Search NASA⌕ Search

SEARCH · Search NASA

Results for “radio transient sources”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Radio and X-Ray Observations of the Luminous Fast Blue Optical Transient AT 2020xnd

We present deep X-ray and radio observations of the fast blue optical transient (FBOT) AT 2020xnd/ZTF 20acigmel at z = 0.2433 from 13 days to 269 days after explosion. AT 2020xnd belongs to the category of optically luminous FBOTs with similarities to the archetypal event AT 2018cow. AT 2020xnd shows luminous radio emission reaching L ν ≈ 8 × 10 29 erg s -1 Hz -1 at 20 GHz and 75 days post-explosion, accompanied by luminous and rapidly fading soft X-ray emission peaking at L X ≈ 6 × 10 42 erg s -1 . Interpreting the radio emission in the context of synchrotron radiation from the explosion's shock interaction with the environment, we find that AT 2020xnd launched a high-velocity outflow (v ~ 0.1c–0.2c) propagating into a dense circumstellar medium (effective $\dot{M}\approx {10}^{-3}\,{M}_{\odot }$ yr -1 for an assumed wind velocity of v w = 1000 km s -1 ). Similar to AT 2018cow, the detected X-ray emission is in excess compared to the extrapolated synchrotron spectrum and constitutes a different emission component, possibly powered by accretion onto a newly formed black hole or neutron star. These properties make AT 2020xnd a high-redshift analog to AT 2018cow, and establish AT 2020xnd as the fourth member of the class of optically luminous FBOTs with luminous multiwavelength counterparts.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2022

FIRST J153350.8+272729: The Radio Afterglow of a Decades-old Tidal Disruption Event

We present the discovery of the fading radio transient FIRST J153350.8+272729. The source had a maximum observed 5 GHz radio luminosity of 8 × 10 39 erg s –1 in 1986, but by 2019 had faded by a factor of nearly 400. It is located at the center of a galaxy (SDSS J153350.89+272729) at 147 Mpc, which shows weak Type II Seyfert activity. We show that a tidal disruption event (TDE) is the preferred scenario for FIRST J153350.8+272729, although it could plausibly be interpreted as the afterglow of a long-duration γ-ray burst. This is only the second TDE candidate to be first discovered at radio wavelengths. Its luminosity fills a gap between the radio afterglows of subrelativistic TDEs in the local universe, and relativistic TDEs at high redshifts. The unusual properties of FIRST J153350.8+272729 (ongoing nuclear activity in the host galaxy, high radio luminosity) motivate more extensive TDE searches in untargeted radio surveys.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2022

Evidence for X-Ray Emission in Excess to the Jet-afterglow Decay 3.5 yr after the Binary Neutron Star Merger GW 170817: A New Emission Component

For the first ~3 yrs after the binary neutron star merger event GW 170817, the radio and X-ray radiation has been dominated by emission from a structured relativistic off-axis jet propagating into a low-density medium with n < 0.01 cm -3 . We report on observational evidence for an excess of X-ray emission at δt > 900 days after the merger. With L x ≈ 5 × 10 38 erg s -1 at 1234 days, the recently detected X-ray emission represents a ≥ 3.2σ (Gaussian equivalent) deviation from the universal post-jet-break model that best fits the multiwavelength afterglow at earlier times. In the context of JetFit afterglow models, current data represent a departure with statistical significance ≥ 3.1σ, depending on the fireball collimation, with the most realistic models showing excesses at the level of ≥ 3.7σ. A lack of detectable 3 GHz radio emission suggests a harder broadband spectrum than the jet afterglow. These properties are consistent with the emergence of a new emission component such as synchrotron radiation from a mildly relativistic shock generated by the expanding merger ejecta, i.e., a kilonova afterglow. In this context, we present a set of ab initio numerical relativity binary neutron star (BNS) merger simulations that show that an X-ray excess supports the presence of a high-velocity tail in the merger ejecta, and argues against the prompt collapse of the merger remnant into a black hole. Radiation from accretion processes on the compact-object remnant represents a viable alternative. Neither a kilonova afterglow nor accretion-powered emission have been observed before, as detections of BNS mergers at this phase of evolution are unprecedented

