arXiv subjects

A. Golden

Publications and source records attributed to A. Golden.

32 records · Page 2Linked to original sources

arXiv2007

Periodic Bursts of Coherent Radio Emission from an Ultracool Dwarf

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.

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arXiv2006

Rotational Modulation of the Radio Emission from the M9 Dwarf TVLM 513-46546: Broadband Coherent Emission at the Substellar Boundary?

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.

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arXiv2005

A MERLIN Observation of PSR B1951+32 and its associated Plerion

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 $σ$, 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.

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arXiv2003

Pulsar PSR B0656+14, the Monogem Ring, and the Origin of the `Knee' in the Primary Cosmic Ray Spectrum

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.

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arXiv2003

The distance and radius of the neutron star PSR B0656+14

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.

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arXiv2002

PSR B1951+32 and PSR J0537-6910 - where are the optical counterparts?

There remain several definitive gamma-ray pulsars that are as yet undetected in the optical regime. A classic case is the pulsar PSR B1951+32, associated with the complex CTB 80 SNR. Previous ground based high speed 2-d optical studies have ruled out candidates to m_V ~ 24. Hester (2000) has reported an analysis of archival HST/WFPC2 observations of the CTB 80 complex which suggest a compact synchrotron nebula coincident with the pulsar's radio position. Performing a similar analysis, we have identified a possible optical counterpart within this synchrotron nebula at m_V ~ 25.5, and another optical counterpart candidate nearby at m_V ~ 24.2. PSR J0537-6910 is a young (canonical age ~ 5000 years), 16ms pulsar in the LMC. We report a search for optical pulsations from the region around the X-ray position. We see no obvious candidate exhibiting optical pulsations at the X-ray period, with a 3 sigma upper limit of m_V ~ 23.6. We have also examined recent Chandra results (Wang et al. 2001) and show that their X-ray-Optical astrometry is in error by about 7''.

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arXiv2002

Why study pulsars optically?

Observations of the five confirmed optical pulsars indicate that the peak emission scales according to the magnetic field strength at the light cylinder. To the accuracy that such low number allows we show that this gives further confirmation that a straightforward synchrotron model such as Pacini & Salvati (1987) still has validity. The derived relationships indicates that the emission mechanism is common across all of the pulsars and towards the edge of the light cylinder. In the future observations should include near and far infra red work to determine the long wave self absorption cut-off and polarization observations of all pulsars to restrict (to first order) emission zone geometry.

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arXiv2002

Detection of new Optical Counterpart candidates to PSR B1951+32 with HST/WFPC2

There remain several definitive gamma-ray pulsars that are as yet undetected in the optical regime. A classic case is the pulsar PSR B1951+32, associated with the complex CTB 80 SNR. Previous ground based high speed 2-d optical studies have ruled out candidates to V ~ 24. Hester (2000a) has reported an analysis of archival HST/WFPC2 observations of the CTB 80 complex which suggest a compact synchrotron nebula coincident with the pulsar's radio position. Performing a similar analysis, we have identified a possible optical counterpart within this synchrotron nebula at V ~ 25.5 - 26, and another optical counterpart candidate nearby at V ~ 24.5. We assess the reality of these counterpart candidates in the context of existing models of pulsar emission.

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arXiv2000

High Speed Phase-Resolved 2-d UBV Photometry of the Crab pulsar

We report a phase-resolved photometric and morphological analysis of UBV data of the Crab pulsar obtained with the 2-d TRIFFID high speed optical photometer mounted on the Russian 6m telescope. By being able to accurately isolate the pulsar from the nebular background at an unprecedented temporal resolution (1 μs), the various light curve components were accurately fluxed via phase-resolved photometry. Within the $UBV$ range, our datasets are consistent with the existing trends reported elsewhere in the literature. In terms of flux and phase duration, both the peak Full Width Half Maxima and Half Width Half Maxima decrease as a function of photon energy. This is similarly the case for the flux associated with the bridge of emission. Power-law fits to the various light curve components are as follows; α= 0.07 \pm 0.19 (peak 1), α= -0.06 \pm 0.19 (peak 2) and α= -0.44 \pm 0.19 (bridge) - the uncertainty here being dominated by the integrated CCD photometry used to independently reference the TRIFFID data. Temporally, the main peaks are coincident to \le 10 μs although an accurate phase lag with respect to the radio main peak is compromised by radio timing uncertainties. The plateau on the Crab's main peak was definitively determined to be \leq 55 μs in extent and may decrease as a function of photon energy. There is no evidence for non-stochastic activity over the light curves or within various phase regions, nor is there evidence of anything akin to the giant pulses noted in the radio. Finally, there is no evidence to support the existence of a reported 60 second modulation suggested to be as a consequence of free precession.

