Abstract
Very long baseline interferometric (VLBI) localizations of repeating fast radio bursts (FRBs) have demonstrated a diversity of local environments: from nearby star-forming regions to ...globular clusters. Here we report the VLBI localization of FRB 20201124A using an ad hoc array of dishes that also participate in the European VLBI Network (EVN). In our campaign, we detected 18 bursts from FRB 20201124A at two separate epochs. By combining the visibilities from both epochs, we were able to localize FRB 20201124A with a 1
σ
uncertainty of 2.7 mas. We use the relatively large burst sample to investigate astrometric accuracy and find that for ≳20 baselines (≳7 dishes) we can robustly reach milliarcsecond precision even using single-burst data sets. Subarcsecond precision is still possible for single bursts, even when only ∼6 baselines (four dishes) are available. In such cases, the limited
uv
coverage for individual bursts results in very high side-lobe levels. Thus, in addition to the peak position from the dirty map, we also explore smoothing the structure in the dirty map by fitting Gaussian functions to the fringe pattern in order to constrain individual burst positions, which we find to be more reliable. Our VLBI work places FRB 20201124A 710 ± 30 mas (1
σ
uncertainty) from the optical center of the host galaxy, consistent with originating from within the recently discovered extended radio structure associated with star formation in the host galaxy. Future high-resolution optical observations, e.g., with Hubble Space Telescope, can determine the proximity of FRB 20201124A’s position to nearby knots of star formation.
ABSTRACT
We present 849 new bursts from FRB 20121102A detected with the 305-m Arecibo Telescope. Observations were conducted as part of our regular campaign to monitor activity and evolution of burst ...properties. The 10 reported observations were carried out between 1150 and $1730\, {\rm MHz}$ and fall in the active period around 2018 November. All bursts were dedispersed at the same dispersion measure and are consistent with a single value of $(562.4 \pm 0.1)\, {\rm pc\, cm^{-3}}$. The rate varies between 0 bursts and 218 ± 16 bursts per hour, the highest rate observed to date. The times between consecutive bursts show a bimodal distribution. We find that a Poisson process with varying rate best describes arrival times with separations ${\gt}{0.1\, {\rm s}}$. Clustering on time-scales of $22\, {\rm ms}$ reflects a characteristic time-scale of the source and possibly the emission mechanism. We analyse the spectro-temporal structure of the bursts by fitting 2D Gaussians with a temporal drift to each sub-burst in the dynamic spectra. We find a linear relationship between the sub-burst’s drift and its duration. At the same time, the drifts are consistent with coming from the sad-trombone effect. This has not been predicted by current models. The energy distribution shows an excess of high-energy bursts and is insufficiently modelled by a single power law even within single observations. We find long-term changes in the energy distribution, the average spectrum, and the sad-trombone drift, compared to earlier and later published observations. Despite the large burst rate, we find no strict short-term periodicity.
We report the detection of GeV gamma -ray emission from the supernova remnant (SNR) Puppis A with the Fermi Gamma-Ray Space Telescope. Puppis A is among the faintest SNRs yet detected at GeV ...energies, with a luminosity of only 2.7 x 10 super(34) (D/2.2 kpc) super(2) erg s super(-1) between 1 and 100 GeV. The gamma -ray emission from the remnant is spatially extended, with a morphology matching that of the radio and X-ray emission, and is well described by a simple power law with an index of 2.1. We attempt to model the broadband spectral energy distribution (SED), from radio to gamma -rays, using standard nonthermal emission mechanisms. To constrain the relativistic electron population we use 7 years of Wilkinson Microwave Anisotropy Probe data to extend the radio spectrum up to 93 GHz. Both leptonic- and hadronic-dominated models can reproduce the nonthermal SED, requiring a total content of cosmic-ray electrons and protons accelerated in Puppis A of at least W sub(CR) approximately (1-5) x 10 super(49) erg.