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS2013

The Evolving Polarized Jet of Black Hole Candidate Swift J1745-26

Swift J1745−26 is an X-ray binary towards the Galactic Centre that was detected when it went into outburst in September 2012. This source is thought to be one of a growing number of sources that display "failed outbursts", in which the self-absorbed radio jets of the transient source are never fully quenched and the thermal emission from the geometrically-thin inner accretion disk never fully dominates the X-ray flux. We present multifrequency data from the Very Large Array, Australia Telescope Compact Array and Karoo Array Telescope (KAT- 7) radio arrays, spanning the entire period of the outburst. Our rich data set exposes radio emission that displays a high level of large scale variability compared to the X-ray emission and deviations from the standard radio-X-ray correlation that are indicative of an unstable jet and confirm the outburst's transition from the canonical hard state to an intermediate state. We also observe steepening of the spectral index and an increase of the linear polarization to a large fraction (is approx. equal to 50%) of the total flux, as well as a rotation of the electric vector position angle. These are consistent with a transformation from a self-absorbed compact jet to optically-thin ejecta - the first time such a discrete ejection has been observed in a failed outburst - and may imply a complex magnetic field geometry.

black↗
NASA NTRS2013

Semi-Supervised Eigenbasis Novelty Detection

Recent discoveries in high-time-resolution radio astronomy data have focused attention on a new class of events. Fast transients are rare pulses of radio frequency energy lasting from microseconds to seconds that might be produced by a variety of exotic astrophysical phenomena. For example, X-ray bursts, neutron stars, and active galactic nuclei are all possible sources of short-duration, transient radio signals. It is difficult to anticipate where such signals might appear, and they are most commonly discovered through analysis of high-time- resolution data that had been collected for other purposes. Transients are often faint and difficult to detect, so improved detection algorithms can directly benefit the science yield of all such commensal monitoring. A new detection algorithm learns a low-dimensional linear manifold for describing the normal data. High reconstruction error indicates a novel signal that does not match the patterns of normal data. One unsupervised portion of the manifold model adapts its representation in response to recent data. A second supervised portion of the model is made of a basis trained in advance using labeled examples of RFI; this prevents false positives due to these events. For a linear model, an orthonormalization operation is used to combine these bases prior to the anomaly detection decision. Another novel aspect of the approach lies in combining basis vectors learned in an unsupervised, online fashion from the data stream with supervised basis vectors learned in advance from known examples of false alarms. Adaptive, data-driven detection is achieved that is also informed by existing domain knowledge about signals that may be statistically anomalous, but are not interesting and should therefore be ignored. The method was evaluated using data from the Parkes Multibeam Survey. This data set was originally collected to search for pulsars, which are astronomical sources that emit radio pulses at regular periods. However, several non-pulsar anomalies have recently been discovered in this dataset, making it a compelling test case. By explicitly filtering known false alarm patterns, the approach yields significantly better performance than current transient detection methods.

Wagstaff, Kiri L.↗
NASA NTRS1989

Deep optical and radio searches for a quiescent counterpart to the optical transient source OTS 1809+31

Hudec (1987) has reported the detection of three bright transient optical images from the same celestial location localized to alpha(1950) = 18 h 9 m 26.29 s + or - 0.03 s, delta (1950) = +31 deg 23 arcmin 19.7 arcsec + or - 0.6 arcsec. This paper reports the result of a deep CCD search in four colors undertaken to find the quiescent counterpart of the optical transients. No source is found at the location of the transients to m(B) = 24.5; however, 18 objects in the vicinity of the reported coordinates which are consistent with the source having a proper motion of 1 arcsec/yr. A search at 6 cm with the VLA revealed no source in the mu less than 1 arcsec/yr circle to a 3 sigma limiting flux of 83 micro-Jy.

