| 1. |
- Broderick, J. W., et al.
(författare)
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LOFAR 144-MHz follow-up observations of GW170817
- 2020
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Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 494:4, s. 5110-5117
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Tidskriftsartikel (refereegranskat)abstract
- We present low-radio-frequency follow-up observations of AT 2017gfo, the electromagnetic counterpart of GW170817, which was the first binary neutron star merger to be detected by Advanced LIGO-Virgo. These data, with a central frequency of 144 MHz, were obtained with LOFAR, the Low-Frequency Array. The maximum elevation of the target is just 13 degrees.7 when observed with LOFAR, making our observations particularly challenging to calibrate and significantly limiting the achievable sensitivity. On time-scales of 130-138 and 371-374 d after the merger event, we obtain 3s upper limits for the afterglow component of 6.6 and 19.5mJy beam(-1), respectively. Using our best upper limit and previously published, contemporaneous higher frequency radio data, we place a limit on any potential steepening of the radio spectrum between 610 and 144 MHz: the two-point spectral index alpha(610)(144) greater than or similar to -2.5. We also show that LOFAR can detect the afterglows of future binary neutron star merger events occurring at more favourable elevations.
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| 2. |
- Fallows, Richard A., et al.
(författare)
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A LOFAR observation of ionospheric scintillation from two simultaneous travelling ionospheric disturbances
- 2020
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Ingår i: Journal of Space Weather and Space Climate. - : EDP Sciences. - 2115-7251. ; 10
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents the results from one of the first observations of ionospheric scintillation taken using the Low-Frequency Array (LOFAR). The observation was of the strong natural radio source Cassiopeia A, taken overnight on 18-19 August 2013, and exhibited moderately strong scattering effects in dynamic spectra of intensity received across an observing bandwidth of 10-80 MHz. Delay-Doppler spectra (the 2-D FFT of the dynamic spectrum) from the first hour of observation showed two discrete parabolic arcs, one with a steep curvature and the other shallow, which can be used to provide estimates of the distance to, and velocity of, the scattering plasma. A cross-correlation analysis of data received by the dense array of stations in the LOFAR "core" reveals two different velocities in the scintillation pattern: a primary velocity of similar to 20-40 ms(-1) with a north-west to south-east direction, associated with the steep parabolic arc and a scattering altitude in the F-region or higher, and a secondary velocity of similar to 110 ms(-1) with a north-east to south-west direction, associated with the shallow arc and a scattering altitude in the D-region. Geomagnetic activity was low in the mid-latitudes at the time, but a weak sub-storm at high latitudes reached its peak at the start of the observation. An analysis of Global Navigation Satellite Systems (GNSS) and ionosonde data from the time reveals a larger-scale travelling ionospheric disturbance (TID), possibly the result of the high-latitude activity, travelling in the north-west to south-east direction, and, simultaneously, a smaller-scale TID travelling in a north-east to south-west direction, which could be associated with atmospheric gravity wave activity. The LOFAR observation shows scattering from both TIDs, at different altitudes and propagating in different directions. To the best of our knowledge this is the first time that such a phenomenon has been reported.
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| 3. |
- Pétri, Jérôme A., et al.
(författare)
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Constraining the magnetic field geometry of the millisecond pulsar PSRJ0030+0451 from joint radio, thermal X-ray, and γ-ray emission
- 2023
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Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 680
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Tidskriftsartikel (refereegranskat)abstract
- Context. With the advent of multi-wavelength electromagnetic observations of neutron stars - spanning many decades in photon energies - from radio wavelengths up to X-rays and γ-rays, it has become possible to significantly constrain the geometry and the location of the associated emission regions. Aims. In this work, we use results from the modelling of thermal X-ray observations of PSR J0030+0451 from the Neutron Star Interior Composition Explorer (NICER) mission and phase-aligned radio and γ-ray pulse profiles to constrain the geometry of an off-centred dipole that is able to reproduce the light curves in these respective bands simultaneously. Methods. To this aim, we deduced a configuration with a simple dipole off-centred from the location of the centre of the thermal X-ray hot spots. We show that the geometry is compatible with independent constraints from radio and -ray pulsations only, leading to a fixed magnetic obliquity of α ≈ 75° and a line-of-sight inclination angle of ζ ≈ 54°. Results. We demonstrate that an off-centred dipole cannot be rejected by accounting for the thermal X-ray pulse profiles. Moreover, the crescent shape of one spot is interpreted as the consequence of a small-scale surface dipole on top of the large-scale off-centred dipole.
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