12 research outputs found
Full Mueller imaging: direction dependent corrections in polarimetric radio imaging
Magnetic fields pervade the universe, spanning a multitude of scales from the dipolar field on Earth, to the largest gravitationally bound structures such as galaxy clusters [1]. The magnetic fields play a vital role in the evolution of these astronomical systems. In addition to the multitude of scales, magnetic fields are present in different astronomical systems of varying strengths. The strongest observed astronomical magnetic fields are in neutron stars with a field strength of â 1015 G [2], far higher than any man-made fields till date. In stark contrast magnetic fields in the interstellar medium while ubiquitous are only a few ”G in field strength. Many fundamental processes in astrophysics have magnetism at their heart, be it cosmic ray particle acceleration, star formation, or the launch of radio galaxy jets, pulsars, etc. One key fundamental process that allows us to detect and characterize cosmic magnetic fields with radio astronomy is the polarization of synchrotron radiation. Synchrotron radiation is intrinsically polarized broadband continuum radiation emitted by relativistic charged particles accelerated by the presence of magnetic fields. The emissivity of the synchrotron radiation is tied to the magnetic field strength B and the spectral index α (defined such that the flux density S â Îœ âα ) such that Δ â B 1+α
A Search for Pulsars Towards the Galactic Center
We present observations from two separate methods for observing the Galactic Center in an attempt to characterize its pulsar and neutron star populations. A persistent puzzle of the past 20 years has been the lack of pulsar detections towards the Galactic Center, specifically within a few parsecs of the central supermassive black hole Sgr A*. This object is bright in the total intensity of its polarized emission, but is very weakly linearly polarized. We take advantage of these circumstances in an experimental search technique where we utilize the Faraday effect in an attempt to detect high rotation measure (RM) point sources towards the Galactic Center, as the few pulsars that have been detected in this region have all been measured at a high RM. We also conduct a wide-field search of the 5âŠarea around the Galactic Center at low frequencies (230-470MHz) and at multiple epochs in an attempt to detect transient sources and other significant emitters of synchrotron radiation
Completing the Census of AGN in GOODS-S/HUDF: New Ultra-Deep Radio Imaging and Predictions for JWST
A global understanding of Active Galactic Nuclei (AGN) and their host
galaxies hinges on completing a census of AGN activity without selection biases
down to the low-luminosity regime. Toward that goal, we identify AGN within
faint radio populations at cosmic noon selected from new ultra-deep, high
resolution imaging from the Karl G. Jansky Very Large Array at 6 and 3 GHz.
These radio data are spatially coincident with the ultra-deep legacy surveys in
the GOODS-S/HUDF region, particularly the unparalleled Chandra 7 Ms X-ray
imaging. Combined, these datasets provide a unique basis for a thorough census
of AGN, allowing simultaneous identification via (1) high X-ray luminosity; (2)
hard X-ray spectra; (3) excess X-ray relative to 6 GHz; (4) mid-IR colors; (5)
SED fitting; (6) radio excess via the radio-infrared relation; (7) flat radio
spectra via multi-band radio; and (8) optical spectroscopy. We uncover AGN in
fully half our faint radio sample, indicating a source density of one AGN
arcmin, with a similar number of radio-undetected AGN identified via
X-ray over the same area. Our radio-detected AGN are majority radio-quiet, with
radio emission consistent with being powered predominantly by star formation.
Nevertheless, we find AGN radio signatures in our sample: with radio
excess indicating radio-loud activity and of radio-quiet AGN
candidates with flat or inverted radio spectra. The latter is a lower limit,
pending our upcoming deeper 3 GHz survey. Finally, despite these extensive
datasets, this work is likely still missing heavily obscured AGN. We discuss in
detail this elusive population and the prospects for completing our AGN census
with JWST/MIRI.Comment: Accepted for publication in ApJ. 30 pages, 13 figures, 2 tables, 2
appendices. Minor update to fix typos and better match published versio
Discovery, Timing, and Multiwavelength Observations of the Black Widow Millisecond Pulsar PSR J1555-2908
We report the discovery of PSR J1555-2908, a 1.79 ms radio and gamma-ray pulsar in a 5.6 hr binary system with a minimum companion mass of 0.052 M â. This fast and energetic ( Ä=3Ă1035 erg s-1) millisecond pulsar was first detected as a gamma-ray point source in Fermi Large Area Telescope (LAT) sky survey observations. Guided by a steep-spectrum radio point source in the Fermi error region, we performed a search at 820 MHz with the Green Bank Telescope that first discovered the pulsations. The initial radio pulse timing observations provided enough information to seed a search for gamma-ray pulsations in the LAT data, from which we derive a timing solution valid for the full Fermi mission. In addition to the discovery and timing of radio and gamma-ray pulsations, we searched for X-ray pulsations using NICER but no significant pulsations were detected. We also obtained time-series r-band photometry that indicates strong heating of the companion star by the pulsar wind. Material blown off the heated companion eclipses the 820 MHz radio pulse during inferior conjunction of the companion for â10% of the orbit, which is twice the angle subtended by its Roche lobe in an edge-on system. © 2022. The Author(s). Published by the American Astronomical Society
