Archival Optical Observations Research Articles

CHIME Discovery of a Binary Pulsar with a Massive Nondegenerate Companion

Of the more than 3000 radio pulsars currently known, only ∼300 are in binary systems, and only five of these consist of young pulsars with massive nondegenerate companions. We present the discovery and initial timing, accomplished using the Canadian Hydrogen Intensity Mapping Experiment (CHIME) telescope, of the sixth such binary pulsar, PSR J2108+4516, a 0.577 s radio pulsar in a 269 day orbit of eccentricity 0.09 with a companion of minimum mass 11 M ⊙. Notably, the pulsar undergoes periods of substantial eclipse, disappearing from the CHIME 400–800 MHz observing band for a large fraction of its orbit, and displays significant dispersion measure and scattering variations throughout its orbit, pointing to the possibility of a circumstellar disk or very dense stellar wind associated with the companion star. Subarcsecond resolution imaging with the Karl G. Jansky Very Large Array unambiguously demonstrates that the companion is a bright, V ≃ 11 OBe star, EM* UHA 138, located at a distance of 3.26(14) kpc. Archival optical observations of EM* UHA 138 approximately suggest a companion mass ranging from 17.5 M ⊙ < M c < 23 M ⊙, in turn constraining the orbital inclination angle to 50.°3 ≲ i ≲ 58.°3. With further multiwavelength follow-up, PSR J2108+4516 promises to serve as another rare laboratory for the exploration of companion winds, circumstellar disks, and short-term evolution through extended-body orbital dynamics.

Revived fossil plasma sources in galaxy clusters

It is well established that particle acceleration by shocks and turbulence in the intra-cluster medium can produce cluster-scale synchrotron emitting sources. However, the detailed physics of these particle acceleration processes is still not well understood. One of the main open questions is the role of fossil relativistic electrons that have been deposited in the intracluster medium (ICM) by radio galaxies. These synchrotron-emitting electrons are very difficult to study as their radiative lifetime is only tens of Myr at gigahertz frequencies, and they are therefore a relatively unexplored population. Despite the typical steep radio spectrum due to synchrotron losses, these fossil electrons are barely visible even at radio frequencies well below the gigahertz level. However, when a pocket of fossil radio plasma is compressed, it boosts the visibility at sub-gigahertz frequencies, creating what are known as radio phoenices. This compression can be the result of bulk motion and shocks in the ICM due to merger activity. In this paper we demonstrate the discovery potential of low-frequency radio sky surveys to find and study revived fossil plasma sources in galaxy clusters. We used the 150 MHz TIFR GMRT Sky Survey and the 1.4 GHz NVSS sky survey to identify candidate radio phoenices. A subset of three candidates was studied in detail using deep multi-band radio observations (LOFAR and GMRT), X-ray obserations (Chandra or XMM-Newton), and archival optical observations. Two of the three sources are new discoveries. Using these observations, we identified common observational properties (radio morphology, ultra-steep spectrum, X-ray luminosity, dynamical state) that will enable us to identify this class of sources more easily, and will help us to understand the physical origin of these sources.

Multiband Optical Light Curves of Black-widow Pulsars

Abstract We collect new and archival optical observations of nine “black-widow” millisecond pulsar binaries. New measurements include direct imaging with the Keck, Gemini-S, MDM, and Las Cumbres Observatory 2 m telescopes. This is supplemented by synthesized colors from Keck long-slit spectra. Four black-widow optical companions are presented here for the first time. Together these data provide multicolor photometry covering a large fraction of the orbital phase. We fit these light curves with a direct (photon) heating model using a version of the ICARUS light-curve modeling code. The fits provide distance and fill-factor estimates, inclinations, and heating powers. We compare the heating powers with the observed GeV luminosities, noting that the ratio is sensitive to pulsar distance and to the gamma-ray beaming. We make a specific correction for “outer gap” model beams, but even then some sources are substantially discrepant, suggesting imperfect beaming corrections and/or errors in the fit distance. The fits prefer large metal abundance for half of the targets, a reasonable result for these wind-stripped secondaries. The companion radii indicate substantial Roche-lobe filling, f c ≈ 0.7−1 except for PSR J0952−0607, which with f c &lt; 0.5 has a companion density ρ ≈ 10 g cm−3, suggesting unusual evolution. We note that the direct-heating fits imply large heating powers and rather small inclinations, and we speculate that unmodeled effects can introduce such bias.

Reconciling Optical and Radio Observations of the Binary Millisecond Pulsar PSR J1640+2224

Abstract Previous optical and radio observations of the binary millisecond pulsar PSR J1640+2224 have come to inconsistent conclusions about the identity of its companion, with some observations suggesting that the companion is a low-mass helium-core (He-core) white dwarf (WD), while others indicate that it is most likely a high-mass carbon–oxygen (CO) WD. Binary evolution models predict PSR J1640+2224 most likely formed in a low-mass X-ray binary based on the pulsar’s short spin period and long-period, low-eccentricity orbit, in which case its companion should be a He-core WD with mass about 0.35–0.39 M ⊙, depending on metallicity. If instead it is a CO WD, it would suggest that the system has an unusual formation history. In this paper we present the first astrometric parallax measurement for this system from observations made with the Very Long Baseline Array (VLBA), from which we determine the distance to be . We use this distance and a reanalysis of archival optical observations originally taken in 1995 with the Wide Field Planetary Camera 2 on the Hubble Space Telescope (HST) to measure the WD’s mass. We also incorporate improvements in calibration, extinction model, and WD cooling models. We find that the existing observations are not sufficient to tightly constrain the companion mass, but we conclude the WD mass is &gt;0.4 M ⊙ with &gt;90% confidence. The limiting factor in our analysis is the low signal-to-noise ratio of the original HST observations.

