7 research outputs found
New upper limits on low-frequency radio emission from isolated neutron stars with LOFAR
Neutron stars that show X-ray and -ray pulsed emission must,
somewhere in the magnetosphere, generate electron-positron pairs. Such pairs
are also required for radio emission, but then why do a number of these sources
appear radio quiet? Here, we carried out a deep radio search towards four such
neutron stars that are isolated X-ray/-ray pulsars but for which no
radio pulsations have been detected yet. These sources are 1RXS
J141256.0+792204 (Calvera), PSR J1958+2846, PSR J1932+1916 and SGR J1907+0919.
Searching at lower radio frequencies, where the radio beam is thought to be
wider, increases the chances of detecting these sources, compared to the
earlier higher-frequency searches. We thus carried a search for periodic and
single-pulse radio emission with the LOFAR radio telescope at 150 MHz. We used
the known periods, and searched a wide range of dispersion measures, as the
distances are not well constrained. We did not detect pulsed emission from any
of the four sources. However, we put very constraining upper limits on the
radio flux density at 150 MHz, of 1.4 mJy.Comment: 7 pages, 2 figures, 1 table. Accepted for publication in Astronomy &
Astrophysic
Chromatic periodic activity down to 120 MHz in a Fast Radio Burst
Fast radio bursts (FRBs) are extragalactic astrophysical transients whose
brightness requires emitters that are highly energetic, yet compact enough to
produce the short, millisecond-duration bursts. FRBs have thus far been
detected between 300 MHz and 8 GHz, but lower-frequency emission has remained
elusive. A subset of FRBs is known to repeat, and one of those sources, FRB
20180916B, does so with a 16.3 day activity period. Using simultaneous Apertif
and LOFAR data, we show that FRB 20180916B emits down to 120 MHz, and that its
activity window is both narrower and earlier at higher frequencies. Binary wind
interaction models predict a narrower periodic activity window at lower
frequencies, which is the opposite of our observations. Our detections
establish that low-frequency FRB emission can escape the local medium. For
bursts of the same fluence, FRB 20180916B is more active below 200 MHz than at
1.4 GHz. Combining our results with previous upper-limits on the all-sky FRB
rate at 150 MHz, we find that there are 3-450 FRBs/sky/day above 50 Jy ms at
90% confidence. We are able to rule out the scenario in which companion winds
cause FRB periodicity. We also demonstrate that some FRBs live in clean
environments that do not absorb or scatter low-frequency radiation.Comment: 50 pages, 14 figures, 3 tables, submitte
Initial characterization of interstellar comet 2I/Borisov
Interstellar comets penetrating through the Solar System had been anticipated
for decades. The discovery of asteroidal-looking 'Oumuamua was thus a huge
surprise and a puzzle. Furthermore, the physical properties of the 'first
scout' turned out to be impossible to reconcile with Solar System objects,
challenging our view of interstellar minor bodies. Here, we report the
identification and early characterization of a new interstellar object, which
has an evidently cometary appearance. The body was discovered by Gennady
Borisov on 30 August 2019 UT and subsequently identified as hyperbolic by our
data mining code in publicly available astrometric data. The initial orbital
solution implies a very high hyperbolic excess speed of ~32 km/s, consistent
with 'Oumuamua and theoretical predictions. Images taken on 10 and 13 September
2019 UT with the William Herschel Telescope and Gemini North Telescope show an
extended coma and a faint, broad tail. We measure a slightly reddish colour
with a g'-r' colour index of 0.66 +/- 0.01 mag, compatible with Solar System
comets. The observed morphology is also unremarkable and best explained by dust
with a power-law size-distribution index of -3.7 +/- 1.8 and a low ejection
speed (44 +/- 14 m/s for = 1 particles, where is the ratio of
the solar gravitational attraction to the solar radiation pressure). The
nucleus is probably ~1 km in radius, again a common value among Solar System
comets, and has a negligible chance of experiencing rotational disruption.
