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At least 127 records · Page 7
All sourcesNASA NTRSDOE OSTI
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↗
DOE OSTI2021

Abell 1430: A merging cluster with exceptional diffuse radio emission

Diffuse radio emission has been found in many galaxy clusters, predominantly in massive systems which are in the state of merging. The radio emission can usually be classified as relic or halo emission, which are believed to be related to merger shocks or volume-filling turbulence, respectively. Recent observations have revealed radio bridges for some pairs of very close galaxy clusters. The mechanisms that may allow one to explain the high specific density of relativistic electrons, which are necessary to explain the radio luminosity of these bridge regions, have been poorly explored until now. When inspecting the first data release of the LOFAR Two-Metre Sky Survey (LoTSS), we discovered diffuse radio emission in the galaxy cluster Abell 1430. Here, for this work, we aim to determine the dynamical state of the cluster and characterise the diffuse radio emission. We analysed the LoTSS data in detail and complemented them with recent Karl G. Jansky Very Large Array observations in the L-band. To study the dynamical state of the cluster, we analysed XMM-Newton data, Chandra data, and Sloan Digital Sky Survey data. Moreover, we compared our results to clusters extracted from THE THREE HUNDRED PROJECT cosmological simulation. We find that Abell 1430 consists of two components, namely A1430-A and A1430-B, with a mass ratio of about 2:1. The massive component shows diffuse radio emission which can be classified as radio halo which shows a low radio power at 1.4 GHz with respect to the mass of the cluster. Most interestingly, there is extended diffuse radio emission in the following dubbed as the ‘Pillow’ according to its morphology, which is apparently related to A1430-B and which is neither typical halo nor typical relic emission. The origin of this emission is puzzling. We speculate that the two components of Abell 1430 undergo an off-axis merger. In this scenario, A1430-B is moving towards the main cluster component and may have compressed and stirred the medium in the filament between the two cluster components. We have discovered evidence for diffuse radio emission related to the low-density intracluster or intergalactic medium in Abell 1430. To date, only a few examples of emission originating from such regions are known. These discoveries are crucial to constrain possible acceleration mechanisms which may allow us to explain the presence of relativistic electrons in these regions. In particular, our results indicate a spectral index of α 144 MHz 1.5 GHz = -1.4±0.5 for the Pillow. If upcoming observations confirm a slope as flat as -1.4 or even flatter, this would pose a challenge for the electron acceleration scenarios.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2021

Probing the Wind Component of Radio Emission in Luminous High-redshift Quasars

We discuss a probe of the contribution of wind-related shocks to the radio emission in otherwise radio-quiet quasars. Given (1) the nonlinear correlation between UV and X-ray luminosity in quasars, (2) that such a correlation leads to higher likelihood of radiation-line-driven winds in more luminous quasars, and (3) that luminous quasars are more abundant at high redshift, deep radio observations of high-redshift quasars are needed to probe potential contributions from accretion disk winds. We target a sample of 50 z ≃ 1.65 color-selected quasars that span the range of expected accretion disk wind properties as traced by broad C iv emission. 3 GHz observations with the Very Large Array to an rms of ≈10 μJy beam -1 probe to star formation rates of ~400 M ⊙ yr -1 , leading to 22 detections. Supplementing these pointed observations are survey data of 388 sources from the LOFAR Two-meter Sky Survey Data Release 1 that reach comparable depth (for a typical radio spectral index), where 123 sources are detected. These combined observations reveal a radio detection fraction that is a nonlinear function of C iv emission-line properties and suggest that the data may require multiple origins of radio emission in radio-quiet quasars. We find evidence for radio emission from weak jets or coronae in radio-quiet quasars with low Eddington ratios, with either (or both) star formation and accretion disk winds playing an important role in optically luminous quasars and correlated with increasing Eddington ratio. Additional pointed radio observations are needed to fully establish the nature of radio emission in radio-quiet quasars.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2022

FLASH pilot survey: detections of associated 21 cm H i absorption in GAMA galaxies at 0.42 < z < 1.00