ABSTRACT We report the Fermi Large Area Telescope detection of extended γ-ray emission from the lobes of the radio galaxy Fornax A using 6.1 years of Pass 8 data. After Centaurus A, this is now the ...second example of an extended γ-ray source attributed to a radio galaxy. Both an extended flat disk morphology and a morphology following the extended radio lobes were preferred over a point-source description, and the core contribution was constrained to be % of the total γ-ray flux. A preferred alignment of the γ-ray elongation with the radio lobes was demonstrated by rotating the radio lobes template. We found no significant evidence for variability on ∼0.5 year timescales. Taken together, these results strongly suggest a lobe origin for the γ-rays. With the extended nature of the γ-ray emission established, we model the source broadband emission considering currently available total lobe radio and millimeter flux measurements, as well as X-ray detections attributed to inverse Compton (IC) emission off the cosmic microwave background (CMB). Unlike the Centaurus A case, we find that a leptonic model involving IC scattering of CMB and extragalactic background light (EBL) photons underpredicts the γ-ray fluxes by factors of about ∼2-3, depending on the EBL model adopted. An additional γ-ray spectral component is thus required, and could be due to hadronic emission arising from proton-proton collisions of cosmic rays with thermal plasma within the radio lobes.
We present a detailed investigation of the gamma -ray emission in the vicinity of the supernova remnant (SNR) W28 (G6.4-0.1) observed by the Large Area Telescope (LAT) on board the Fermi Gamma-ray ...Space Telescope. We detected significant gamma -ray emission spatially coincident with TeV sources HESS J1800-240A, B, and C, located outside the radio boundary of the SNR. Their spectra in the 2-100 GeV band are consistent with the extrapolation of the power-law spectra of the TeV sources. We also identified a new source of GeV emission, dubbed Source W, which lies outside the boundary of TeV sources and coincides with radio emission from the western part of W28. All of the GeV gamma -ray sources overlap with molecular clouds in the velocity range from 0 to 20 km s super(-1). Under the assumption that the gamma -ray emission toward HESS J1800-240A, B, and C comes from pi super(0) decay due to the interaction between the molecular clouds and cosmic rays (CRs) escaping from W28, they can be naturally explained by a single model in which the CR diffusion coefficient is smaller than the theoretical expectation in the interstellar space. The total energy of the CRs escaping from W28 is constrained through the same modeling to be larger than ~2 x 10 super(49) erg. The emission from Source W can also be explained with the same CR escape scenario.
We report on the discovery of gamma-ray pulsations from five millisecond pulsars (MSPs) using the Fermi Large Area Telescope (LAT) and timing ephemerides provided by various radio observatories. We ...also present confirmation of the gamma-ray pulsations from a sixth source, PSR J2051−0827. Five of these six MSPs are in binary systems: PSRs J1713+0747, J1741+1351, J1600−3053 and the two black widow binary pulsars PSRs J0610−2100 and J2051−0827. The only isolated MSP is the nearby PSR J1024−0719, which is also known to emit X-rays. We present X-ray observations in the direction of PSRs J1600−3053 and J2051−0827. While PSR J2051−0827 is firmly detected, we can only give upper limits for the X-ray flux of PSR J1600−3053. There are no dedicated X-ray observations available for the other three objects.
The MSPs mentioned above, together with most of the MSPs detected by Fermi, are used to put together a sample of 30 gamma-ray MSPs. This sample is used to study the morphology and phase connection of radio and gamma-ray pulse profiles. We show that MSPs with pulsed gamma-ray emission which is phase-aligned with the radio emission present the steepest radio spectra and the largest magnetic fields at the light cylinder among all MSPs. Also, we observe a trend towards very low, or undetectable, radio linear polarization levels. These properties could be attributed to caustic radio emission produced at a range of different altitudes in the magnetosphere. We note that most of these characteristics are also observed in the Crab pulsar, the only other radio pulsar known to exhibit phase-aligned radio and gamma-ray emission.
Four high-resolution regional climate models (RCMs) have been set up for the area of Greenland, with the aim of providing future projections of Greenland ice sheet surface mass balance (SMB), and its ...contribution to sea level rise, with greater accuracy than is possible from coarser-resolution general circulation models (GCMs). This is the first time an intercomparison has been carried out of RCM results for Greenland climate and SMB. Output from RCM simulations for the recent past with the four RCMs is evaluated against available observations. The evaluation highlights the importance of using a detailed snow physics scheme, especially regarding the representations of albedo and meltwater refreezing. Simulations with three of the RCMs for the 21st century using SRES scenario A1B from two GCMs produce trends of between −5.5 and −1.1 Gt yr−2 in SMB (equivalent to +0.015 and +0.003 mm sea level equivalent yr−2), with trends of smaller magnitude for scenario E1, in which emissions are mitigated. Results from one of the RCMs whose present-day simulation is most realistic indicate that an annual mean near-surface air temperature increase over Greenland of ~ 2°C would be required for the mass loss to increase such that it exceeds accumulation, thereby causing the SMB to become negative, which has been suggested as a threshold beyond which the ice sheet would eventually be eliminated.