Based on these early characteristics, and putting its hyperbolic orbit aside,
2I/Borisov appears indistinguishable from the native Solar System comets.Comment: Published in Nature Astronomy on 14 October 201
A fast radio burst with submillisecond quasi-periodic structure
Fast radio bursts (FRBs) are extragalactic radio transients of extraordinary luminosity. Studying the diverse temporal and spectral behaviour recently observed in a number of FRBs may help to determine the nature of the entire class. For example, a fast spinning or highly magnetised neutron star (NS) might generate the rotation-powered acceleration required to explain the bright emission. Periodic, subsecond components suggesting such rotation were recently reported in one FRB, and may also exist in two more. Here we report the discovery of FRB 20201020A with Apertif, an FRB that shows five components regularly spaced by 0.411 ms. This submillisecond structure in FRB 20201020A carries important clues about the progenitor of this FRB specifically, and potentially about the progenitors of FRBs in general. We therefore contrast its features to what is seen in other FRBs and pulsars, and to the predictions of some FRB models. We present a timing analysis of the FRB 20201020A components carried out in order to determine the significance of the periodicity. We compare these against the timing properties of the previously reported CHIME FRBs with subsecond quasi-periodic components, and against two Apertif bursts from repeating FRB 20180916B, which show complex time-frequency structure. We find the periodicity of FRB 20201020A to be marginally significant at 2.4σ. Its repeating subcomponents cannot be explained as pulsar rotation because the required spin rate of over 2 kHz exceeds the limits set by typical NS equations of state and observations. The fast periodicity is also in conflict with a compact object merger scenario. However, these quasi-periodic components could be caused by equidistant emitting regions in the magnetosphere of a magnetar. The submillisecond spacing of the components in FRB 20201020A, the smallest observed so far in a one-off FRB, may rule out both a NS spin period and binary mergers as the direct source of quasi-periodic FRB structure
The Apertif Radio Transient System (ARTS): Design, Commissioning, Data Release, and Detection of the first 5 Fast Radio Bursts
Fast Radio Bursts must be powered by uniquely energetic emission mechanisms.
This requirement has eliminated a number of possible source types, but several
remain. Identifying the physical nature of Fast Radio Burst (FRB) emitters
arguably requires good localisation of more detections, and broadband studies
enabled by real-time alerting. We here present the Apertif Radio Transient
System (ARTS), a supercomputing radio-telescope instrument that performs
real-time FRB detection and localisation on the Westerbork Synthesis Radio
Telescope (WSRT) interferometer. It reaches coherent-addition sensitivity over
the entire field of the view of the primary dish beam. After commissioning
results verified the system performed as planned, we initiated the Apertif FRB
survey (ALERT). Over the first 5 weeks we observed at design sensitivity in
2019, we detected 5 new FRBs, and interferometrically localised each of these
to 0.4--10 sq. arcmin. All detections are broad band and very narrow, of order
1 ms duration, and unscattered. Dispersion measures are generally high. Only
through the very high time and frequency resolution of ARTS are these
hard-to-find FRBs detected, producing an unbiased view of the intrinsic
population properties. Most localisation regions are small enough to rule out
the presence of associated persistent radio sources. Three FRBs cut through the
halos of M31 and M33. We demonstrate that Apertif can localise one-off FRBs
with an accuracy that maps magneto-ionic material along well-defined lines of
sight. The rate of 1 every ~7 days next ensures a considerable number of new
sources are detected for such study. The combination of detection rate and
localisation accuracy exemplified by the 5 first ARTS FRBs thus marks a new
phase in which a growing number of bursts can be used to probe our Universe
The Apertif Radio Transient System (ARTS): Design, commissioning, data release, and detection of the first five fast radio bursts
Fast radio bursts (FRBs) must be powered by uniquely energetic emission mechanisms. This requirement has eliminated a number of possible source types, but several remain. Identifying the physical nature of FRB emitters arguably requires good localisation of more detections, as well as broad-band studies enabled by real-time alerting. In this paper, we present the Apertif Radio Transient System (ARTS), a supercomputing radio-telescope instrument that performs real-time FRB detection and localisation on the Westerbork Synthesis Radio Telescope (WSRT) interferometer. It reaches coherent-addition sensitivity over the entire field of the view of the primary-dish beam. After commissioning results verified that the system performed as planned, we initiated the Apertif FRB survey (ALERT). Over the first 5 weeks we observed at design sensitivity in 2019, we detected five new FRBs, and interferometrically localised each of them to 0.4–10 sq. arcmin. All detections are broad band, very narrow, of the order of 1 ms in duration, and unscattered. Dispersion measures are generally high. Only through the very high time and frequency resolution of ARTS are these hard-to-find FRBs detected, producing an unbiased view of the intrinsic population properties. Most localisation regions are small enough to rule out the presence of associated persistent radio sources. Three FRBs cut through the halos of M31 and M33. We demonstrate that Apertif can localise one-off FRBs with an accuracy that maps magneto-ionic material along well-defined lines of sight. The rate of one every ~7 days ensures a considerable number of new sources are detected for such a study. The combination of the detection rate and localisation accuracy exemplified by the first five ARTS FRBs thus marks a new phase in which a growing number of bursts can be used to probe our Universe