation mechanism to reach high amplitudes. Furthermore, they have the property that a fraction of the accretion energy can be emitted toward the corona: this would explain that these QPO are seen in a spectral state where Comptonized emission from the corona is always present. Their existence depends critically on the existence of a magnetic structure, formed by poloidal flux advected in the accretion process, in the central region between the disk and the black hole.

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arXiv2005

A Possible Rossby Wave Instability Origin for the Flares in Sagittarius A*

In recent years, near-IR and X-ray flares have been detected from the Galaxy's central radio point source, Sagittarius A* (Sgr A*), believed to be a \~3.10^6 solar masses supermassive black hole. In some cases, the transient emission appears to be modulated with a (quasi-)periodic oscillation (QPO) of ~ 17-20 minutes. The implied ~ 3 r_S size of the emitter (where r_S = 2GM/c^2 is the Schwarzschild radius) points to an instability - possibly induced by accretion - near the Marginally Stable Orbit (MSO) of a slowly spinning object. But Sgr A* is not accreting via a large, `standard' disk; instead, the low density environment surrounding it apparently feeds the black hole with low angular momentum clumps of plasma that circularize within ~ 10-300 r_S and merge onto a compact, hot disk. In this Letter, we follow the evolution of the disk following such an event, and show that a Rossby wave instability, particularly in its magnetohydrodynamic (MHD) form, grows rapidly and produces a period of enhanced accretion lasting several hours. Both the amplitude of this response, and its duration, match the observed flare characteristics rather well.

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arXiv2004

Magnetic floods: a scenario for the variability of the microquasar GRS 1915+105

We present a scenario for the variability of the microquasar GRS 1915+105. This starts from previous works, leading to the tentative identification of the Accretion-Ejection Instability as the source of the low-frequency Quasi-Periodic Oscillation of microquasars and other accreting sources. We follow the physics of this instability: its conditions (the magnetic field and geometry adapted to MHD jet models), its instability criterion, and its consequences (cooling down of the disk, heating and excitation of the corona). Comparing them to the observed properties of the source, in particular the detailed properties of its spectral states, we first derive a model for the $\sim$ 30 minutes cycles often exhibited by GRS 1915+105. In our model this is a limit cycle determined by the advection of poloidal magnetic flux to the inner region of the disk, and its destruction by reconnection (leading to relativistic ejections) with the magnetic flux trapped in the vicinity of the central source. We show how it leads to natural explanations for observed behaviors of GRS 1915+105, including the three basic states of Belloni et al (2000). We then discuss how this could be extrapolated further to understand the longer-term variability of this and other microquasars.

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arXiv2002

Accretion-ejection instability in magnetized disks: Feeding the corona with Alfven waves

We present a detailed calculation of the mechanism by which the Accretion-Ejection Instability can extract accretion energy and angular momentum from a magnetized disk, and redirect them to its corona. In a disk threaded by a poloidal magnetic field of the order of equipartition with the gas pressure, the instability is composed of a spiral wave (analogous to galactic ones) and a Rossby vortex. The mechanism detailed here describes how the vortex, twisting the footpoints of field lines threading the disk, generates Alfven waves propagating to the corona. We find that this is a very efficient mechanism, providing to the corona (where it could feed a jet or a wind) a substantial fraction of the accretion energy.

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arXiv2001

On Rossby waves and vortices with differential rotation

We present a simplified model for linearized perturbations in a fluid with both differential rotation and differential vorticity. Without the latter the model reduces to the classical Shearing Sheet used in the description of spiral density waves in astrophysical disks. Without the former it reduces to the $β$-plane approximation, used in the description of Rossby waves. Retaining both, our model allows one in general to discuss the coupling between density waves and Rossby waves, resulting in what is known as the ``corotation resonance'' for density waves. Here we derive, as a first application of this model, the properties of Rossby waves in a differentially rotating disk. We find that their propagation is quenched by differential rotation: after a limited number of oscillations, a Rossby wave collapses to a singular vortex, as fluid elements are sheared apart by differential rotation. In a keplerian disk, this number of oscillations is always lower than one. We also describe how, in a similar manner, a vortex is sheared in a very short time.

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arXiv2000

Numerical simulations of the accretion-ejection instability

The Accretion-Ejection Instability (AEI) is explored numerically using a global 2d model of the inner region of a magnetised accretion disk. The disk is initially currentless but threaded by an external vertical magnetic field created by external currents, and frozen in the flow. In agreement with the theory a spiral instability, similar in many ways to those observed in self-gravitating disks, but driven by magnetic stresses, develops when the magnetic field is close to equipartition with the disk thermal pressure. The present non-linear simulations give good evidence that such an instability can occur in the inner region of accretion disks.

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arXiv2000

The Accretion-Ejection Instability and a "Magnetic Flood" scenario for GRS 1915+105

I present a global view of recent results on the Accretion-Ejection Instability (AEI), described in more details in other contributions to this workshop. These results address essentially the characteristics of the AEI as a good candidate to explain the low-frequency QPO of X-ray binaries, in particular (at $\sim 1-10$ Hz) of micro-quasars. I then discuss how, if the AEI is considered as the source of the QPO, a possible scenario can be considered where the $\sim 30$ mn. cycles of GRS 1915+105 are controlled by the evolution of magnetic flux in the disk.

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arXiv1999

Accretion-Ejection Instability and a "Magnetic Flood" scenario for GRS 1915+105

We present an instability, occurring in the inner region of magnetized accretion disks, which seems to be a good candidate to explain the low-frequency QPO observed in many X-ray binaries. We then briefly show how, in the remarkable case of the microquasar GRS 1915+105, identifying this QPO with our instability leads to a scenario for the $\sim$ 30 mn cycles of this source. In this scenario the cycles are controlled by the build-up of magnetic flux in the disk.

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arXiv1995

The Parker-Shearing instability in azimuthaly magnetized discs

We describe the effects of both magnetic buoyancy and differential rotation on a disc of isothermal gas embedded in a purely azimuthal magnetic field, in order to study the evolution and interplay of Parker and shearing instabilities. Both instabilities occur on the slow MHD branch of the dispersion relation, and can affect the same waves. We put a stress on the natural polarization properties of the slow MHD waves to get a better understanding of the physics involved. The mechanism of the shearing instability is described in details. Waves with a vertical wavelength shorter than the scale height of the disc are subject to both the Parker and the transient shearing instabilities. They occur in different ranges of radial wavenumbers, \ie at different times in the shearing evolution ; these ranges can overlap or, on the contrary, be separated by a phase of wave-like oscillations, depending on the strength of differential rotation. Our results can be applied to both galactic and accretion discs.

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