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Simon Johnston

Publications and source records attributed to Simon Johnston.

106 records · Page 6Linked to original sources
arXiv2003

Giant Pulses from PSR B0540-69 in the Large Magellanic Cloud

We report the discovery of the first giant pulses from an extragalactic radio pulsar. Observations of PSR B0540-69 in the Large Magellanic Cloud made with the Parkes radio telescope at 1.38 GHz show single pulses with energy more than 5000 times that of the average pulse energy. This is only the second young pulsar, after the Crab, to show giant pulse emission. Similar to the Crab pulsar, the giant pulses occur in two distinct phase ranges and have significant arrival time jitter within these ranges. The location of the giant pulses appears to lag the peak of the (sinusoidal) X-ray profile by 0.37 and 0.64 phase, although absolute timing between the radio and X-ray data is not yet secure. The dispersion measure of the giant pulses is 146.5 pc/cc, in agreement with the detection of the pulsar at 0.64 GHz by Manchester et al. (1993). The giant pulses are scatter broadened at 1.4 GHz with an exponential scattering time of 0.4 ms and have an emission bandwidth of at least 256 MHz. In 8 hr of integration we have failed to detect any integrated flux density from the pulsar to a level of 13 microJy, assuming a duty cycle of 10%. This implies the spectral index between 0.64 and 1.38 GHz is steeper than -4.4.

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arXiv2003

Radio Observations of Two Isolated Neutron Stars, RXJ0720.4-3125 and RX J0806.4-4132

Radio observations of two isolated neutron stars, RXJ0720.4-3125 and RX J0806.4-4132, have been made with the Australia Telescope Compact Array at a frequency of 1.4 GHz. No continuum emission is detected from either object with a 3 sigma upper limit of 0.2 mJy. The data were also folded synchronously with the known rotation periods of 8.4 and 11.4 s respectively. No pulsed emission was detected. If the pulse duty cycle is small, the upper limit on pulsed emission can be reduced still further to 0.04 mJy. The best evidence seems to indicate that the isolated neutron stars detected by ROSAT all-sky survey are relatively young objects, born with a very high magnetic field.

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arXiv2003

Multi-epoch HI line measurements of 4 southern pulsars

We have measured 21-cm absorption spectra in the direction of three southern pulsars, PSRs B0736-40, B1451-68 and B1641-45, in three separate epochs spread over 2.5 yr. We see no evidence for any changes in the absorption spectra over this time span in spite of good velocity resolution and sensitivity. Towards PSR B1641-45 we place an upper limit of 10^19 per cm^2 on the change in the column density of the cold, neutral gas. In addition, we observed PSR B1557-50 and compared its HI absorption spectrum with spectra taken in 1980 and 1994. A prominent deep absorption feature seen at -110 km/s in 1994 is weaker in 2000 and likely was not present at all in 1980. Using the standard interpretation which links the distance traversed by the pulsar to a physical cloud size, this results in a cloud size of ~1000 AU and density of 2x10^4 per cc, parameters typical of those seen in other observations. These results are also consistent with an alternative model by Deshpande (2000) who expects the largest variations in optical depth to be seen against the longest time intervals.

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arXiv2002

Single Dish Polarization Calibration

Using the formalism of Hamaker et al. (1996), I derive a method for the polarization calibration of observations made with a single radio telescope. This method is particularly appropriate for observations of pulsars, where the sign and magnitude of the circular polarization are useful for understanding the emission processes at work. I apply the method to observations of PSR J1359-6038 made using the multibeam receiver on the Parkes radio telescope.

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arXiv2002

A search for giant pulses in Vela-like pulsars

We have carried out a survey for `giant pulses' in 6 young, Vela-like pulsars. In no cases did we find single pulses with flux densities more than 10 times the mean flux density. However, in PSR B1706-44 we have detected giant micro-pulses very similar to those seen in the Vela pulsar. In PSR B1706-44 these giant micro-pulses appear on the trailing edge of the profile and have an intrinsic width of ~1 ms. The cumulative probability distribution of their intensities is best described by a power-law. If the power-law continues to higher intensities, then 3.7x10^6 rotations are required to obtain a pulse with 20 times the mean pulse flux. This number is similar to the giant pulse rate in PSR B1937+21 and PSR B1821-24 but significantly higher than that for the Crab.