ABSTRACT We present the results of a search for associated 21 cm H i absorption at redshift 0.42 < z < 1.00 in radio-loud galaxies from three Galaxy And Mass Assembly (GAMA) survey fields. These observations were carried out as part of a pilot survey for the Australian SKA Pathfinder (ASKAP) First Large Absorption Survey in H i (FLASH). From a sample of 326 radio sources with 855.5 MHz peak flux density above 10 mJy, we detected two associated H i absorption systems, in SDSS J090331+010847 at z = 0.522 and SDSS J113622+004852 at z = 0.563. Both galaxies are massive (stellar mass $\gt 10^{11}\, \mathrm{M}_\odot$) and have optical spectra characteristic of luminous red galaxies, though spectral energy distribution fitting implies that SDSS J113622+004852 contains a dust-obscured starburst with star formation rate ∼69 M⊙ yr−1. The H i absorption lines have a high optical depth, with τpk of 1.77 ± 0.16 for SDSS J090331+010847 (the highest value for any z > 0.1 associated system found to date) and 0.14 ± 0.01 for SDSS J113622+004852. In the redshift range probed by our ASKAP observations, the detection rate for associated H i absorption lines (with τpk > 0.1 and at least 3σ significance) is $2.9_{-2.6}^{+9.7}$ per cent. Although the current sample is small, this rate is consistent with a trend seen in other studies for a lower detection rate of associated 21 cm H i absorption systems at higher redshift. We also searched for OH absorption lines at 0.67 < z < 1.34, but no detection was made in the 145 radio sources searched.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2021

Very Large Array imaging rules out precessing radio jets in three DES–SDSS-selected candidate periodic quasars

ABSTRACT Periodic quasars have been suggested as candidates for hosting binary supermassive black holes (SMBHs), although alternative scenarios remain possible to explain the optical light-curve periodicity. To test the alternative hypothesis of precessing radio jet, we present deep 6 GHz radio imaging conducted with NSF’s Karl G. Jansky Very Large Array (VLA) in its C configuration for the three candidate periodic quasars, DES J024703.24−010032.0, DES J024944.66−000036.8, and DES J025214.67−002813.7. Our targets were selected based on their optical variability using 20 yr long multicolour light curves from the Dark Energy Survey (DES) and the Sloan Digital Sky Survey (SDSS). The new VLA observations show that all three periodic quasars are radio-quiet with the radio loudness parameters measured to be $R\equiv f_{6\, {\rm cm}}/f_{{\rm 2500}}$ of ≲1.0–1.5 and the k-corrected luminosities νLν[6 GHz] of ≲5–21 × 1039 erg s−1. They are in stark contrast to previously known periodic quasars proposed as binary SMBH candidates such as the blazar OJ 287 and PG1302−102. Our results rule out optical emission contributed from precessing radio jets as the origin of the optical periodicity in the three DES–SDSS-selected candidate periodic quasars. Future continued optical monitoring and complementary multiwavelength observations are still needed to further test the binary SMBH hypothesis as well as other competing scenarios to explain the optical periodicity.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2022

Possible jet reorientation in low-frequency radio structures of blazars

ABSTRACT Blazars are active galactic nuclei (AGNs) observed at small viewing angles to the observer’s line of sight; hence most of them are compact sources in the radio regime. At low radio frequencies, we may observe additional extended structures, considered to be lobes propagating directly towards Earth. However, there are rare cases of blazars whose morphologies are interpreted in the framework of an episodic jet activity with a reorientation of the jet axis. In this paper, we aim to find other candidates for sources of this kind and present Low Frequency Array (LOFAR) observations of seven blazars with rare morphologies composed of kiloparsec-scale jets and elongated multihundred kiloparsec-scale features. Based on their radio images and physical parameters, we propose that jets could change the direction of propagation out of the sky plane during the lifetime of at least some of these sources and with such we may observe morphologically different AGN types simultaneously, similar to the previously discovered blazar SBS B1646+499. Several plausible origin scenarios are presented, including fast realignment and precession. We also derive the 144 MHz fluxes of the compact and extended structures, the total radio luminosities, the spectral indices for the diffuse haloes, and the core-dominance parameters. We also discuss the possibility that these objects are actually giant blazars with rare Fanaroff–Riley I (FR I) morphology. The previously discovered blazar SBS B1646+499 should be classified as a giant independently of its inclination with regard to the observer.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2022