Magnetically gated accretion in an accreting 'non-magnetic' white dwarf.

White dwarfs are often found in binary systems with orbital periods ranging from tens of minutes to hours in which they can accrete gas from their companion stars. In about 15 per cent of these binaries, the magnetic field of the white dwarf is strong enough (at 106 gauss or more) to channel the accreted matter along field lines onto the magnetic poles. The remaining systems are referred to as 'non-magnetic', because until now there has been no evidence that they have a magnetic field that is strong enough to affect the accretion dynamics. Here we report an analysis of archival optical observations of the 'non-magnetic' accreting white dwarf in the binary system MV Lyrae, whose light curve displays quasi-periodic bursts of about 30 minutes duration roughly every 2 hours. The timescale and amplitude of these bursts indicate the presence of an unstable, magnetically regulated accretion mode, which in turn implies the existence of magnetically gated accretion, in which disk material builds up around the magnetospheric boundary (at the co-rotation radius) and then accretes onto the white dwarf, producing bursts powered by the release of gravitational potential energy. We infer a surface magnetic field strength for the white dwarf in MV Lyrae of between 2 × 104 gauss and 1 × 105 gauss, too low to be detectable by other current methods. Our discovery provides a new way of studying the strength and evolution of magnetic fields in accreting white dwarfs and extends the connections between accretion onto white dwarfs, young stellar objects and neutron stars, for which similar magnetically gated accretion cycles have been identified.

HZ HER: STELLAR RADIUS FROM X-RAY ECLIPSE OBSERVATIONS, EVOLUTIONARY STATE, AND A NEW DISTANCE

Observations of HZ Her/Her X-1 by the Rossi X-ray Timing Explorer (RXTE) covering high state eclipses of the neutron star are analyzed here. Models of the eclipse are used to measure the radius and atmospheric scale height of HZ Her, the stellar companion to the neutron star. The radius is 2.58 to 3.01 $\times10^{11}$ cm, depending on system inclination and mass ratio(q), with accuracy of $\sim$1 part in 1000 for given inclination and $q$. We fit Kurucz model stellar atmosphere models to archival optical observations. The resulting effective temperature ($T_{eff}$) of the unheated face of HZ Her is determined to be in the 2$\sigma$ range 7720K to 7865K, and metallicity ($log(Z/Z_{\odot})$) in the range -0.27 to +.03. The model atmosphere surface flux and new radius yield a new distance to HZ Her/ Her X-1, depending on system inclination and $q$: a best-fit value of 6.1 kpc with upper and lower limits of 5.7 kpc and 7.0 kpc. We calculate stellar evolution models for the range of allowed masses (from orbital parameters), and allowed metallicities (from optical spectrum fits). The stellar models agree with $T_{eff}$ and radius of HZ Her for two narrow ranges of mass: 2.15 to 2.20 $M_{\odot}$ and 2.35 to 2.45 $M_{\odot}$ This lower mass range implies a low neutron star mass (1.3 $M_{\odot}$), whereas the higher mass range implies a high neutron star mass (1.5-1.7 $M_{\odot}$).

VLT/FORS2 observations of four high-luminosity ULX candidates★

We obtained Very Large Telescope/FOcal Reducer and low dispersion Spectrograph spectra of the optical counterparts of four high-luminosity (LX ≥ 1040 erg s−1) Ultraluminous X-ray sources (ULX) candidates from the catalogue of Walton et al. We first determined accurate positions for the X-ray sources from archival Chandra observations and identified counterparts in archival optical observations that are sufficiently bright for spectroscopy with an 8 m telescope. From the spectra we determine the redshifts to the optical counterparts and emission line ratios. One of the candidate ULXs, in the spiral galaxy ESO 306−003, appears to be a bona fide ULX in an H ii region. The other three sources, near the elliptical galaxies NGC 533 and NGC 741 and in the ring galaxy AM 0644−741, turn out to be background active galactic nuclei (AGN) with redshifts of 1.85, 0.88 or 1.75 and 1.40, respectively. Our findings confirm the trend of a high probability of finding background AGN for systems with a ratio of log(FX/Fopt) in the range of −1–1.

Long-term timing of four millisecond pulsars

We have timed four millisecond pulses, PSRs J1721-2457, J1745-0952, J1810-2005, and J1918-0642, for up to a total of 10.5 years each using multiple telescopes in the European Pulsar Timing Array network: the Westerbork Synthesis Radio Telescope in The Netherlands, the Nancay Radio Telescope in France and the Lovell telescope at Jodrell Bank in the UK. The long time span has enabled us to measure the proper motions of J1745-0952 and J1918-0642, indicating that they have transverse velocities of 200(50) and 54(7) km/s respectively. We have obtained upper limits on the proper motion of J1721-2457 and J1810-2005, which imply that they have transverse velocities less than 140 and 400 km/s respectively. In all cases, the velocities lie in the range typical of millisecond pulsars. We present pulse profiles for each pulsar taken from observations at multiple frequencies in the range of 350 to 2600 MHz, and show that J1810-2005 shows significant profile evolution in this range. Using our multi-frequency observations, we measured the spectral indices for all four pulsars, and for J1810-2005 it appears to be very flat. The flux density of J1918-0642 shows extensive modulation which we attribute to the combined effects of refractive and diffractive scintillation. We discuss the possible use of including J1721-2457 or J1918-0642 in a pulsar timing array, and find that J1918-0642 will be useful to include when the timing precision of this pulsar is improved over the next few years. We have searched archival optical observations to detect companions of the binary pulsars, but none were detected. However, we provide lower limits on the masses of the white dwarf companions of PSRs J1745-0952 and J1918-0642.

High-resolution optical and infrared spectroscopic observations of Cir X-1

High-resolution optical and infrared spectroscopic observations of Cir X-1