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arXiv1999

Unpulsed UBV Optical Emission from the Crab Pulsar

Based on observations of the Crab pulsar using the TRIFFID high speed imaging photometer in the UBV bands using the Special Astrophysical Observatory's 6m telescope in the Russian Caucasus, we report the detection of pronounced emission during the so-called `off' phase of emission. Following de-extinction, this unpulsed component of emission is shown to be consistent with a power law with an exponent of alpha = -0.60 +/- 0.37, the uncertainty being dominated by the error associated with the independent CCD photometry used to reference the TRIFFID data. This suggests a steeper power law form than that reported elsewhere in the literature for the total integrated spectrum, which is essentially flat with alpha ~ 0.1, although the difference in this case is only significant at the ~ 2 sigma level. Deeper reference integrated and TRIFFID phase-resolved photometry in these bands in conjunction with further observations in the UV and R region would constrain this fit further.

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arXiv1998

Radius & Distance Estimates of the Isolated Neutron Stars Geminga & PSR B0656+14 using Optical Photometry

Integrated ground-based and HST optical studies of isolated neutron stars have provided important independent datasets in the determination of emission activity, particularly in the fitting of anticipated Rayleigh-Jeans extrapolations from EUV/soft X-ray datasets, despite their intrinsic faintness. Differentiation of the pulsed and unpulsed fluxes and consequently of the nonthermal and thermal modes of emission could provide definitive data with which to constrain this blackbody continuum. Based upon high speed photometric observations of Geminga and PSR B0656+14 in the B band, we have combined upper limits of unpulsed emission with recently published model-fits with a view to assessing possible implications for the R/d parameter. For Geminga, with a known distance of ~ 160 pc, we find that R_{infty} < 9.5 km for a ~ blackbody source, and R_{\infty} < 10.0 km with the presence of a magnetized H atmosphere. In addition, we suggest that PSR B0656+14 is some ~ 4 - 5 times closer than the 760 pc estimated from DM measurements alone.

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arXiv1998

A Search for the Optical Counterpart of PSR B1951+32 in the Supernova Remnant CTB 80

Using time-resolved two-dimensional aperture photometry we have put upper limits on the pulsed emission from two proposed optical counterparts for PSR B1951+32. Our pulsed upper limits of m_{vpulsed}>23.3, m_{bpulsed}>24.4$, for the first candidate and m_{vpulsed}>23.6$, m_{bpulsed}>24.3 for the second, make it unlikely that either of these is, in fact, the pulsar. We discuss three further candidates, but also reject these on the basis of timing results. A search of a 5.5arcs x 5.5arcs area centred close to these stars failed to find any significant pulsations at the reported pulsar period.

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arXiv1998

Time-Resolved Optical Observations of PSR 1509-58

Using time resolved 2-dimensional aperture photometry we have established that the optical candidate for PSR 1509-58 does not pulse. Our pulsed upper limits (m_V = 24.3 and m_B = 25.7) put severe constraints on this being the optical counterpart. Furthermore the colours of the candidate star are consistent with a main sequence star at a distance of 2-4 kpc. The probability of a chance coincidence with a normal star and the difficulty of explaining the lack of pulsed emission leads us to conclude that this object is an intermediate field star.

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arXiv1998

Pulsed optical emission from Geminga

We present optical data which shows that G", the optical counterpart of the gamma-ray pulsar Geminga, pulses in B with a period of 0.237 seconds. The similarity between the optical pulse shape and the gamma-ray light curve indicates that a large fraction of the optical emission is non-thermal in origin - contrary to recent suggestions based upon the total optical flux. The derived magnitude of the pulsed emission is m_B = 26.0 +/- 0.4. Whilst it is not possible to give an accurate figure for the pulsed fraction (due to variations in the sky background) we can give an upper limit of m_B ~ 27 for the unpulsed fraction.

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