Isoprene is the most important biogenic volatile organic
compound in the atmosphere. Its calculated impact on ozone (O3) is
critically dependent on the model isoprene oxidation chemical scheme, in
...particular the way the isoprene-derived organic nitrates (IN) are treated.
By combining gas chromatography with mass spectrometry, we have developed a
system capable of separating and unambiguously measuring individual IN
isomers. In this paper we use measurements from its first field deployment,
which took place in Beijing as part of the Atmospheric Pollution and Human
Health in a Chinese Megacity programme, to test understanding of the
isoprene chemistry as simulated in the Master Chemical Mechanism (MCM)
(v.3.3.1). Seven individual isoprene nitrates were identified and quantified
during the campaign: two β-hydroxy nitrates (IHN), four δ-carbonyl nitrates (ICN), and propanone nitrate. Our measurements show that in the summertime conditions experienced in
Beijing the ratio of (1-OH, 2-ONO2)-IHN to (4-OH, 3-ONO2)-IHN (the
numbers indicate the carbon atom in the isoprene chain to which the radical
is added) increases at NO mixing ratios below 2 ppb. This provides
observational field evidence of the redistribution of the peroxy radicals
derived from OH oxidation of isoprene away from the kinetic ratio towards a
new thermodynamic equilibrium consistent with box model calculations. The
observed amounts of δ-ICN demonstrate the importance of daytime
addition of NO3 to isoprene in Beijing but suggest that the predominant
source of the δ-ICN in the model (reaction of NO with δ-nitrooxy peroxy radicals) may be too large. Our speciated measurements of
the four δ-ICN exhibit a mean C1 : C4 isomer ratio of 1.4 and a mean
trans : cis isomer ratio of 7 and provide insight into the isomeric distribution of the
δ-nitrooxy peroxy radicals. Together our measurements and model
results indicate that propanone nitrate was formed from the OH oxidation of
δ-ICN both during the day and night, as well as from NO3
addition to propene at night. This study demonstrates the value of speciated IN measurements in testing
understanding of the isoprene degradation chemistry and shows how more
extensive measurements would provide greater constraints. It highlights
areas of the isoprene chemistry that warrant further study, in particular
the impact of NO on the formation of the IHN and the NO3-initiated
isoprene degradation chemistry, as well as the need for further laboratory
studies on the formation and the losses of IN, in particular via photolysis
of δ-ICN and hydrolysis.
ABSTRACT We report a discovery of shocked gas from the supernova remnant (SNR) G357.7+0.3. Our millimeter and submillimeter observations reveal broad molecular lines of CO(2-1), CO(3-2), CO(4-3), ...13CO (2-1), and 13CO (3-2), HCO+, and HCN using the Heinrich Hertz Submillimeter Telescope, the Arizona 12 m Telescope, APEX, and the MOPRA Telescope. The widths of the broad lines are 15-30 km s−1, and the detection of such broad lines is unambiguous, dynamic evidence showing that the SNR G357.7+0.3 is interacting with molecular clouds. The broad lines appear in extended regions (>4 5 × 5′). We also present the detection of shocked H2 emission in the mid-infrared but lacking ionic lines using Spitzer/IRS observations to map a few-arcminute area. The H2 excitation diagram shows a best fit with a two-temperature local thermal equilibrium model with the temperatures of ∼200 and 660 K. We observed C ii at 158 m and high-J CO(11-10) with the German Receiver for Astronomy at Terahertz Frequencies (GREAT) on the Stratospheric Observatory for Infrared Astronomy. The GREAT spectrum of C ii, a 3 detection, shows a broad line profile with a width of 15.7 km−1 that is similar to those of broad CO molecular lines. The line width of C ii implies that ionic lines can come from a low-velocity C-shock. Comparison of H2 emission with shock models shows that a combination of two C-shock models is favored over a combination of C- and J-shocks or a single shock. We estimate the CO density, column density, and temperature using a RADEX model. The best-fit model with n(H2) = 1.7 × 104 cm−3, N(CO) = 5.6 × 1016 cm−2, and T = 75 K can reproduce the observed millimeter CO brightnesses.
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