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تسجيل مستخدم جديدUnlocking the Origins of Ultracool Dwarf Radio Emission
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Empirical trends in stellar X-ray and radio luminosities suggest that low mass ultracool dwarfs (UCDs) should not produce significant radio emission. Defying these expectations, strong non-thermal emission has been observed in a few UCDs in the 1-10 GHz range, with a variable component often attributed to global aurorae and a steady component attributed to other processes such as gyrosynchrotron emission. While both auroral and gyrosynchrotron emission peak near the critical frequency, only the latter radiation is expected to extend into millimeter wavelengths. We present ALMA 97.5 GHz and VLA 33 GHz observations of a small survey of 5 UCDs. LP 349-25, LSR J1835+3259, and NLTT 33370 were detected at 97.5 GHz, while LP 423-31 and LP 415-20 resulted in non-detections at 33 GHz. A significant flare was observed in NLTT 33370 that reached a peak flux of 4880 +/- 360 microJy, exceeding the quiescent flux by nearly an order of magnitude, and lasting 20 seconds. These ALMA observations show bright 97.5 GHz emission with spectral indices ranging from alpha = -0.76 to alpha = -0.29, suggestive of optically thin gyrosynchrotron emission. If such emission traces magnetic reconnection events, then this could have consequences for both UCD magnetic models and the atmospheric stability of planets in orbit around them. Overall, our results provide confirmation that gyrosynchrotron radiation in radio loud UCDs can remain detectable into the millimeter regime.
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Recently unanticipated magnetic activity in ultracool dwarfs (UCDs, spectral classes later than M7) have emerged from a number of radio observations. The highly (up to 100%) circularly polarized nature and high brightness temperature of the emission
has been interpreted as an effective amplification mechanism of the high-frequency electromagnetic waves, the electron cyclotron maser instability (ECMI). In order to understand the magnetic topology and the properties of the radio emitting region and associated plasmas in these ultracool dwarfs and interpret the origin of radio pulses and their radiation mechanism, we built an active region model, based on the rotation of the UCD and the ECMI mechanism. ECMI mechanism is responsible for the radio bursts from the magnetic tubes and the rotation of the dwarf can modulate the integral of flux with respect to time. The high degree of variability in the brightness and the diverse profile of pulses can be interpreted in terms of a large-scale hot active region with extended magnetic structure existing in the magnetosphere of TVLM 513-46546. We suggest the time profile of the radio light curve is in the form of power law in the model. The radio emitting region consists of complicated substructure. With this model, we can determine the nature (e.g. size, temperature, density) of the radio emitting region and plasma. The magnetic topology can also be constrained. We compare our predicted X-ray flux with Chandra X-ray observation of TVLM 513-46546. Although the X-ray detection is only marginally significant, our predicted flux is significantly lower than the observed flux. We suggest more observations at multi-wavelength will help us understand the magnetic field structure and plasma behavior on the ultracool dwarf.
We present the numerical simulations for an electron-beam-driven and loss-cone-driven electron-cyclotron maser (ECM) with different plasma parameters and different magnetic field strengths for a relatively small region and short time-scale in an atte
mpt to interpret the recent discovered intense radio emission from ultracool dwarfs. We find that a large amount of electromagnetic field energy can be effectively released from the beam-driven ECM, which rapidly heats the surrounding plasma. A rapidly developed high-energy tail of electrons in velocity space (resulting from the heating process of the ECM) may produce the radio continuum depending on the initial strength of the external magnetic field and the electron beam current. Both significant linear polarization and circular polarization of electromagnetic waves can be obtained from the simulations. The spectral energy distributions of the simulated radio waves show that harmonics may appear from 10 to 70$ u_{rm pe}$ ($ u_{rm pe}$ is the electron plasma frequency) in the non-relativistic case and from 10 to 600$ u_{rm pe}$ in the relativistic case, which makes it difficult to find the fundamental cyclotron frequency in the observed radio frequencies. A wide frequency band should therefore be covered by future radio observations.
We report the detection of periodic (p = 1.96 hours) bursts of extremely bright, 100% circularly polarized, coherent radio emission from the M9 dwarf TVLM 513-46546. Simultaneous photometric monitoring observations have established this periodicity t
o be the rotation period of the dwarf. These bursts, which were not present in previous observations of this target, confirm that ultracool dwarfs can generate persistent levels of broadband, coherent radio emission, associated with the presence of kG magnetic fields in a large-scale, stable configuration. Compact sources located at the magnetic polar regions produce highly beamed emission generated by the electron cyclotron maser instability, the same mechanism known to generate planetary coherent radio emission in our solar system. The narrow beams of radiation pass our line of sight as the dwarf rotates, producing the associated periodic bursts. The resulting radio light curves are analogous to the periodic light curves associated with pulsar radio emission highlighting TVLM 513-46546 as the prototype of a new class of transient radio source.
A number of fast-rotating ultra cool dwarfs (UCDs) emit pulsed coherent radiation, attributed to the electron cyclotron maser instability, a phenomenon that occurs in the solar system at planets with strong auroral emission. In this paper we examine
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Recently, a number of ultracool dwarfs have been found to produce periodic radio bursts with high brightness temperature and polarization degree; the emission properties are similar to the auroral radio emissions of the magnetized planets of the Sola
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