10 research outputs found
A dispersion excess from pulsar wind nebulae and supernova remnants: Implications for pulsars and FRBs
Young pulsars and the pulsar wind nebulae (PWNe) or supernova remnants (SNRs)
that surround them are some of the most dynamic and high-powered environments
in our Universe. With the rise of more sensitive observations, the number of
pulsar-SNR and PWN associations (hereafter, SNR/PWN) has increased, yet we do
not understand to which extent this environment influences the pulsars'
impulsive radio signals. We studied the dispersive contribution of SNRs and
PWNe on Galactic pulsars, and considered their relevance to fast radio bursts
(FRBs) such as FRB 121102. We investigated the dispersion measure (DM)
contribution of SNRs and PWNe by comparing the measured DMs of Galactic pulsars
in a SNR/PWN to the DM expected only from the intervening interstellar
electrons, using the NE2001 model. We find that a two- DM contribution
of SNRs and PWNe to the pulsar signal exists, amounting to pc
cm. The control sample of pulsars unassociated with a SNR/PWN shows no
excess. We model the SNR and PWN electron densities for each young pulsar in
our sample and show that these indeed predict an excess of this magnitude. By
extrapolating to the kind of fast-spinning, high magnetic field, young pulsars
that may power FRBs, we show their SNR and PWN are capable of significantly
contributing to the observed DM.Comment: 7 pages, 4 figures, 2 tables. Accepted for publication in A&
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
HESS J1943+213: a non-classical high-frequency-peaked BL Lac object
HESS J1943+213 is an unidentified TeV source that is likely a
high-frequency-peaked BL Lac (HBL) object but also compatible with a pulsar
wind nebula (PWN) nature. Each of these enormously different astronomical
interpretations is supported by some of the observed unusual characteristics.
In order to finally classify and understand this object we took a three-pronged
approach, through time-domain, high angular resolution, and multi-frequency
radio studies. First, our deep time-domain observations with the Arecibo
telescope failed to uncover the putative pulsar powering the proposed PWN. We
conclude with ~70% certainty that HESS J1943+213 does not host a pulsar.
Second, long-baseline interferometry of the source with e-MERLIN at 1.5- and 5-
GHz, shows only a core, a point source at ~ 1 - 100 milli-arcsecond resolution.
Its 2013 flux density is about one-third lower than detected in 2011
observations with similar resolution. This radio variability of the core
strengthens the HBL object hypothesis. More evidence against the PWN scenario
comes, third, from the radio spectrum we compiled. The extended structure
follows a power-law behavior with spectral index alpha = -0.54 +- 0.04 while
the core component is flat spectrum (alpha = -0.03 +- 0.03). In contrast, the
radio synchrotron emission of PWNe predicts a single power-law distribution.
Overall we rule out the PWN hypothesis and conclude the source is a BL Lac
object. The consistently high fraction (70%) of the flux density from the
extended structure then leads us to conclude that HESS J1943+213 must be a
non-classical HBL object.Comment: 8 pages, 4 figures, ApJ submitte
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
Discovery of GeV Gamma-Ray Emission from Pulsar Wind Nebula Kes 75 and PSR J1846–0258
We report the detection of gamma-ray emission from pulsar wind nebula (PWN) Kes 75 and PSR J1846−0258. Through modeling the spectral energy distribution incorporating the new Fermi-LAT data, we find that the observed gamma-ray emission is likely a combination of both the PWN and pulsar magnetosphere. The spectral shape of this magnetospheric emission is similar to the γ -ray spectrum of rotation-powered pulsars detected by Fermi-LAT, and the results from our best-fit model suggest that the pulsar’s magnetospheric emission accounts for 1% of the current spin-down luminosity. Prior works attempted to characterize the properties of this system and found a low supernova (SN) explosion energy and low SN ejecta mass. We reanalyze the broadband emission incorporating the new Fermi emission and compare the implications of our results to prior reports. The best-fit gamma-ray emission model suggests a second very hot photon field possibly generated by the stellar wind of a Wolf–Rayet star embedded within the nebula, which supports the low ejecta mass found for the progenitor in prior reports and here in the scenario of binary mass transfer
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