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ABSTRACT: We conducted a volume-limited survey at 4.9 GHz of 32 nearby ultracool dwarfs
with spectral types covering the range M7 -- T8. A statistical analysis was
performed on the combined data from the present survey and previous radio
observations of ultracool dwarfs. Whilst no radio emission was detected from
any of the targets, significant upper limits were placed on the radio
luminosities that are below the luminosities of previously detected ultracool
dwarfs. Combining our results with those from the literature gives a detection
rate for dwarfs in the spectral range M7 -- L3.5 of ~ 9%. In comparison, only
one dwarf later than L3.5 is detected in 53 observations. We report the
observed detection rate as a function of spectral type, and the number
distribution of the dwarfs as a function of spectral type and rotation
velocity. The radio observations to date point to a drop in the detection rate
toward the ultracool dwarfs. However, the emission levels of detected ultracool
dwarfs are comparable to those of earlier type active M dwarfs, which may imply
that a mildly relativistic electron beam or a strong magnetic field can exist
in ultracool dwarfs. Fast rotation may be a sufficient condition to produce
magnetic fields strengths of several hundreds Gauss to several kilo Gauss, as
suggested by the data for the active ultracool dwarfs with known rotation
rates. A possible reason for the non-detection of radio emission from some
dwarfs is that maybe the centrifugal acceleration mechanism in these dwarfs is
weak (due to a low rotation rate) and thus cannot provide the necessary density
and/or energy of accelerated electrons. An alternative explanation could be
long-term variability, as is the case for several ultracool dwarfs whose radio
emission varies considerably over long periods with emission levels dropping
below the detection limit in some instances.
12/2012;
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ABSTRACT: Hot Jupiters have been proposed as a likely population of low frequency radio
sources due to electron cyclotron maser emission of similar nature to that
detected from the auroral regions of magnetized solar system planets. Such
emission will likely be confined to specific ranges of orbital/rotational phase
due to a narrowly beamed radiation pattern. We report on GMRT 150 MHz radio
observations of the hot Jupiter Tau Bootis b, consisting of 40 hours carefully
scheduled to maximize coverage of the planet's 79.5 hour orbital/rotational
period in an effort to detect such rotationally modulated emission. The
resulting image is the deepest yet published at these frequencies and leads to
a 3-sigma upper limit on the flux density from the planet of 1.2 mJy, two
orders of magnitude lower than predictions derived from scaling laws based on
solar system planetary radio emission. This represents the most stringent upper
limits for both quiescent and rotationally modulated radio emission from a hot
Jupiter yet achieved and suggests that either a) the magnetic dipole moment of
Tau Bootis b is insufficient to generate the surface field strengths of > 50
Gauss required for detection at 150 MHz or b) Earth lies outside the beaming
pattern of the radio emission from the planet.
10/2012;
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ABSTRACT: 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 attempt 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$\nu_{\rm pe}$ ($\nu_{\rm pe}$ is the electron plasma frequency) in the
non-relativistic case and from 10 to 600$\nu_{\rm pe}$ 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.
04/2012;
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ABSTRACT: 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 Solar System. We simulate the dynamic spectra of
radio emission from ultracool dwarfs. The emission is assumed to be generated
due to the electron-cyclotron maser instability. We consider two source models:
the emission caused by interaction with a satellite and the emission from a
narrow sector of active longitudes; the stellar magnetic field is modeled by a
tilted dipole. We have found that for the dwarf TVLM 513-46546, the model of
the satellite-induced emission is inconsistent with the observations. On the
other hand, the model of emission from an active sector is able to reproduce
qualitatively the main features of the radio light curves of this dwarf; the
magnetic dipole seems to be highly tilted (by about 60 degrees) with respect to
the rotation axis.