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arXiv2001

Multiwavelength Studies of PSR J1420-6048, a Young Pulsar in the Kookaburra

We present X-ray, radio, and infrared observations of the 68 ms pulsar PSR J1420-6048 and its surrounding nebula, a possible counterpart of the gamma-ray source GeV J1417-6100/3EG J1420-6038. Pulsed X-ray emission at the radio period is marginally detected by ASCA from a source embedded in the hard spectrum X-ray nebula AX J1420.1-6049. At radio wavelengths, the pulsar is found to be strongly linearly and circularly polarized, and the polarization sweep is measured. A comparison of high resolution ATCA radio imaging of the Kookaburra's upper wing (G313.6+0.3), which contains the pulsar and the X-ray nebula, with infrared images suggests the radio emission is partly non-thermal.

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arXiv2001

Giant Pulses from the Millisecond Pulsar B1821-24

We have carried out a survey for `giant pulses' in 5 millisecond pulsars. We detect individual pulses from the high Edot pulsar PSR B1821-24 with energies exceeding 50x the mean pulse energy. These giant pulses are concentrated in a narrow phase window coincident with the power-law non-thermal pulse seen in hard X-rays. This is the third example of the giant pulse phenomenon. It supports the idea that large B fields in the outer magnetosphere are critical to the formation of such pulses and further suggests a direct connection between giant pulses and high energy emission.

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arXiv2001

X-Ray and Radio Observations of Bright GeV Sources

We present X-ray and radio studies of sources which are brightabove 1 GeV (F_{>1GeV} > 4e-8 ph/cm^2/s. Only 11 out of ~30 of these gamma-ray sources have been identified with lower energy counterparts: 5 blazars and 6 pulsars. Three of these pulsars are surrounded by radio pulsar wind nebulae (PWN), two of which are also seen as bright, extended X-ray synchrotron nebulae. The ASCA X-ray telescope has observed 28 of the bright GeV sources, revealing an excess of F_{2-10keV} > 10e-12 ergs/cm^2/s sources within the {\it EGRET} error contours of the unidentified sources. Although several supernova remnants are positionally coincident with these sources, we find no X-ray evidence of high energy particle production in SNR shell shocks consistent with the GeV positions. We also present initial results from follow on radio imaging studies of several fields containing unidentified sources. We have discovered new X-ray/radio nebulae in three of these fields which are strong candidates for PWN. These sources, along with a similar nebula in CTA 1 and the PWN around PSR B1853+01 in W44, are all positionally coincident with variable EGRET sources. This suggests a class of variable gamma-ray sources associated with synchrotron emitting regions powered by the winds of young pulsars.

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arXiv2001

High time-resolution observations of the Vela pulsar

We present high time resolution observations of single pulses from the Vela pulsar (PSR B0833-45) made with a baseband recording system at observing frequencies of 660 and 1413 MHz. We have discovered two startling features in the 1413 MHz single pulse data. The first is the presence of giant micro-pulses which are confined to the leading edge of the pulse profile. One of these pulses has a peak flux density in excess of 2500 Jy, more than 40 times the integrated pulse peak. The second new result is the presence of a large amplitude gaussian component on the trailing edge of the pulse profile. This component can exceed the main pulse in intensity but is switched on only relatively rarely. Fluctutation spectra reveal a possible periodicity in this feature of 140 pulse periods. Unlike the rest of the profile, this component has low net polarization and emits predominantly in the orthogonal mode. This feature appears to be unique to the Vela pulsar. We have also detected microstructure in the Vela pulsar for the first time. These same features are present in the 660 MHz data. We suggest that the full width of the Vela pulse profile might be as large as 10 ms but that the conal edges emit only rarely.

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arXiv2000

HI line measurements of pulsars towards the Galactic Centre and the electron density in the inner Galaxy

We have measured 21-cm absorption and emission spectra in the direction of a further 7 southern pulsars with the Parkes telescope to derive their kinematic distances and to study the interstellar medium. For the first time we have successfully obtained HI absorption measurements for PSRs J1602-5100, J1740-3015 and J1745-3040. We have also significantly improved the sensitivity and resolution on PSRs J1600-5044, J1752-2806, and J1825-0935, whose spectra have previously been measured, and have corrected an error in the published distance to PSR J1824-1945. Since the Frail & Weisberg summary of pulsar distances in 1990, a further 23 pulsars now have measured HI distances, mainly through the efforts of the current group. We discuss the Taylor & Cordes electron density model in light of these new measurements and find that, although the model towards the Galactic Centre appears good, the line of sight through the Carina spiral arm is poorly fit by the model.

