LOFAR 144-MHz follow-up observations of GW170817

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${_{.}^{\circ}}$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 3σ upper limits for the afterglow component of 6.6 and 19.5 mJy 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 $\alpha ^{610}_{144} \gtrsim$ −2.5. We also show that LOFAR can detect the afterglows of future binary neutron star merger events occurring at more favourable elevations.</jats:p

LOFAR detections of low-frequency radio recombination lines towards Cassiopeia A

Cassiopeia A was observed using the Low-Band Antennas of the LOw Frequency ARray (LOFAR) with high spectral resolution. This allowed a search for radio recombination lines (RRLs) along the line-of-sight to this source. Five carbon-alpha RRLs were detected in absorption between 40 and 50 MHz with a signal-to-noise ratio of > 5 from two independent LOFAR datasets. The derived line velocities (v_LSR ~ -50 km/s) and integrated optical depths (~ 13 s^-1) of the RRLs in our spectra, extracted over the whole supernova remnant, are consistent within each LOFAR dataset and with those previously reported. For the first time, we are able to extract spectra against the brightest hotspot of the remnant at frequencies below 330 MHz. These spectra show significantly higher (15-80 %) integrated optical depths, indicating that there is small-scale angular structure on the order of ~1 pc in the absorbing gas distribution over the face of the remnant. We also place an upper limit of 3 x 10^-4 on the peak optical depths of hydrogen and helium RRLs. These results demonstrate that LOFAR has the desired spectral stability and sensitivity to study faint recombination lines in the decameter band

The brightness and spatial distributions of terrestrial radio sources

Faint undetected sources of radio-frequency interference (RFI) might become visible in long radio observations when they are consistently present over time. Thereby, they might obstruct the detection of the weak astronomical signals of interest. This issue is especially important for Epoch of Reionisation (EoR) projects that try to detect the faint redshifted HI signals from the time of the earliest structures in the Universe. We explore the RFI situation at 30-163 MHz by studying brightness histograms of visibility data observed with LOFAR, similar to radio-source-count analyses that are used in cosmology. An empirical RFI distribution model is derived that allows the simulation of RFI in radio observations. The brightness histograms show an RFI distribution that follows a power-law distribution with an estimated exponent around -1.5. With several assumptions, this can be explained with a uniform distribution of terrestrial radio sources whose radiation follows existing propagation models. Extrapolation of the power law implies that the current LOFAR EoR observations should be severely RFI limited if the strength of RFI sources remains strong after time integration. This is in contrast with actual observations, which almost reach the thermal noise and are thought not to be limited by RFI. Therefore, we conclude that it is unlikely that there are undetected RFI sources that will become visible in long observations. Consequently, there is no indication that RFI will prevent an EoR detection with LOFAR

Upper limits on the 21 cm epoch of reionization power spectrum from one night with LOFAR

We present the first limits on the Epoch of Reionization 21 cm H i power spectra, in the redshift range z = 7.9–10.6, using the Low-Frequency Array (LOFAR) High-Band Antenna (HBA). In total, 13.0 hr of data were used from observations centered on the North Celestial Pole. After subtraction of the sky model and the noise bias, we detect a non-zero ${{\rm{\Delta }}}_{{\rm{I}}}^{2}={(56\pm 13\mathrm{mK})}^{2}$ (1-σ) excess variance and a best 2-σ upper limit of ${{\rm{\Delta }}}_{21}^{2}\lt {(79.6\mathrm{mK})}^{2}$ at k = 0.053 h cMpc−1 in the range z = 9.6–10.6. The excess variance decreases when optimizing the smoothness of the direction- and frequency-dependent gain calibration, and with increasing the completeness of the sky model. It is likely caused by (i) residual side-lobe noise on calibration baselines, (ii) leverage due to nonlinear effects, (iii) noise and ionosphere-induced gain errors, or a combination thereof. Further analyses of the excess variance will be discussed in forthcoming publications

LOFAR tied-array imaging and spectroscopy of solar S bursts

© 2015 ESO. The Sun is an active source of radio emission that is often associated with energetic phenomena ranging from nanoflares to coronal mass ejections (CMEs). At low radio frequencies (-1 and a wide range of circular polarisation degrees (2-8 times more polarised than the accompanying Type III bursts). Extrapolation of the photospheric magnetic field using the potential field source surface (PFSS) model suggests that S bursts are associated with a trans-equatorial loop system that connects an active region in the southern hemisphere to a bipolar region of plage in the northern hemisphere. Conclusions. We have identified polarised, short-lived solar radio bursts that have never been imaged before. They are observed at a height and frequency range where plasma emission is the dominant emission mechanism, however, they possess some of the characteristics of electron-cyclotron maser emission.6 pages, 4 figuresstatus: publishe