11/2011;
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ABSTRACT: 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. Comment: 11 pages, 7 figures, accepted for publication in Astronomy and Astrophysics, language edited
09/2010;
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A. Shearer,
R. M. Redfern,
G. Gorman,
R. Butler, A. Golden,
P. O'Kane,
G. M. Beskin,
S. I. Neizvestny,
V. V. Neustroev,
V. L. Plokhotnichenko,
and M. Cullum
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ABSTRACT: Using data collected with the Special Astrophysical Observatory (SAO) 6 m telescope and the University College Galway (UCG) Transputer Instrument for Fast Image Deconvolution (TRIFFID) imaging photometer, we show that the radio, X-ray, and γ-ray pulsar PSR 0656+14 exhibits pulsed optical emission. We observed that the pulsed fraction was consistent with 100% and that the flux was higher than that expected from a thermal source. The magnitude of PSR 0656+14 in the B band was observed to be 25.1 ± 0.3, consistent with previous CCD observations. The peak of the optical signal is at phase 0.2 compared to the radio and in phase with the weak γ-ray pulse.
The Astrophysical Journal 01/2009; 487(2):L181. · 6.02 Impact Factor
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ABSTRACT: Nonthermal optical emission from pulsars has been observed definitively from five objects. The model that most accurately predicts the luminosity of such emissions is that of Pacini and Salvati, a model based on synchrotron radiation originating at a constant fraction of the light cylinder. We parameterize optical emission in a similar way, obtaining the solution to the radiative transfer equation that yields expressions for the expected monochromatic luminosity and synchrotron self-absorption frequency. If due to synchrotron self-absorption, we investigate whether the rollover at IR wavelengths observed for the Crab pulsar is a process likely to be common to all the synchrotron active pulsars. Although the low-frequency turnover in the Crab pulsar spectrum may be due to a low-energy cutoff in the underlying emitting particle population or to particles emitting below their critical frequency, a test of the current self-absorption model is provided by PSR B0540-69 and the 16 ms "Crab-like" pulsar J0537-69. Our model, scaled relative to the Crab pulsar, predicts that PSR B0540-69 should exhibit a rollover between the L and I bands and that PSR J0537-69 may be self-absorbed at optical wavelengths with mV ~ 24 (dependent on the spectral index, α, where Fν ~ ν-α). Our model is applicable to frequency regions well outside the optical and is limited to "young," "fast" (<100 ms) pulsars.
The Astrophysical Journal 12/2008; 631(1):471. · 6.02 Impact Factor
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ABSTRACT: A selection of ultracool dwarfs are known to be radio active, with both gyrosynchrotron emission and the electron cyclotron maser instability being given as likely emission mechanisms. To explore whether ultracool dwarfs previously undetected at 8.5 GHz may be detectable at a lower frequency. We select a sample of fast rotating ultracool dwarfs with no detectable radio activity at 8.5 GHz, observing each of them at 4.9 GHz. From the 8 dwarfs in our sample, we detect emission from 2MASS J07464256+2000321, with a mean flux level of 286 $\pm$ 24 $\mu Jy$. The light-curve of 2MASS J07464256+2000321, is dominated towards the end of the observation by a very bright, $\approx $100 % left circularly polarized burst during which the flux reached 2.4 mJy. The burst was preceded by a raise in the level of activity, with the average flux being $\approx$ 160 $\mu Jy$ in the first hour of observation rising to $\approx$ 400 $\mu Jy$ in the 40 minutes before the burst. During both periods, there is significant variability. The detection of 100% circular polarization in the emission at 4.9 GHz points towards the electron cyclotron maser as the emission mechanism. However, the observations at 4.9 GHz and 8.5 GHz were not simultaneous, thus the actual fraction of dwarfs capable of producing radio emission, as well as the fraction of those that show periodic pulsations is still unclear, as indeed are the relative roles played by the electron cyclotron maser instability versus gyrosynchrotron emission, therefore we cannot assert if the previous non-detection at 8.5 GHz was due to a cut-off in emission between 4.9 and 8.4 GHz, or due to long term variability.
