D. Carbone

Limits on Fast Radio Bursts and other transient sources at 182 MHz using the Murchison Widefield Array

We present a survey for transient and variable sources, on time-scales from 28 s to ∼1 yr, using the Murchison Widefield Array (MWA) at 182 MHz. Down to a detection threshold of 0.285 Jy, no transient candidates were identified, making this the most constraining low-frequency survey to date and placing a limit on the surface density of transients of <4.1 × 10−7 deg−2 for the shortest time-scale considered. At these frequencies, emission from Fast Radio Bursts (FRBs) is expected to be detectable in the shortest time-scale images without any corrections for interstellar or intergalactic dispersion. At an FRB limiting flux density of 7980 Jy, we find a rate of <82 FRBs per sky per day for dispersion measures <700 pc cm−3. Assuming a cosmological population of standard candles, our rate limits are consistent with the FRB rates obtained by Thornton et al. if they have a flat spectral slope. Finally, we conduct an initial variability survey of sources in the field with flux densities ≳0.5 Jy and identify no sources with significant variability in their light curves. However, we note that substantial further work is required to fully characterize both the short-term and low-level variability within this field.

Calibrating the absolute amplitude scale for air showers measured at LOFAR

Air showers induced by cosmic rays create nanosecond pulses detectable at radio frequencies. These pulses have been measured successfully in the past few years at the LOw-Frequency ARray (LOFAR) and are used to study the properties of cosmic rays. For a complete understanding of this phenomenon and the underlying physical processes, an absolute calibration of the detecting antenna system is needed. We present three approaches that were used to check and improve the antenna model of LOFAR and to provide an absolute calibration of the whole system for air shower measurements. Two methods are based on calibrated reference sources and one on a calibration approach using the diffuse radio emission of the Galaxy, optimized for short data-sets. An accuracy of 19% in amplitude is reached. The absolute calibration is also compared to predictions from air shower simulations. These results are used to set an absolute energy scale for air shower measurements and can be used as a basis for an absolute scale for the measurement of astronomical transients with LOFAR.

Pulsar polarisation below 200 MHz: Average profiles and propagation effects

We present the highest-quality polarisation profiles to date of 16 non-recycled pulsars and four millisecond pulsars, observed below 200 MHz with the LOFAR high-band antennas. Based on the observed profiles, we perform an initial investigation of expected observational effects resulting from the propagation of polarised emission in the pulsar magnetosphere and the interstellar medium. The predictions of magnetospheric birefringence in pulsars have been tested using spectra of the pulse width and fractional polarisation from multifrequency data. The derived spectra offer only partial support for the expected effects of birefringence on the polarisation properties, with only about half of our sample being consistent with the models predictions. It is noted that for some pulsars these measurements are contaminated by the effects of interstellar scattering. For a number of pulsars in our sample, we have observed significant variations in the amount of Faraday rotation as a function of pulse phase, which is possibly an artefact of scattering. These variations are typically two orders of magnitude smaller than that observed at 1400 MHz by Noutsos et al. (2009), for a different sample of southern pulsars. In this paper we present a possible explanation for the difference in magnitude of this effect between the two frequencies, based on scattering. Finally, we have estimated the magnetospheric emission heights of low-frequency radiation from four pulsars, based on the phase lags between the flux-density and the PA profiles, and the theoretical framework of Blaskiewicz, Cordes & Wasserman (1991). These estimates yielded heights of a few hundred km; at least for PSR B1133+16, this is consistent with emission heights derived based on radius-to-frequency mapping, but is up to a few times larger than the recent upper limit based on pulsar timing.

The peculiar radio galaxy 4C 35.06: a case for recurrent AGN activity?

Using observations obtained with the LOw Fequency ARray (LOFAR), the Westerbork Synthesis Radio Telescope (WSRT) and archival Very Large Array (VLA) data, we have traced the radio emission to large scales in the complex source 4C 35.06 located in the core of the galaxy cluster Abell 407. At higher spatial resolution (~ 4″), the source was known to have two inner radio lobes spanning 31 kpc and a diffuse, low-brightness extension running parallel to them, offset by about 11 kpc (in projection). At 62 MHz, we detect the radio emission of this structure extending out to 210 kpc. At 1.4 GHz and intermediate spatial resolution (~ 30″), the structure appears to have a helical morphology. We have derived the characteristics of the radio spectral index across the source. We show that the source morphology is most likely the result of at least two episodes of AGN activity separated by a dormant period of around 35 Myr. The outermost regions of radio emission have a steep spectral index (α< − 1), indicative of old plasma. We connect the spectral index properties of the resolved source structure with the integrated fluxdensity spectral index of 4C 35.06 and suggest an explanation for its unusual integrated flux density spectral shape (a moderately steep power law with no discernible spectral break), possibly providing a proxy for future studies of more distant radio sources through inferring their detailed spectral index properties and activity history from their integrated spectral indices. The AGN is hosted by one of the galaxies located in the cluster core of Abell 407. We propose that it is intermittently active as it moves in the dense environment in the cluster core. In this scenario, the AGN turned on sometime in the past, and has produced the helical pattern of emission, possibly a sign of jet precession/merger during that episode of activity. Using LOFAR, we can trace the relic plasma from that episode of activity out to greater distances from the core than ever before. Using the the WSRT, we detect H I in absorption against the center of the radio source. The absorption profile is relatively broad (FWHM of 288 kms-1), similar to what is found in other clusters. The derived column density is NHI ~ 4 × 1020 cm-2 for a Tspin = 100 K. This detection supports the connection – already suggested for other restarted radio sources – between the presence of cold gas and restarting activity. The cold gas appears to be dominated by a blue-shifted component although the broad H I profile could also include gas with different kinematics. Understanding the duty cycle of the radio emission as well as the triggering mechanism for starting (or restarting) the radio-loud activity can provide important constraints to quantify the impact of AGN feedback on galaxy evolution. The study of these mechanisms at low frequencies using morphological and spectral information promises to bring new important insights in this field.

