A LOFAR search for steep-spectrum pulsars in Supernova Remnants and Pulsar Wind Nebulae

Pinpointing a pulsar in its parent supernova remnant (SNR) or resulting pulsar wind nebula (PWN) is key for understanding its formation history, and the pulsar wind mechanism. Yet, only about half the SNRs and PWNe appear associated with a pulsar. We aim to find the pulsars in a sample of eight known and new SNRs and PWNe. Using the LOFAR radio telescope at 150 MHz, each source was observed for 3 hours. We covered the entire remnants where needed, by employing many tied-array beams to tile out even the largest objects. For objects with a confirmed point source or PWN we constrained our search to those lines of sight. We identify a promising radio pulsar candidate towards PWN G141.2+5.0. The candidate, PSR J0337+61, has a period of 94 ms and a DM of 226 pc cm$^{-3}$. We re-observed the source twice with increased sensitivities of 30% and 50% but did not re-detect it. It thus remains unconfirmed. For our other sources we obtain very stringent upper limits of 0.8-3.1 mJy at 150 MHz. Generally we can rule out that the pulsars travelled out of the remnant. From these strict limits we conclude our non-detections towards point-sources and PWNe are the result of beaming and propagation effects. Some of the remaining SNRs should host a black hole rather than a neutron star.Comment: 11 pages, 3 figures, Accepted for publication in Astronomy & Astrophysic

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-$\sigma$ DM contribution of SNRs and PWNe to the pulsar signal exists, amounting to $21.1 \pm 10.6$ pc cm$^{-3}$. 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 $\gamma$-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/$\gamma$-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 $\lesssim$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

MeV Emission from Pulsar Wind Nebulae: Understanding Extreme Particle Acceleration in Highly Relativistic Outflows

The Earth is constantly bombarded from outer space by energetic particles. Where and how these "cosmic rays" are produced is poorly understood, with various particle types and energies likely originating from different sources. Particularly mysterious is the source of high-energy e+/- produced in our Galaxy, especially those responsible for both the high fraction of e+ in the GeV cosmic ray lepton spectrum and the e+/- and observed excess of microwaves and gamma-rays detected towards the Galactic center and bulge. While these particles could be evidence for exotic forms of dark matter, they might also be produced by "normal" astrophysical sources such as pulsars the strongly magnetized, rapidly rotating neutron stars whose rotational energy powers an ultra-relativistic outflow (commonly referred to as a "pulsar wind") whose interaction with the surrounding medium creates a pulsar wind nebula .While the detection of TeV emission from numerous PWNe strongly suggest they contain e+/- with PeV or higher energies, how and to what energies these particles are produced is unknown, let alone their dependence on the properties of the pulsar, pulsar wind, and surrounding medium. A major reason for this uncertainty is the lack of information concerning their MeV properties, since the synchrotron emission from the highest energy e+/- peaks in this waveband. Only by combining the MeV spectrum of PWNe measured by proposed missions with that obtained at lower (primarily radio and X-ray) and higher (TeV) photon energies by current and hopefully future facilities is it possible to measure the full spectrum of e+/- in these sources. The resultant insights into the underlying acceleration mechanism would significantly impact many areas of astrophysics from indirect searches for dark matter to the origin of cosmic rays to the physics of relativistic outflows observed from active galactic nuclei, gamma-ray bursts, and some gravitational wave events

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

Broadband X-Ray Spectroscopy of the Pulsar Wind Nebula in HESS J1640-465

We present updated measurements of the X-ray properties of the pulsar wind nebula associated with the TeV γ -ray source HESS J1640-465 derived from Chandra and Nuclear Spectroscopic Telescope Array data. We report a high N _H value along the line of sight, consistent with previous work, which led us to incorporate the effects of dust scattering in our spectral analysis. Due to uncertainties in the dust scattering, we report a range of values for the PWN properties (photon index and unabsorbed flux). In addition, we fit the broadband spectrum of this source and found evidence for spectral softening and decreasing unasborbed flux as we go to higher photon energies. We then used a one-zone time-dependent evolutionary model to reproduce the dynamical and multiwavelength spectral properties of our source. Our model suggests a short spin-down timescale, a relatively higher than average magnetized pulsar wind, a strong pulsar wind nebula magnetic field and maximum electron energy up to PeV, suggesting HESS J1640-465 could be a PeVatron candidate

Fermi-LAT Gamma-ray Emission Discovered from the Composite Supernova Remnant B0453-685 in the Large Magellanic Cloud

International audienceWe report the second extragalactic pulsar wind nebula (PWN) to be detected in the MeV-GeV band by the Fermi-LAT, located within the Large Magellanic Cloud (LMC). The only other known PWN to emit in the Fermi band outside of the Milky Way Galaxy is N 157B which lies to the west of the newly detected gamma-ray emission at an angular distance of 4 degrees. Faint, point-like gamma-ray emission is discovered at the location of the composite supernova remnant (SNR) B0453-685 with a ~ 4 sigma significance from energies 300 MeV - 2 TeV. We present the Fermi-LAT data analysis of the new gamma-ray source, coupled with a detailed multi-wavelength investigation to understand the nature of the observed emission. Combining the observed characteristics of the SNR and the physical implications from broadband modeling, we argue it is unlikely the SNR is responsible for the gamma-ray emission. While the gamma-ray emission is too faint for a pulsation search, we try to distinguish between any pulsar and PWN component of SNR B0453-685 that would be responsible for the observed gamma-ray emission using semi-analytic models. We determine the most likely scenario is that the old PWN (t ~ 14,000 years) within B0453-685 has been impacted by the return of the SNR reverse shock with a possible substantial pulsar component below 5 GeV