Pulsed Optical Emission Research Articles

A Magnetic White Dwarf Accretion Model for the Anomalous X-Ray Pulsar 4U 0142+61

Abstract The quiescent emission of the anomalous X-ray pulsar (AXP) 4U 0142+61 extends over a broad range of energy, from radio up to hard X-rays. In particular, this object is unique among soft gamma-ray repeaters (SGRs) and AXPs in presenting simultaneously mid-infrared emission and pulsed optical emission. In spite of the many propositions to explain this wide range of emission, it still lacks one that reproduces all of the observations. Filling this gap, we present a model to reproduce the quiescent spectral energy distribution of 4U 0142+61 from mid-infrared up to hard X-rays using plausible physical components and parameters. We propose that the persistent emission comes from a magnetic accreting white dwarf (WD) surrounded by a debris disk. This model assumes that (i) the hard X-rays are due to the bremsstrahlung emission from the postshock region of the accretion column, (ii) the soft X-rays are originated by hot spots on the WD surface, and (iii) the optical and infrared emissions are caused by an optically thick dusty disk, the WD photosphere, and the tail of the postshock region emission. In this scenario, the fitted model parameters indicate that 4U 0142+61 harbors a fast-rotator magnetic near-Chandrasekhar WD, which is very hot and hence young. Such a WD can be the recent outcome of a merger of two less massive WDs. In this case, 4U 0142+61 can evolve into a supernova Ia and hence give hints of the origin of these important astrophysical events. Additionally, we also present a new estimate of 4U 0142+61’s distance, kpc, based on the measured hydrogen column density and new interstellar extinction 3D maps.

On the Polarized Nonthermal Emission from AR Scorpii

We study linear polarization of optical emission from white dwarf (WD) binary system AR~Scorpii. The optical emission from this binary is modulating with the beat frequency of the system, and it is highly polarized, with the degree of the polarization reaching $\sim 40$\%. The angle of the polarization monotonically increases with the spin phase, and the total swing angle can reach $360^{\circ}$ over one spin phase. It is also observed that the morphology of the pulse profile and the degree of linear polarization evolve with the orbital phase. These polarization properties can constrain the scenario for nonthermal emission from AR Scorpii. In this paper, we study the polarization properties predicted by the emission model, in which (i) the pulsed optical emission is produced by the synchrotron emission from relativistic electrons trapped by magnetic field lines of the WD and (ii) the emission is mainly produced at magnetic mirror points of the electron motion. We find that this model can reproduce the large swing of the polarization angle, provided that the distribution of the initial pitch angle of the electrons that are leaving the M-type star is biased to a smaller angle rather than a uniform distribution. The observed direction of the swing suggests that the Earth viewing angle is less than $90^{\circ}$ measured from the WD spin axis. The current model prefers an Earth viewing angle of $50^{\circ}-60^{\circ}$ and a magnetic inclination angle of $50^{\circ}-60^{\circ}$ (or $120^{\circ}-130^{\circ}$). We discuss that the different contribution of the emission from M-type star to total emission causes a large variation in the pulsed fraction and the degree of the linear polarization along the orbital phase.

On the Infrared, Optical, and High‐Energy Emission from the Anomalous X‐Ray Pulsar 4U 0142+61

We show that the observed pulsed optical emission of the anomalous X-ray pulsar 4U 0142+61 can be accounted for by both the magnetar outer gap models and the disk-star dynamo gap models; therefore, there is no evidence favoring only one of these models as its responsible mechanism. Nevertheless, the estimated high-energy gamma-ray spectra from these models have different power-law indices and can be tested by future observations with the Gamma-Ray Large Area Space Telescope (GLAST). Furthermore, we show by analytical estimations that the expectations of a standard disk model are in agreement with the observed unpulsed optical and infrared luminosities of the AXP 4U 0142+61.

Pulsed optical emission by radio pulsars

The luminosity L of radio pulsars due to synchrotron radiation by the primary beam at the magnetosphere periphery is derived. There is a strong correlation between the observed optical luminosities of radio pulsars and the parameter $$\dot P/P^4$$ (where P is the pulsar period). This correlation predicts appreciable optical emission from several dozen pulsars, in particular, from all those with P<0.1 s. Agreement with optical observations can be achieved for Lorentz factors of the secondary plasma γp=2–13. Plasma with such energies can be produced only when the magnetic-field structure near the neutron-star surface deviates substantially from a dipolar field. The peak frequency of the synchrotron spectrum should shift toward higher values as the pulsar period P decreases; this is, in agreement with observational data for 27 radio pulsars for which emission has been detected outside the radio band.

Implications of the Optical Observations of Isolated Neutron Stars

We show that observations of pulsars with pulsed optical emission indicate that the peak flux scales according to the magnetic field strength at the light cylinder. The derived relationships indicate that the emission mechanism is common across all of the observed pulsars with periods ranging from 33 to 385 ms and ages of 1000-300,000 yr. It is noted that similar trends exist for γ-ray pulsars. Furthermore, the model proposed by Pacini and developed by Pacini and Salvati still has validity and gives an adequate explanation of the optical phenomena.

CLYPOS: the Cananea-Ljubljana Young Pulsar Optical Survey

AbstractThe Cananea-Ljubljana Young Pulsar Optical Survey, or CLYPOS, is an observational study of ≳ 30 known radio pulsars, in search for pulsed optical emission. The survey is being carried out at the 2.12m telescope of the Observatorio Astrofísico Guillermo Haro, near Cananea, Sonora, using the stroboscopic camera of the University of Ljubljana. We restricted the study to young pulsars with declinationδ&amp;gt; −35°, prioritized in terms ofE/d2. Our strategy consists in spending two hours of observing time per pulsar, scanning the whole pulsed-phase interval three times. Further details are given below.