05/2008;
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ABSTRACT: We report on radio observations of the M8.5 dwarf LSR J1835+3259 and the L3.5 dwarf 2MASS J00361617+1821104, which provide the strongest evidence to date that the electron cyclotron maser instability is the dominant mechanism producing radio emission in the magnetospheres of ultracool dwarfs. As has previously been reported for the M9 dwarf TVLM 513-46546, periodic pulses of 100% circularly polarized, coherent radio emission are detected from both dwarfs with periods of 2.84 +/- 0.01 and 3.08 +/- 0.05 hours respectively for LSR J1835+3259 and 2MASS J00361617+1821104. Importantly, periodic unpolarized radio emission is also detected from 2MASS J00361617+1821104, and brightness temperature limitations rule out gyrosynchrotron radiation as a source of this radio emission. The unpolarized emission from this and other ultracool dwarfs is also attributed to electron cyclotron maser emission, which has become depolarized on traversing the ultracool dwarf magnetosphere, possibly due to propagations effects such as scattering. Based on available v sin i data in the literature and rotation periods derived from the periodic radio data for the three confirmed sources of electron cyclotron maser emission, TVLM 513-46546, LSR J1835+3259 and 2MASS J00361617+1821104, we determine that the rotation axes of all three dwarfs are close to perpendicular to our line of sight. This suggests a possible geometrical selection effect due to the inherent directivity of electron cyclotron maser emission, that may account for the previously reported relationship between radio activity and v sin i observed for ultracool dwarfs. We also determine the radius of the dwarf LSR J1835+3259 to be > 0.117 +/- 0.012 R_Sol. (abridged) Comment: 11 pages, 2 tables, 4 figures, accepted for publication in ApJ
05/2008;
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ABSTRACT: Radio activity has been observed in a large variety of stellar objects, including in the last few years, ultra-cool dwarfs. To explore the extent of long-term radio activity in ultra-cool dwarfs, we use data taken over an extended period of 9 hr from the Very Large Array of the source 2MASS J05233822-1403022 in September 2006, plus data taken in 2004. The observation taken in September 2006 failed to detect any radio activity at 8.46 GHz. A closer inspection of earlier data reveals that the source varied from a null detection on 3 May 2004, to $\approx$95 $\mu$Jy on 17 May 2004, to 230 $\mu$Jy on 18 June 2004. The lack of detection in September 2006 suggests at least a factor of ten flux variability at 8.46 GHz. Three short photometric runs did not reveal any optical variability. In addition to the observed pulsing nature of the radio flux from another ultra-cool source, the present observations suggests that ultra-cool dwarfs may not just be pulsing but can also display long-term sporadic variability in their levels of quiescent radio emission. The lack of optical photometric variability suggests an absence of large-scale spots at the time of the latest VLA observations, although small very high latitude spots combined with a low inclination could cause very low amplitude rotational modulation which may not be measurable. We discuss this large variability in the radio emission within the context of both gyrosynchrotron emission and the electron-cyclotron maser, favoring the latter mechanism. Comment: 7 pages, 2 figures, 1 table, accepted for publication in A&A Letters
07/2007;
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ABSTRACT: 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 to 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. Comment: 12 pages, 3 figures, accepted for publication in ApJ Letters
05/2007;
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ABSTRACT: The Very Large Array was used to observe the ultracool, rapidly rotating M9 dwarf TVLM 513-46546 simultaneously at 4.88 GHz and 8.44 GHz. The radio emission was determined to be persistent, variable and periodic at both frequencies with a period of ~2 hours. This periodicity is in excellent agreement with the estimated period of rotation of the dwarf based on its v sin i of ~60 km/s. This rotational modulation places strong constraints on the source size of the radio emitting region and hence the brightness temperature of the associated emission. We find the resulting high brightness temperature, together with the inherent directivity of the rotationally modulated component of the emission, difficult to reconcile with incoherent gyrosynchrotron radiation. We conclude that a more likely source is coherent, electron cyclotron maser emission from the low density regions above the magnetic poles. This model requires the magnetic field of TVLM 513-46546 to take the form of a large-scale, stable, dipole or multipole with surface field strengths up to at least 3kG. We discuss a mechanism by which broadband, persistent electron cyclotron maser emission can be sustained in the low density regions of the magnetospheres of ultracool dwarfs. A second nonvarying, unpolarized component of the emission may be due to depolarization of the coherent electron cyclotron maser emission or alternatively, incoherent gyrosynchrotron or synchrotron radiation from a population of electrons trapped in the large-scale magnetic field.