Ricker, George R.↗
DOE OSTI2023

A matched-filter approach to radio variability and transients: searching for orphan afterglows in the VAST Pilot Survey

Radio transient searches using traditional variability metrics struggle to recover sources whose evolution time-scale is significantly longer than the survey cadence. Motivated by the recent observations of slowly evolving radio afterglows at gigahertz frequency, we present the results of a search for radio variables and transients using an alternative matched-filter approach. We designed our matched-filter to recover sources with radio light curves that have a high-significance fit to power-law and smoothly broken power-law functions; light curves following these functions are characteristic of synchrotron transients, including ‘orphan’ gamma-ray burst afterglows, which were the primary targets of our search. Applying this matched-filter approach to data from Variables and Slow Transients Pilot Survey conducted using the Australian SKA Pathfinder, we produced five candidates in our search. Subsequent Australia Telescope Compact Array observations and analysis revealed that: one is likely a synchrotron transient; one is likely a flaring active galactic nucleus, exhibiting a flat-to-steep spectral transition over 4 months; one is associated with a starburst galaxy, with the radio emission originating from either star formation or an underlying slowly evolving transient; and the remaining two are likely extrinsic variables caused by interstellar scintillation. The synchrotron transient, VAST J175036.1–181454, has a multifrequency light curve, peak spectral luminosity, and volumetric rate that is consistent with both an off-axis afterglow and an off-axis tidal disruption event; interpreted as an off-axis afterglow would imply an average inverse beaming factor $\langle f^{-1}_{\text{b}} \rangle = 860^{+1980}_{-710}$, or equivalently, an average jet opening angle of $\langle \theta _{\textrm {j}} \rangle = 3^{+4}_{-1}\,$ deg.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS2005

Aspects of Neutron Star and Black Hole Transients in RXTE Observations

Scans of the the galactic center with the Proportional Counter Array on the Rossi X-ray Timing Explorer have discovered the outbursts of many neutron star and black hole binary transients early in the outbursts. We have had observations during the rise of the transients which allow us to study the spectral and timing characteristics. Some of the transients are also identified as radio sources, with compact cores and jet ejections. A recent example is H 1743-322, a recurrent black hole candidate, in which the ejection of radio emitting plasma is correlated with changes in the spectral characteristics. I will review the properties of black hole transients compare the characteristics of neutron star transients.

Swank, J.↗
NASA NTRS2020

The Fast Radio Burst Luminosity Function and Death Line in the Low-Twist Magnetar Model

We explore the burst energy distribution of fast radio bursts(FRBs)in the low-twist magnetar model ofWadiasingh & Timokhin(WT19). Motivated by the power-lawfluence distributions of FRB121102, we proposean elementary model for the FRB luminosity function of individual repeaters with an inversion protocol thatdirectly relates the power-law distribution index of magnetar short burstfluences to that for FRBs. The protocolindicates that the FRB energy scales virtually linearly with crust/field dislocation amplitude, if magnetar shortbursts prevail in the magnetoelastic regime. Charge starvation in the magnetosphere during bursts(required inWT19)for individual repeaters implies the predicted burstfluence distribution is narrow,3 decades for yieldingstrains and oscillation frequencies feasible in magnetar crusts. Requiring magnetic confinement and chargestarvation, we obtain a death line for FRBs, which segregates magnetars from the normal pulsar population,suggesting only the former will host recurrent FRBs. We convolve the burst energy distribution for individualmagnetars to define the distribution of luminosities in evolved magnetar populations. The broken power-lawluminosity function’s low-energy character depends on the population model, while the high-energy index tracesthat of individual repeaters. Independent of the evolved population, the broken power-law isotropic-equivalentenergy/luminosity function peaks at∼1037–1040erg with a low-energy cutoff at∼1037erg. Lastly, we considerthe localfluence distribution of FRBs andfind that it can constrain the subset of FRB-producing magnetarprogenitors. Our model suggests that improvements in sensitivity may reveal aflattening of the global FRBfluencedistribution and saturation in FRB rates.