All-purpose, all-sky photometric redshifts for the Legacy Imaging Surveys Data Release 8

ABSTRACT In this paper, we present photometric redshift (photo-z) estimates for the Dark Energy Spectroscopic Instrument (DESI) Legacy Imaging Surveys, currently the most sensitive optical survey covering the majority of the extragalactic sky. Our photo-z methodology is based on a machine-learning approach, using sparse Gaussian processes augmented with Gaussian mixture models (GMMs) that allow regions of parameter space to be identified and trained separately in a purely data-driven way. The same GMMs are also used to calculate cost-sensitive learning weights that mitigate biases in the spectroscopic training sample. By design, this approach aims to produce reliable and unbiased predictions for all parts of the parameter space present in wide area surveys. Compared to previous literature estimates using the same underlying photometry, our photo-zs are significantly less biased and more accurate at z > 1, with negligible loss in precision or reliability for resolved galaxies at z < 1. Our photo-z estimates offer accurate predictions for rare high-value populations within the parent sample, including optically selected quasars at the highest redshifts (z > 6), as well as X-ray or radio continuum selected populations across a broad range of flux (densities) and redshift. Deriving photo-z estimates for the full Legacy Imaging Surveys Data Release 8, the catalogues provided in this work offer photo-z estimates predicted to be of high quality for ≳9 × 108 galaxies over ${\sim}19\, 400\, \text{deg}^{2}$ and spanning 0 < z ≲ 7, offering one of the most extensive samples of redshift estimates ever produced.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2023

Measuring the giant radio galaxy length distribution with the LoTSS

Many massive galaxies launch jets from the accretion disk of their central black hole, but only ~10 3 instances are known in which the associated outflows form giant radio galaxies (GRGs, or giants): luminous structures of megaparsec extent that consist of atomic nuclei, relativistic electrons, and magnetic fields. Large samples are imperative to understanding the enigmatic growth of giants, and recent systematic searches in homogeneous surveys constitute a promising development. For the first time, it is possible to perform meaningful precision statistics with GRG lengths, but a framework to do so is missing. We measured the intrinsic GRG length distribution by combining a novel statistical framework with a LOFAR Two-metre Sky Survey (LoTSS) sample of freshly discovered giants. In turn, this allowed us to answer an array of questions on giants. For example, we can now assess how rare a 5 Mpc giant is compared with one of 1 Mpc, and how much larger – given a projected length – the corresponding intrinsic length is expected to be. Notably, we can now also infer the GRG number density in the Local Universe. We assumed the intrinsic GRG length distribution to be Paretian (i.e. of power-law form) with tail index ξ, and predicted the observed distribution by modelling projection and selection effects. To infer ξ, we also systematically searched the LoTSS for hitherto unknown giants and compiled the largest catalogue of giants to date. We show that if intrinsic GRG lengths are Pareto distributed with index ξ, then projected GRG lengths are also Pareto distributed with index ξ. Selection effects induce curvature in the observed projected GRG length distribution: angular length selection flattens it towards the lower end, while surface brightness selection steepens it towards the higher end. We explicitly derived a GRG’s posterior over intrinsic lengths given its projected length, laying bare the ξ dependence. We also discovered 2060 giants within LoTSS DR2 pipeline products; our sample more than doubles the known population. Spectacular discoveries include the largest, second-largest, and fourth-largest GRG known (l p = 5.1 Mpc, l p = 5.0 Mpc, and l p = 4.8 Mpc), the largest GRG known hosted by a spiral galaxy (l p = 2.5 Mpc), and the largest secure GRG known beyond redshift 1 (l p = 3.9 Mpc). We increase the number of known giants whose angular length exceeds that of the Moon from 10 to 23; among the discoveries is the angularly largest known radio galaxy in the Northern Sky, which is also the angularly largest known GRG ($\phi$ = 2°). Combining theory and data, we determined that intrinsic GRG lengths are well described by a Pareto distribution, and measured the index ξ = –3.5 ± 0.5. This implies that, given its projected length, a GRG’s intrinsic length is expected to be just 15% larger. Finally, we determined the comoving number density of giants in the Local Universe to be n GRG = 5 ± 2(100 Mpc) –3 . We developed a practical mathematical framework that elucidates the statistics of giant radio galaxy lengths. Through a LoTSS search, we also discovered 2060 new giants. By combining both advances, we determined that intrinsic GRG lengths are well described by a Pareto distribution with index ξ = –3.5 ± 0.5, and that giants are truly rare in a cosmological sense: most clusters and filaments of the Cosmic Web are not currently home to a giant. Thus, our work yields new observational constraints for analytical models and simulations featuring radio galaxy growth.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2023