09/2006;
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ABSTRACT: In an investigative 16 hour L band observation using the MERLIN radio interferometric array, we have resolved both the pulsar PSR B1951+32 and structure within the flat spectral radio continuum region, believed to be the synchrotron nebula associated with the interaction of the pulsar and its `host' supernova remnant CTB 80. The extended structure we see, significant at $\sim$ 4.5 $\sigma$, is of dimensions 2.5" $\times$ 0.75", and suggests a sharp bow shaped arc of shocked emission, which is correlated with similar structure observed in lower resolution radio maps and X-ray images. Using this MERLIN data as a new astrometric reference for other multiwavelength data we can place the pulsar at one edge of the HST reported optical synchrotron knot, ruling out previous suggested optical counterparts, and allowing an elementary analysis of the optical synchrotron emission which appears to trail the pulsar. The latter is possibly a consequence of pulsar wind replenishment, and we suggest that the knot is a result of magnetohydrodynamic (MHD) instabilities. These being so, it suggests a dynamical nature to the optical knot, which will require high resolution optical observations to confirm. Comment: 12 pages, 2 figures. Accepted for publication in ApJL
12/2005;
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Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series; 09/2004
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Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series; 09/2004
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Astronomical Data Analysis Software and Systems (ADASS) XIII; 07/2004
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Astronomical Data Analysis Software and Systems (ADASS) XIII; 07/2004
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ABSTRACT: We detected a correlation between optical and giant radio pulse emission from the Crab pulsar. Optical pulses coincident with the giant radio pulses were on average 3% brighter than those coincident with normal radio pulses. Combined with the lack of any other pulse profile changes, this result indicates that both the giant radio pulses and the increased optical emission are linked to an increase in the electron-positron plasma density.
Science 08/2003; 301(5632):493-5. · 31.20 Impact Factor
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ABSTRACT: The Monogem ring is a bright, diffuse, 25-degree-diameter supernova remnant easily visible in soft X-ray images of the sky. Projected within the ring is a young radio pulsar, PSR B0656+14. An association between the remnant and pulsar has been considered, but was seemingly ruled out by the direction and magnitude of the pulsar proper motion and by a distance estimate that placed the pulsar twice as far from Earth as the remnant. Here we show that in fact the pulsar was born very close to the center of the expanding remnant, both in distance and projection. The inferred pulsar and remnant ages are in good agreement. The conclusion that the pulsar and remnant were born in the same supernova explosion is nearly inescapable. The remnant distance and age are in remarkable concordance with the predictions of a model for the primary cosmic ray energy spectrum in which the `knee' feature is produced by a single dominant source. Comment: 4 pages, to appear in the Astrophys. J. Lett. Full size color figure can be found at http://www.thorsett.org/research
06/2003;
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ABSTRACT: We present the result of astrometric observations of the radio pulsar PSR B0656+14, made using the Very Long Baseline Array. The parallax of the pulsar is pi = 3.47 +- 0.36 mas, yielding a distance of 288 +33 -27 pc. This independent distance estimate has been used to constrain existing models of thermal x-ray emission from the neutron star's photosphere. Simple blackbody fits to the x-ray data formally yield a neutron star radius R_inf ~ 7-8.5 km. With more realistic fits to a magnetized hydrogen atmosphere, any radius between ~13 and ~20 km is allowed. Comment: 7 pages including 1 figure. Submitted to ApJL. AAStex
06/2003;