The LOFAR long baseline snapshot calibrator survey

Aims. An efficient means of locating calibrator sources for International LOFAR is developed and used to determine the average density of usable calibrator sources on the sky for subarcsecond observations at 140 MHz. Methods. We used the multi-beaming capability of LOFAR to conduct a fast and computationally inexpensive survey with the full International LOFAR array. Sources were pre-selected on the basis of 325 MHz arcminute-scale flux density using existing catalogues. By observing 30 different sources in each of the 12 sets of pointings per hour, we were able to inspect 630 sources in two hours to determine if they possess a sufficiently bright compact component to be usable as LOFAR delay calibrators. Results. Over 40% of the observed sources are detected on multiple baselines between international stations and 86 are classified as satisfactory calibrators. We show that a flat low-frequency spectrum (from 74 to 325 MHz) is the best predictor of compactness at 140 MHz. We extrapolate from our sample to show that the density of calibrators on the sky that are sufficiently bright to calibrate dispersive and non-dispersive delays for the International LOFAR using existing methods is 1.0 per square degree. Conclusions. The observed density of satisfactory delay calibrator sources means that observations with International LOFAR should be possible at virtually any point in the sky, provided that a fast and efficient search using the methodology described here is conducted prior to the observation to identify the best calibrator.

LOFAR MSSS: Discovery of a 2.56 Mpc giant radio galaxy associated with a disturbed galaxy group

This document is the Accepted Manuscript of an article accepted for publication in Astronomy & Astrophysics following peer review. The version of record [A. O. Clarke, et al, LOFAR MSSS: Discovery of a 2.56 Mpc giant radio galaxy associated with a disturbed galaxy group, A&A, Vol 601, May 2017), is available online at DOI https://doi.org/10.1051/0004-6361/201630152. Published by EDP Sciences.

LOFAR MSSS: The scaling relation between AGN cavity power and radio luminosity at low radio frequencies

This article has been accepted for publication in a forthcoming issue of Astronomy & Astrophysics. Reproduced with permission from Astronomy & Astrophysics.

Orbital and superorbital variability of LS I +61 303 at low radio frequencies with GMRT and LOFAR

LS I +61 303 is a gamma-ray binary that exhibits an outburst at GHz frequencies each orbitalcycle of 26:5 d and a superorbital modulation with a period of 4.6 yr. We have performeda detailed study of the low-frequency radio emission of LS I +61 303 by analysing all thearchival GMRT data at 150, 235 and 610 MHz, and conducting regular LOFAR observationswithin the Radio Sky Monitor (RSM) at 150 MHz. We have detected the source for the firsttime at 150 MHz, which is also the first detection of a gamma-ray binary at such a low frequency.We have obtained the light-curves of the source at 150, 235 and 610 MHz, all of themshowing orbital modulation. The light-curves at 235 and 610 MHz also show the existenceof superorbital variability. A comparison with contemporaneous 15-GHz data shows remarkabledierences with these light-curves. At 15 GHz we see clear outbursts, whereas at lowfrequencies we see variability with wide maxima. The light-curve at 235 MHz seems to beanticorrelated with the one at 610 MHz, implying a shift of 0.5 orbital phases in the maxima.We model the shifts between the maxima at dierent frequencies as due to changes in thephysical parameters of the emitting region assuming either free-free absorption or synchrotronself-absorption, obtaining expansion velocities for this region close to the stellar wind velocitywith both mechanisms.

Calculating transient rates from surveys

We have developed a method to determine the transient surface density and transient rate for any given survey, using Monte Carlo simulations. This method allows us to determine the transient rate as a function of both the flux and the duration of the transients in the whole flux–duration plane rather than one or a few points as currently available methods do. It is applicable to every survey strategy that is monitoring the same part of the sky, regardless the instrument or wavelength of the survey, or the target sources. We have simulated both top-hat and Fast Rise Exponential Decay light curves, highlighting how the shape of the light curve might affect the detectability of transients. Another application for this method is to estimate the number of transients of a given kind that are expected to be detected by a survey, provided that their rate is known.

Measuring the expansion velocity of the outflows of LS I +61 303 through low-frequency radio observations

LS I +61 303 is a gamma-ray binary that exhibits an outburst at GHz frequencies each orbital cycle of 26.5 d and a superorbital modulation with a period of 4.6 yr. We have performed a detailed study of the low-frequency radio emission of LS I +61 303 by analyzing data from the Giant Metrewave Radio Telescope (GMRT) at 150, 235 and 610 MHz, and from the Low Frequency Array (LOFAR) at 150 MHz. We have detected the source for the first time at 150 MHz, which is also the first detection of a gamma-ray binary at such a low frequency. We have obtained the light-curves of the source at 150, 235 and 610 MHz, all of them showing orbital modulation. The light-curves at 235 and 610 MHz also show the existence of superorbital variability. A comparison with contemporaneous 15-GHz data shows remarkable differences with these light-curves. At 15 GHz we see clear outbursts, whereas at low frequencies we see variability with wide maxima. The light-curve at 235 MHz seems to be anticorrelated with the one at 610 MHz, implying a shift of about 0.5 orbital phases in the maxima. We model the shifts between the maxima at different frequencies as due to changes in the physical parameters of the emitting region assuming either free-free absorption or synchrotron self-absorption, obtaining expansion velocities for this region close to the stellar wind velocity with both mechanisms.