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arXiv2000

The 1997 periastron passage of the binary pulsar PSR B1259-63

We report here on multifrequency radio observations of the pulsed emission from PSR B1259-63 around the time of the closest approach to its B2e companion star. There was a general increase in the pulsar's dispersion measure and scatter broadening, and a decrease in the flux density towards periastron although changes in these parameters were seen on timescales as short as minutes. The pulsed emission disappeared 16 days prior to periastron and remained undetectable until 16 days after periastron. The observations are used to determine the parameters of the wind from the Be star. We show that a simple model, in which the wind density varies with radius as r^-2, provides a good fit to the data. The wind is clumpy with size scales <= 10^10 cm, densities of 10^6 cm^-3 and a velocity of 2000 km/s at a distance of 20 - 50 stellar radii. We find a correlation between dispersion measure variations and the pulse scattering times, suggesting that the same electrons are responsible for both effects.

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arXiv1999

Pulsar Braking Indices Revisited

Using the standard equation for the slowdown of a neutron star, we derive a formula for the braking index via integration rather than the conventional differentiation. The new formula negates the need to measure the second time derivative of the rotation frequency. We show that the method gives similar braking indices for PSR B1509-58 and the Crab pulsar to those already in the literature. We point out that our method is useful for obtaining the braking indices of moderate aged pulsars without the need for long, phase-connected timing solutions. We applied the method to 20 pulsars and discuss the implications of the results. We find that virtually all the derived braking indices are dominated by the effects of (unseen) glitches, the recovery from which corrupts the value of the frequency first derivative. However, any real, large, positive braking index has implications for magnetic field decay and offers support to recent models of pulsar evolution.

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arXiv1999

Origin of the transient unpulsed radio emission from the PSR B1259-63 binary system

We discuss the interpretation of transient, unpulsed radio emission detected from the unique pulsar/Be-star binary system PSR B1259-63. Extensive monitoring of the 1994 and 1997 periastron passages has shown that the source flares over a 100-day interval around periastron, varying on time-scales as short as a day and peaking at 60 mJy (~100 times the apastron flux density) at 1.4 GHz. Interpreting the emission as synchrotron radiation, we show that (i) the observed variations in flux density are too large to be caused by the shock interaction between the pulsar wind and an isotropic, radiatively driven, Be-star wind, and (ii) the radio emitting electrons do not originate from the pulsar wind. We argue instead that the radio electrons originate from the circumstellar disk of the Be star and are accelerated at two epochs, one before and one after periastron, when the pulsar passes through the disk. A simple model incorporating two epochs of impulsive acceleration followed by synchrotron cooling reproduces the essential features of the radio light curve and spectrum and is consistent with the system geometry inferred from pulsed radio data.

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arXiv1998

The `Rabbit': A Potential Radio Counterpart of GeV J1417-6100

We have mapped the radio emission in the error ellipse of GeV J1417-6100 (2EGS J1418-6049) at 13cm and 20cm using the Australia Telescope Compact Array. We find a large shell with extended wings, at the edge of which is a non-thermal, polarized structure with a center filled morphology (the `Rabbit'), coincident with an extended, hard X-ray source. We discuss the various sources seen within the ellipse as potential counterparts of the gamma-ray source. We conclude that the most likely scenario is that the Rabbit is a wind nebula surrounding a radio-quiet gamma-ray pulsar.

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arXiv1997

Diffractive Scintillation of the Pulsar PSR B1259-63

PSR B1259-63 is in a highly eccentric 3.4 yr orbit around the Be-star SS2883. The system is located in the direction of the Sagittarius-Carina spiral arm at a distance of $\sim$1.5 kpc. We have made scintillation observations of the pulsar far from periastron at 4.8 GHz and 8.4 GHz, determining the diffractive bandwidth, $Δν_{\rm d}$, and timescale, $Δt_{\rm d}$, at both frequencies. We find no dependency on orbital phase until within 30 days of periastron. The data indicate that the scintillation is caused, not by the circumstellar environment, but by an HII region within the Sagittarius-Carina spiral arm located at least three-quarters of the way to the pulsar. Close to periastron, when the line-of-sight to the pulsar intersects the disk of the Be star, the electron densities within the disk are sufficient to overcome the `lever-arm effect' and produce a reduction in the scintillation bandwidth by six orders of magnitude.

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