Multi-scale VLBI observations of the candidate host galaxy of GRB 200716C

We present the discovery and the subsequent follow up of radio emission from SDSS J130402.36+293840.6 (J1304+2938), the candidate host galaxy of the gamma-ray burst (GRB) GRB 200716C. The galaxy is detected in the RACS (0.89 GHz), the NVSS, the Apertif imaging survey, and the FIRST (1.4 GHz), the VLASS (3 GHz), and in public LOFAR (130–170 MHz), WISE (3.4–22 μm), and SDSS (z, i, r, g, u filters) data. The luminosity inferred at 1.4 GHz is (5.1 ± 0.2) × 10 30 erg s -1 Hz -1 . To characterise the emission and distinguish between different components within the galaxy, we performed dedicated, high-sensitivity and high-resolution observations with the European VLBI Network (EVN) + e-MERLIN at 1.6 and 5 GHz. We did not detect any emission from a compact core, suggesting that the presence of a radio-loud active galactic nucleus (AGN) is unlikely, and therefore we ascribe the emission observed in the public surveys to star-forming regions within the galaxy. We confirm and refine the redshift estimate, z = 0.341 ± 0.004, with a dedicated Telescopio Nazionale Galileo (TNG) spectroscopic observation. Finally, we compiled a list of all the known hosts of GRB afterglows detected in radio and computed the corresponding radio luminosity: if GRB 200716C belongs to J1304+2938, this is the third most radio-luminous host of a GRB, implying one of the highest star-formation rates (SFRs) currently known, namely SFR ~ 324±61 M ⊙ yr -1 . On the other hand, through the analysis of the prompt emission light curve, recent works suggest that GRB 200716C might be a short-duration GRB located beyond J1304+2938 and gravitationally lensed by an intermediate-mass black hole (IMBH) hosted by the galaxy. Neither the public data nor our Very Long Baseline Interferometry (VLBI) observations can confirm or rule out the presence of an IMBH acting as a (milli-)lens hosted by the galaxy, a scenario still compatible with the set of radio observations presented in this work.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2023

Star formation efficiency and AGN feedback in narrow-line Seyfert 1 galaxies with fast X-ray nuclear winds

ABSTRACT We present the first systematic study of the molecular gas and star formation efficiency in a sample of ten narrow-line Seyfert 1 galaxies selected to have X-ray Ultra Fast Outflows and, therefore, to potentially show AGN feedback effects. CO observations were obtained with the IRAM 30-m telescope in six galaxies and from the literature for four galaxies. We derived the stellar mass, star formation rate, AGN, and FIR dust luminosities by fitting the multi-band spectral energy distributions with the CIGALE code. Most of the galaxies in our sample lie above the main sequence (MS), and the molecular depletion time is one to two orders of magnitude shorter than the one typically measured in local star-forming galaxies. Moreover, we found a promising correlation between the star formation efficiency and the Eddington ratio, as well as a tentative correlation with the AGN luminosity. The role played by the AGN activity in the regulation of star formation within the host galaxies of our sample remains uncertain (little or no effect? positive feedback?). Nevertheless, we can conclude that quenching by the AGN activity is minor and that star formation will likely stop in a short time due to gas exhaustion by the current starburst episode.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2022