Discovery, Timing, and Multiwavelength Observations of the Black Widow Millisecond Pulsar PSR J1555-2908
We report the discovery of PSR J1555-2908, a 1.79 ms radio and gamma-ray
pulsar in a 5.6 hr binary system with a minimum companion mass of 0.052
. This fast and energetic ( erg/s)
millisecond pulsar was first detected as a gamma-ray point source in Fermi LAT
sky survey observations. Guided by a steep spectrum radio point source in the
Fermi error region, we performed a search at 820 MHz with the Green Bank
Telescope that first discovered the pulsations. The initial radio pulse timing
observations provided enough information to seed a search for gamma-ray
pulsations in the LAT data, from which we derive a timing solution valid for
the full Fermi mission. In addition to the radio and gamma-ray pulsation
discovery and timing, we searched for X-ray pulsations using NICER but no
significant pulsations were detected. We also obtained time-series r-band
photometry that indicates strong heating of the companion star by the pulsar
wind. Material blown off the heated companion eclipses the 820 MHz radio pulse
during inferior conjunction of the companion for ~10% of the orbit, which is
twice the angle subtended by its Roche lobe in an edge-on system.Comment: 15 pages, 6 figures, accepted by Ap
Alignments of Radio Source in the ELAIS N1 Deep Field
Results of the AGN jets position angle alignments in the ELAIS N1 Deep radio survey are presented here. The ELAIS N1 deep radio survey was carried out with the Giant Meter- wave Radio Telescope at 615 MHz. The deep field is a seven pointing mosaic that covers 1.2 sq.deg centred at α2000 = 16h10m35s and ÎŽ2000 = 54d35âČ with an average angular resolution of 5â Ă 5â across the mosaic. The average sensitivity of the mosaicked image is 10ÎŒJy/beam in Stokes I. There were 65 extended radio galaxy jets extracted from the image of which 33 galaxies have redshift information available. The positions angles of radio galaxy jets are expected to be uniform. The radio galaxy jets in our sample were found to deviate from uniform distribution of positions angles to 99 percent significance level. Further testing with angular covariance reveals alignments in radio position angles, across angular scales of 1.2 to 1.8 Ă . Position angle correlations at scales of 1.2 to 1.8 translates to a comoving scale of 50-75 Mpc/h at z = 1. These results corroborate prior evidence for large scale alignments in quasar optical polarization derived by Hutsem Ìekers (1998)
Direction-dependent corrections in polarimetric radio imaging. Iii. A-to-Z solverâ Modeling the full jones antenna aperture illumination pattern
In this third paper of a series describing direction-dependent corrections for polarimetric radio imaging, we present
the the A-to-Z solver methodology to model the full Jones antenna aperture illumination pattern (AIP) using
Zernike polynomials. In order to achieve accurate, thermal noise-limited imaging with modern radio
interferometers, it is necessary to correct for the instrumental effects of the antenna primary beam (PB) as a
function of time, frequency, and polarization. The algorithm employs the orthonormal, circular Zernike polynomial
basis to model the full Jones AIP response, which is obtained by a Fourier transform of corresponding antenna
holography measurements. These full Jones models are then used to reconstruct the full Mueller AIP response of
an antenna, in principle accounting for all the off-axis frequency-dependent leakage effects of the PB. The A-to-Z
solver is general enough to accommodate any interferometer for which holographic measurements exist, and we
have successfully modeled the AIP of the VLA, MeerKAT, and ALMA as a demonstration of its versatility. We
show that our models capture the PB morphology to high accuracy within the first two side lobes, and show the
viability of full Mueller gridding and deconvolution for any telescope given high-quality holographic
measurements
Multi-Wavelength Study of a Proto-BCG at z = 1.7
In this work we performed a spectral energy distribution (SED) analysis in the optical/infrared band of the host galaxy of a proto-brightest bluster galaxy (BCG, NVSS J103023 + 052426) in a proto-cluster at z = 1.7. We found that it features a vigorous star formation rate (SFR) of ∼570 M⊙/yr and a stellar mass of M*∼3.7×1011M⊙; the high corresponding specific SFR = 1.5±0.5Gyr−1 classifies this object as a starburst galaxy that will deplete its molecular gas reservoir in ∼3.5×108 yr. Thus, this system represents a rare example of a proto-BCG caught during the short phase of its major stellar mass assembly. Moreover, we investigated the nature of the host galaxy emission at 3.3 mm. We found that it originates from the cold dust in the interstellar medium, even though a minor non-thermal AGN contribution cannot be completely ruled out. Finally, we studied the polarized emission of the lobes at 1.4 GHz. We unveiled a patchy structure where the polarization fraction increases in the regions in which the total intensity shows a bending morphology; in addition, the magnetic field orientation follows the direction of the bendings. We interpret these features as possible indications of an interaction with the intracluster medium. This strengthens the hypothesis of positive AGN feedback, as inferred in previous studies of this object on the basis of X-ray/mm/radio analysis. In this scenario, the proto-BCG heats the surrounding medium and possibly enhances the SFR in nearby galaxies