Cosmology↗
NASA NTRS1992

VLA, PHOENIX, and BATSE observations of an X1 flare

We present observations of an X1 flare (18 Jul. 1991) detected simultaneously with the Very Large Array (VLA), the PHOENIX Digital Radio Spectrometer and the Burst and Transient Source Experiment (BATSE) aboard the Gamma Ray Observatory (GRO). The VLA was used to produce snapshot maps of the impulsive acceleration in the higher corona several minutes before the onset of the hard x ray burst detected by BATSE. Comparisons with high spectral and temporal observations by PHOENIX reveal a variety of radio bursts at 20 cm, such as type 3 bursts, intermediate drift bursts, and quasi-periodic pulsations during different stages of the X1 flare. From the drift rates of these radio bursts we derive information on local density scale heights, the speed of radio exciters, and the local magnetic field. Radio emission at 90 cm shows a type 4 burst moving outward with a constant velocity of 240 km/s. The described X1 flare is unique in the sense that it appeared at the east limb (N06/E88), providing the most accurate information on the vertical structure of different flare tracers visible in radio wavelengths.

Willson, Robert F.↗
NASA NTRS1978

Nuclear gamma rays from compact objects

Accreting compact objects may be important gamma ray line sources and may explain recent observations of celestial gamma-ray line emission from a transient source in the direction of the galactic anti-center, from the galactic center, and possibly from the radio galaxy Centaurus A. The identification of the lines from the transient source requires a strong redshift. Such a redshift permits the identification of these lines with the most intense nuclear emission lines expected in nature, positron annihilation, and neutron capture on hydrogen and iron. Their production as a result of nuclear interactions in accreting gas around a neutron star is proposed. The gamma-ray line emission from the galactic center and possibly Centaurus A appears to have a surprisingly high luminosity, amounting to perhaps as much as 10% of the total luminosity of these sources. Such high gamma-ray line emission efficiencies could result from nuclear interactions in accreting gas around a massive black hole.

Lingenfelter, R. E.↗
NASA NTRS1979

Helios-2 Vela-Ariel-5 gamma-ray burst source position

The gamma-ray burst of 28 January 1976, one of 18 events thus far detected in interplanetary space with Helios-2, was also observed with the Vela-5A, -6A and the Ariel-5 satellites. A small source field is obtained from the intersection of the region derived from the observed time delays between Helios-2 and Vela-5A and -6A with the source region independently found with the Ariel-5 X-ray detector. This area contains neither any steady X-ray source as scanned by HEAO-A nor any previously catalogued X-ray, radio or infrared sources, X-ray transients, quasars, seyferts, globular clusters, flare stars, pulsars, white dwarfs or high energy gamma-ray sources. The region is however, within the source field of a gamma-ray transient observed in 1974, which exhibited nuclear gamma-ray line structure.

Cline, T. L.↗
NASA NTRS1979

Helios 2-Vela-Ariel 5 gamma-ray burst source position

The gamma-ray burst of January 28, 1976, one of 18 events thus far detected in interplanetary space with Helios 2, was also observed with the Vela 5A and 6A and the Ariel 5 satellites. A small source field is obtained from the intersection of the region derived from the observed time delays between Helios 2 and Vela 5A and 6A, with the source region independently found with the Ariel 5 X-ray detector. This area contains neither any steady X-ray source as scanned by HEAO 1 nor any previously cataloged X-ray, radio, or infrared sources, X-ray transients, quasars, Seyferts, globular clusters, flare stars, pulsars, white dwarfs, or high enery gamma-ray sources. The region is, however, within the source field of a gamma-ray transient observed in 1974 by Jacobson et al. (1978) which exhibited nuclear gamma-ray line structure.

Cline, T. L.↗
NASA NTRS2012

A VLA (Very Large Array) Search for 5 GHz Radio Transients and Variables at Low Galactic Latitudes