Disk Evolution Study through Imaging of Nearby Young Stars (DESTINYS): A Panchromatic View of DO Tau’s Complex Kilo-astronomical-unit Environment

While protoplanetary disks are often treated as isolated systems in planet formation models, observations increasingly suggest that vigorous interactions between Class II disks and their environments are not rare. DO Tau is a T Tauri star that has previously been hypothesized to have undergone a close encounter with the HV Tau system. As part of the DESTINYS ESO Large Programme, we present new Very Large Telescope (VLT)/SPHERE polarimetric observations of DO Tau and combine them with archival Hubble Space Telescope (HST) scattered-light images and Atacama Large Millimeter/submillimeter Array (ALMA) observations of CO isotopologues and CS to map a network of complex structures. The SPHERE and ALMA observations show that the circumstellar disk is connected to arms extending out to several hundred astronomical units. HST and ALMA also reveal stream-like structures northeast of DO Tau, some of which are at least several thousand astronomical units long. These streams appear not to be gravitationally bound to DO Tau, and comparisons with previous Herschel far-IR observations suggest that the streams are part of a bridge-like structure connecting DO Tau and HV Tau. We also detect a fainter redshifted counterpart to a previously known blueshifted CO outflow. While some of DO Tau's complex structures could be attributed to a recent disk–disk encounter, they might be explained alternatively by interactions with remnant material from the star formation process. These panchromatic observations of DO Tau highlight the need to contextualize the evolution of Class II disks by examining processes occurring over a wide range of size scales.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2024

First Sagittarius A* Event Horizon Telescope Results. VII. Polarization of the Ring

Abstract The Event Horizon Telescope observed the horizon-scale synchrotron emission region around the Galactic center supermassive black hole, Sagittarius A* (Sgr A*), in 2017. These observations revealed a bright, thick ring morphology with a diameter of 51.8 ± 2.3 μas and modest azimuthal brightness asymmetry, consistent with the expected appearance of a black hole with mass M ≈ 4 × 106 M ⊙. From these observations, we present the first resolved linear and circular polarimetric images of Sgr A*. The linear polarization images demonstrate that the emission ring is highly polarized, exhibiting a prominent spiral electric vector polarization angle pattern with a peak fractional polarization of ∼40% in the western portion of the ring. The circular polarization images feature a modestly (∼5%–10%) polarized dipole structure along the emission ring, with negative circular polarization in the western region and positive circular polarization in the eastern region, although our methods exhibit stronger disagreement than for linear polarization. We analyze the data using multiple independent imaging and modeling methods, each of which is validated using a standardized suite of synthetic data sets. While the detailed spatial distribution of the linear polarization along the ring remains uncertain owing to the intrinsic variability of the source, the spiraling polarization structure is robust to methodological choices. The degree and orientation of the linear polarization provide stringent constraints for the black hole and its surrounding magnetic fields, which we discuss in an accompanying publication.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2023

Disentangling the AGN and Star formation Contributions to the Radio–X-Ray Emission of Radio-loud Quasars at 1 < Z < 2

We constrain the emission mechanisms responsible for the prodigious electromagnetic output generated by active galactic nuclei (AGNs) and their host galaxies with a novel state-of-the-art AGN radio-to-X-ray spectral energy distribution model fitting code (ARXSED). ARXSED combines multiple components to fit the spectral energy distributions (SEDs) of AGNs and their host galaxies. Emission components include radio structures such as lobes and jets, infrared emission from the AGN torus, visible-to-X-ray emission from the accretion disk, and radio-to-ultraviolet emission from the host galaxy. Applying ARXSED to the radio SEDs of 20 3CRR quasars at 1 < z < 2 verifies the need for more than a simple power law when compact radio structures are present. The nonthermal emission contributes 91%–57% of the observed-frame 1.25 mm to 850 μm flux, and this component must be accounted for when using these wavelengths to estimate star formation properties. We predict the presence of strong radio-linked X-ray emission in more than half the sample sources. ARXSED estimates median (and the associated first and third quartile ranges) BH mass of ${2.9}_{1.7}^{6.0}\times {10}^{9}\,{M}_{\odot }$ , logarithm of Eddington ratio of $-{1.0}_{-1.2}^{-0.6}$ , and spin of ${0.98}_{0.94}^{0.99}$ for our sample. The inferred AGN torus and accretion disk parameters agree with those estimated from spectroscopic analyses of similar samples in the literature. We present the median intrinsic SED of the luminous radio-loud quasars at 1 < z ≲ 2; this SED represents a significant improvement in the way each component is modeled.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2024