Measuring a Cherenkov ring in the radio emission from air showers at 110-190 MHz with LOFAR

Measuring radio emission from air showers offers a novel way to determine properties of the primary cosmic rays such as their mass and energy. Theory predicts that relativistic time compression effects lead to a ring of amplified emission which starts to dominate the emission pattern for frequencies above ∼100 MHz. In this article we present the first detailed measurements of this structure. Ring structures in the radio emission of air showers are measured with the LOFAR radio telescope in the frequency range of 110-190 MHz. These data are well described by CoREAS simulations. They clearly confirm the importance of including the index of refraction of air as a function of height. Furthermore, the presence of the Cherenkov ring offers the possibility for a geometrical measurement of the depth of shower maximum, which in turn depends on the mass of the primary particle

Needle-like structures discovered on positively charged lightning branches

\u3cp\u3e Lightning is a dangerous yet poorly understood natural phenomenon. Lightning forms a network of plasma channels propagating away from the initiation point with both positively and negatively charged ends—called positive and negative leaders \u3csup\u3e1\u3c/sup\u3e . Negative leaders propagate in discrete steps, emitting copious radio pulses in the 30–300-megahertz frequency band \u3csup\u3e2–8\u3c/sup\u3e that can be remotely sensed and imaged with high spatial and temporal resolution \u3csup\u3e9–11\u3c/sup\u3e . Positive leaders propagate more continuously and thus emit very little high-frequency radiation \u3csup\u3e12\u3c/sup\u3e . Radio emission from positive leaders has nevertheless been mapped \u3csup\u3e13–15\u3c/sup\u3e , and exhibits a pattern that is different from that of negative leaders \u3csup\u3e11–13,16,17\u3c/sup\u3e . Furthermore, it has been inferred that positive leaders can become transiently disconnected from negative leaders \u3csup\u3e9,12,16,18–20\u3c/sup\u3e , which may lead to current pulses that both reconnect positive leaders to negative leaders \u3csup\u3e11,16,17,20–22\u3c/sup\u3e and cause multiple cloud-to-ground lightning events \u3csup\u3e1\u3c/sup\u3e . The disconnection process is thought to be due to negative differential resistance \u3csup\u3e18\u3c/sup\u3e , but this does not explain why the disconnections form primarily on positive leaders \u3csup\u3e22\u3c/sup\u3e , or why the current in cloud-to-ground lightning never goes to zero \u3csup\u3e23\u3c/sup\u3e . Indeed, it is still not understood how positive leaders emit radio-frequency radiation or why they behave differently from negative leaders. Here we report three-dimensional radio interferometric observations of lightning over the Netherlands with unprecedented spatiotemporal resolution. We find small plasma structures—which we call ‘needles’—that are the dominant source of radio emission from the positive leaders. These structures appear to drain charge from the leader, and are probably the reason why positive leaders disconnect from negative ones, and why cloud-to-ground lightning connects to the ground multiple times. \u3c/p\u3

LBCS: The LOFAR Long-Baseline Calibrator Survey

We outline the LOFAR Long-Baseline Calibrator Survey (LBCS), whose aim is to identify sources suitable for calibrating the highest-resolution observations made with the International LOFAR Telescope, which include baselines &gt;1000 km. Suitable sources must contain significant correlated flux density (≳ 50 − 100 mJy) at frequencies around 110−190 MHz on scales of a few hundred milliarcseconds. At least for the 200−300-km international baselines, we find around 1 suitable calibrator source per square degree over a large part of the northern sky, in agreement with previous work. This should allow a randomly selected target to be successfully phase calibrated on the international baselines in over 50% of cases. Products of the survey include calibrator source lists and fringe-rate and delay maps of wide areas – typically a few degrees – around each source. The density of sources with significant correlated flux declines noticeably with baseline length over the range 200−600 km, with good calibrators on the longest baselines appearing only at the rate of 0.5 per sq. deg. Coherence times decrease from 1−3 min on 200-km baselines to about 1 min on 600-km baselines, suggesting that ionospheric phase variations contain components with scales of a few hundred kilometres. The longest median coherence time, at just over 3 min, is seen on the DE609 baseline, which at 227 km is close to being the shortest. We see median coherence times of between 80 and 110 s on the four longest baselines (580−600 km), and about 2 min for the other baselines. The success of phase transfer from calibrator to target is shown to be influenced by distance, in a manner that suggests a coherence patch at 150-MHz of the order of 1 deg. Although source structures cannot be measured in these observations, we deduce that phase transfer is affected if the calibrator source structure is not known. We give suggestions for calibration strategies and choice of calibrator sources, and describe the access to the online catalogue and data products