Emission spectroscopic studies of CO 2-laser induced graphite plasmas

Using a pulsed high-energy CO 2 laser under focussed condition to evaporate graphite in vacuo, as well as in helium and nitrogen atmosphere, we observed pulsed optical emission spectra in the visible region. In the spectra atomic and diatomic carbon emission, atomic helium and nitrogen lines, as well as broad continua were observed. From CN molecular emission, an approximate plasma temperature of 5000 K is deduced, and the sources for the continuum background are considered.

Pulsed Optical Emission from PSR 0656+14

Using data collected with the Special Astrophysical Observatory (SAO) 6 m telescope and the University College Galway (UCG) Transputer Instrument for Fast Image Deconvolution (TRIFFID) imaging photometer, we show that the radio, X-ray, and γ-ray pulsar PSR 0656+14 exhibits pulsed optical emission. We observed that the pulsed fraction was consistent with 100% and that the flux was higher than that expected from a thermal source. The magnitude of PSR 0656+14 in the B band was observed to be 25.1 ± 0.3, consistent with previous CCD observations. The peak of the optical signal is at phase 0.2 compared to the radio and in phase with the weak γ-ray pulse.

On the origin of pulsed emission from the young supernova remnant SN 1987A

To overcome difficulties in understanding the origin of the sub-msec optical pulses from SN 1987A, a model similar to that of Kundt and Krotscheck (1977) for pulsed synchotron emission from the Crab is applied. The interaction of the expected ultrarelativistic electron-positron pulsar wind with pulsar dipole EM wave or wind-carried toroidal magnetic field reflected from the walls of the expected pulsar cavity within the SN 1987A nebula can generate pulsed optical emission with efficiency at most 0.001. The maximum luminosity of the source is reproduced, and other observational constraints can be satisfied for an average wind energy flow of about 10 to the 38th erg/sec sr and for wind electron Lorentz factor gamma of about 100,000. This model applied to the Crab yields pulsations of much lower luminosity and frequency.

H2215-086/FO AQR: The "king" of intermediate polars spins down

Our 5-color photometric observations of the intermediate polar H2215-086/FO Aqr which were collected between 1982 and the end of 1985 reveal that the 20.9 min rotational periodincreases with a spin-down rate of (1.0±0.2) 10−10 ss−1. This is in marked contrast to the spin-up behaviour of other well studied DQ Her systems. The pulse profile of the optical modulation as a function of wavelength changes drastically across the Balmer jump suggesting the presence of two distinct components of pulsed optical emission which furthermore are shifted in phase with respect to the X-ray maxima observed with EXOSAT. The UV, optical and IR flux distribution can adequately be accounted for by an accretion disc model with a mass-flow rate $$\dot M$$ = 1017 g s−1 and an inner boundary which is ≈ 10 times larger than the radius of the white dwarf. Our results support the magnetic accretion torque theory worked out by Ghosh and Lamb which can account for both spin-up and spin-down of the accreting star depending on the value of the fastness parameters ωs. We derive a magnetic moment μ ≃ 1.0 1033 G cm3 which will allow H2216-086 to evolve into an AM Her system once it has crossed the 2–3 h period gap.

H2215−086 / Fo Aqr: The “King” of Intermediate Polars Spins Down

AbstractOur 5-color photometric observations of the intermediate polar H2215−086/FO Aqr which were collected between 1982 and the end of 1985 reveal that the 20.9 min rotational period increases with a spin-down rate of (1.0±0.2) 10−10 ss−1. This is in marked contrast to the spin-up behaviour of other well studied DQ Her systems.The pulse profile of the optical modulation as a function of wavelength changes drastically across the Balmer jump suggesting the presence of two distinct components of pulsed optical emission which furthermore are shifted in phase with respect to the X-ray maxima observed with EXOSAT.The UV, optical and IR flux distribution can adequately be accounted for by an accretion disc model with a mass-flow rate Ṁ = 1017 g s−1 and an inner boundary which is ≈10 times larger than the radius of the white dwarf. Our results support the magnetic accretion torque theory worked out by Ghosh and Lamb which can account for both spin-up and spin-down of the accreting star depending on the value of the fastness parameter ωs.We derive a magnetic moment μ ≈ 1.0 1033 G cm3 which will allow H2215−086 to evolve into an AM Her system once it has crossed the 2–3 h period gap.

Optical pulsations in the Large Magellanic Cloud remnant 0540–69.3

We have detected pulsed optical emission from the Large Magellanic Cloud (LMC) X-ray pulsar PSR 0540-693 (Seward et al. 1984). The pulsed emission has a time averaged magnitude of approximately 22.7. The X-ray pulsar was discovered in the LMC remnant, 0540-69.3 as a pulse repetition period of approx. 50 milliseconds (ms) in Einstein Obsrvatory data (Seward et al. 1984). Earlier, Clark et al. (1982) had noted that this remnant resembles the Crab Nebula because of the X-ray power law spectrum, and suggested that the nebular emission was synchrotron radiation powered by a central pulsar. After the announcement of X-ray pulsed emission, Chanan et al. (1984) measured the broad optical band properties of the nebula and found evidence for synchrotron emission. They reported that the 4.5 arc second continuum emission remnant has only a tenth the luminosity of the Crab Nebula. We have recorded broad-band optical time-series data at 1 ms intervals with the 4-m and 1.5-m Cerro Tololo telescopes and have found strong pulsations, employing the usual Fourier transform methods. A summary of the observations, including magnitudes, barycentric frequencies and times of arrival is given.

Detection of optical pulses from the Vela pulsar

PULSED optical emission has been detected from the Vela pulsar, PSR 0833–45. Double pulses were detected from an area which included Lasker's1 star M. The pulsed emission has a time-averaged blue magnitude of approximately 25.2 min.