Comparisons Between Resolved Star Formation Rate and Gas Tracers in the Strongly Lensed Galaxy SDSS J0901+1814 at Cosmic Noon

We report new radio observations of SDSS J090122.37+181432.3, a strongly lensed star-forming galaxy at $z=2.26$. We image 1.4 GHz (L-band) and 3 GHz (S-band) continuum using the VLA and 1.2 mm (band 6) continuum with ALMA, in addition to the CO(7-6) and CI(${\rm ^3P_2\rightarrow ^3\!P_1}$) lines, all at $\lesssim1.^{\prime\prime}7$ resolution. Based on the VLA integrated flux densities, we decompose the radio spectrum into its free-free (FF) and non-thermal components. The infrared-radio correlation (IRRC) parameter $q_{\rm TIR}=2.65_{-0.31}^{+0.24}$ is consistent with expectations for star forming galaxies. We obtain radio continuum-derived SFRs that are free of dust extinction, finding $\rm {620}_{-220}^{+280}\,M_\odot\,yr^{-1}$, $\rm {230}_{-160}^{+570}\,M_\odot\,yr^{-1}$, and $\rm {280}_{-120}^{+460}\,M_\odot\,yr^{-1}$ from the FF emission, non-thermal emission, and when accounting for both emission processes, respectively, in agreement with previous results. We estimate the gas mass from the CI(${\rm ^3P_2\rightarrow ^3\!P_1}$) line as $M_{\rm gas}=(1.2\pm0.2)\times10^{11}\,M_\odot$, which is consistent with prior CO(1-0)-derived gas masses. Using our new IR and radio continuum data to map the SFR, we assess the dependence of the Schmidt-Kennicutt relation on choices of SFR and gas tracer for $\sim{\rm kpc}$ scales. The different SFR tracers yield different slopes, with the IR being the steepest, potentially due to highly obscured star formation in J0901. The radio continuum maps have the lowest slopes and overall fidelity for mapping the SFR, despite producing consistent total SFRs. We also find that the Schmidt-Kennicutt relation slope is flattest when using CO(7-6) or CI(${\rm ^3P_2\rightarrow ^3\!P_1}$) to trace gas mass, suggesting that those transitions are not suitable for tracing the bulk molecular gas in galaxies like J0901.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2024

A Model for Pair Production Limit Cycles in Pulsar Magnetospheres

Abstract It was recently proposed that the electric field oscillation as a result of self-consistent e ± pair production may be the source of coherent radio emission from pulsars. Direct particle-in-cell simulations of this process have shown that the screening of the parallel electric field by this pair cascade manifests as a limit cycle, as the parallel electric field is recurrently induced when pairs produced in the cascade escape from the gap region. In this work, we develop a simplified time-dependent kinetic model of e ± pair cascades in pulsar magnetospheres that can reproduce the limit-cycle behavior of pair production and electric field screening. This model includes the effects of a magnetospheric current, the escape of e ± , as well as the dynamic dependence of pair production rate on the plasma density and energy. Using this simple theoretical model, we show that the power spectrum of electric field oscillations averaged over many limit cycles is compatible with the observed pulsar radio spectrum.