We present the results of a 5GHz survey with the Very Large Array (VLA) and the expanded VLA, designed to search for short-lived (approx < 1 day) transients and to characterize the variability of radio sources at milli-Jansky levels. A total sky area of 2.66 sq. deg, spread over 141 fields at low Galactic latitudes (b approx equals 6 - 8 deg) was observed 16 times with a cadence that was chosen to sample timescales of days, months and years. Most of the data were reduced, analyzed and searched for transients in near real time. Interesting candidates were followed up using visible light telescopes (typical delays of 1 - 2 hr) and the X-Ray Telescope on board the Swift satellite. The final processing of the data revealed a single possible transient with a flux density of f(sub v) approx equals 2.4mJy. This implies a transients sky surface density of kappa(f(sub v) > 1.8mJy) = 0.039(exp +0.13,+0.18) (sub .0.032,.0.038) / sq. deg (1, 2 sigma confidence errors). This areal density is consistent with the sky surface density of transients from the Bower et al. survey extrapolated to 1.8mJy. Our observed transient areal density is consistent with a Neutron Stars (NSs) origin for these events. Furthermore, we use the data to measure the sources variability on days to years time scales, and we present the variability structure function of 5GHz sources. The mean structure function shows a fast increase on approximately 1 day time scale, followed by a slower increase on time scales of up to 10 days. On time scales between 10 - 60 days the structure function is roughly constant. We find that approx > 30% of the unresolved sources brighter than 1.8mJy are variable at the > 4-sigma confidence level, presumably due mainly to refractive scintillation.

Ofek, E. O.↗
NASA NTRS1979

Radio and white-light observations of the 1973 August 21 coronal transient

A coronal transient, which occurred on August 21, 1973, has been observed simultaneously in white light and at decameter radio wavelengths. The radio observations were obtained with a two-dimensional swept-frequency array (called the Teepee Tee). The white-light observations consist of a series of photographs taken by the High Altitude Observatory's coronagraph aboard Skylab. The radio emission associated with the transient was continuum in nature and lasted for almost 5 hours. The source of emission was observed to be cospatial with the lower part of one of the secondary white-light loops. A lower limit of 0.6 solar radius for the depth (extension along the line of sight) of this part of the transient was derived from considerations of radio-wave propagation in the corona. The radio source showed no dispersion of height with frequency, and, therefore, the emission is attributed to gyrosynchrotron radiation. Based on this assumption, the magnetic-field strength in the lower part of the loop is estimated to be in the 2.0-4.5-gauss range at a height of 2.1 solar radii from the center of the sun.

Gergely, T. E.↗
NASA NTRS1994

Characteristics of the persistent emission of SGR 1806-20

Earlier, using the X-ray satellite ASCA, we had identified the soft gamma-ray burst repeater (SGR) 1806-20 with a persistent X-ray source, AX 1805.7-2025 which is located close to the center of the radio supernova remnant G10.0-0.3. A burst seen by Burst and Transient Source Experiment (BATSE) was shown to be centered on G10.0-0.3. Here we report detailed temporal and spectral analysis of the persistent source. We find that AX 1805.7-2025 is a very compact source, size less than 30 sec in diameter, with a power-law spectrum. The count rate from AX 1805.7-2025 is steady on timescales of minutes, days, and 1 week. Most supernova remnants have extended X-ray emission ascribed to shocked gas. We set an upper limit to an extended X-ray emission both in continuum and line emission. We conclude that an isolated neutron star, most likely a pulsar, powers the radio remnant G10.0-0.3. Finally, we note that the persistent spectrum appears to be less absorbed than the burst spectrum.

Sonobe, T.↗
NASA NTRS1995

RE J1255+266: Detection of an extremely bright EUV transient

During a pointed ROSAT observation in the direction of the Coma cluster of galaxies an exceptionally bright EUV source, RE J1255+266, was detected serendipitously. The source is located close to the Galactic North Pole, at b(sub II) is approximately or equal to 89 deg. Its observed EUV flux (62 - 110 eV) at the time of the detection was of order 7 x 10(exp -9) ergs/sq cm, making RE J1255+266 temporarily one of the brightest EUV sources on the sky. The EUV flare of RE J1255+266 has a light curve with a decay time of about 0.86 days. With respect to earlier non-detections, the source brightened by a factor of approximately 7000. Such a behavior has not been observed before. Thus, it is unclear what type of source RE J1255+266 might be. Up to now no positive identification with any known source could be obtained. Emission at the position of the source was previously only detected in the 1987 Green Bank radio continuum survey. Simultaneous observations with Compton Gamma Ray Observatory/Burst and Transient Source Experiment (CGRO/BATSE) resulted in non-detections of the source in the 8 - 50 keV energy range.

Dahlem, M.↗