Astronomy & Astrophysics↗
DOE OSTI2023

The 2022 High-energy Outburst and Radio Disappearing Act of the Magnetar 1E 1547.0–5408

We report the radio and high-energy properties of a new outburst from the radio-loud magnetar 1E 1547.0–5408. Following the detection of a short burst from the source with Swift-BAT on 2022 April 7, observations by NICER detected an increased flux peaking at (6.0 ± 0.4) × 10 –11 erg s –1 cm –2 in the soft X-ray band, falling to a baseline level of 1.7 × 10 –11 erg s –1 cm –2 over a 17 day period. Joint spectroscopic measurements by NICER and NuSTAR indicated no change in the hard nonthermal tail despite the prominent increase in soft X-rays. Observations at radio wavelengths with Murriyang, the 64 m Parkes radio telescope, revealed that the persistent radio emission from the magnetar disappeared at least 22 days prior to the initial Swift-BAT detection and was redetected two weeks later. Such behavior is unprecedented in a radio-loud magnetar, and may point to an unnoticed slow rise in the high-energy activity prior to the detected short bursts. Finally, our combined radio and X-ray timing revealed the outburst coincided with a spin-up glitch, where the spin frequency and spin-down rate increased by 0.2 ± 0.1 μHz and (–2.4 ± 0.1) × 10 –12 s –2 , respectively. A linear increase in the spin-down rate of (–2.0 ± 0.1) × 10 –19 s –3 was also observed over 147 days of postoutburst timing. Our results suggest that the outburst may have been associated with a reconfiguration of the quasi-polar field lines, likely signaling a changing twist, accompanied by spatially broader heating of the surface and a brief quenching of the radio signal, yet without any measurable impact on the hard X-ray properties.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2020

Observations of the Hydrogen Cyanide in Comet 46P/Wirtanen at a 3.4 mm Wavelength

We searched for rotational lines of H{sup 12}C{sup 14}N in comet 46P/Wirtanen, using the Purple Mountain Observatory 13.7 m radio telescope. The J = 1–0 lines of HCN (3.4 mm wavelength) were unambiguously detected in 46P during the 2018 December 14–15 period, with a beam resolution of 62.″2 and a velocity resolution of ∼0.21 km s{sup −1} per channel. The molecular production rates were computed. The mean HCN(1–0) production rate of 46P was (6.45 ± 1.31) × 10{sup 24} molec s{sup −1} in the above two days. We found possible indications of time variation in the outgassing activity. The average HCN abundance relative to water Q {sub [HCN]}/Q{sub [H{sub 2O]}} of 46P was (0.09 ± 0.01)%. The HCN line shapes gave primary information on the kinematics of the cometary atmosphere. The HCN linewidths provided a direct determination of the coma expansion velocity, which was about 0.5 ± 0.05 km s{sup −1} at post-perihelion (1.056 au).

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2020

Time-variable Radio Recombination Line Emission in W49A

We present new Jansky Very Large Array (VLA) images of the central region of the W49A star-forming region at 3.6 cm and at 7 mm at resolutions of 0.″15 (1650 au) and 0.″04 (440 au), respectively. The 3.6 cm data reveal new morphological detail in the ultracompact H ii region population, as well as several previously unknown and unresolved sources. In particular, source A shows elongated, edge-brightened bipolar lobes, indicative of a collimated outflow, and source E is resolved into three spherical components. We also present VLA observations of radio recombination lines at 3.6 cm and 7 mm, and IRAM Northern Extended Millimeter Array (NOEMA) observations at 1.2 mm. Three of the smallest ultracompact H ii regions (sources A, B2, and G2) all show broad kinematic linewidths, with ΔV {sub FWHM} ≳ 40 km s{sup −1}. A multi-line analysis indicates that broad linewidths remain after correcting for pressure broadening effects, suggesting the presence of supersonic flows. Substantial changes in linewidth over the 21 yr time baseline at both 3.6 cm and 7 mm are found for source G2. At 3.6 cm, the linewidth of G2 changed from 31.7 ± 1.8 km s{sup −1} to 55.6 ± 2.7 km s{sup −1}, an increase of +23.9 ± 3.4 km s{sup −1}. The G2 source was previously reported to have shown a 3.6 cm continuum flux density decrease of 40% between 1994 and 2015. This source sits near the center of a very young bipolar outflow whose variability may have produced these changes.

79 ASTRONOMY AND ASTROPHYSICS↗