Superior Conjunction Research Articles

NuSTAR and Parkes observations of the transitional millisecond pulsar binary XSS J12270–4859 in the rotation-powered state

ABSTRACT We report on the first NuSTAR observation of the transitional millisecond pulsar binary XSS J12270–4859 during its current rotation-powered state, complemented with a 2.5 yr-long radio monitoring at Parkes telescope and archival XMM–Newton and Swift X-ray and optical data. The radio pulsar is mainly detected at 1.4 GHz displaying eclipses over $\sim 40{{\ \rm per\ cent}}$ of the 6.91 h orbital cycle. We derive a new updated radio ephemeris to study the 3–79 keV light curve that displays a significant orbital modulation with fractional amplitude of $28\pm 3{{\ \rm per\ cent}}$, a structured maximum centred at the inferior conjunction of the pulsar and no cycle-to-cycle or low–high-flaring mode variabilities. The average X-ray spectrum, extending up to ∼70 keV without a spectral break, is well described by a simple power law with photon index Γ = 1.17 ± 0.08 giving a 3–79 keV luminosity of $\rm 7.6_{-0.8}^{+3.8} \times 10^{32}\, erg\, s^{-1}$ for a distance of 1.37$_{-0.15}^{+0.69}$ kpc. Energy resolved orbital light curves reveal that the modulation is not energy dependent from 3 to 25 keV and is undetected with an upper limit of ${\sim} 10{{\ \rm per\ cent}}$ above 25 keV. Comparison with previous X-ray XMM–Newton observations in common energy ranges confirms that the modulation amplitudes vary on time-scales of a few months, indicative of a non-stationary contribution of the intrabinary shock (IBS) formed by the colliding winds of the pulsar and the companion. A more detailed inspection of energy resolved modulations than previously reported gives hints of a mild softening at superior conjunction of the pulsar below 3 keV, likely due to the contribution of the thermal emission from the neutron star. The IBS emission, if extending into the MeV range, would be energetically capable alone to irradiate the donor star.

VERITAS Detection of LS 5039 and HESS J1825-137

With 8 h of observations, VERITAS confirms the detection of two very high energy gamma-ray sources. The gamma-ray binary LS 5039 is detected with a statistical significance of 8.8σ. The measured flux above 1 TeV is (2.5±0.4)×10−12cm−2s−1 near inferior conjunction and (7.8±2.8)×10−13cm−2s−1 near superior conjunction. The pulsar wind nebula HESS J1825-137 is detected with a statistical significance of 6.7σ and a measured flux above 1 TeV of (3.9±0.8)×10−12cm−2s−1.

Timing of PSR J2055+3829, an eclipsing black widow pulsar discovered with the Nançay Radio Telescope

We report on the timing observations of the millisecond pulsar PSR J2055+3829 originally discovered as part of the SPAN512 survey conducted with the Nançay Radio Telescope. The pulsar has a rotational period of 2.089 ms and is in a tight 3.1 h orbit around a very low mass (0.023 ≤ mc ≲ 0.053 M⊙, 90% c.l.) companion. Our 1.4 GHz observations reveal the presence of eclipses of the radio signal of the pulsar, caused by the outflow of material from the companion, for a few minutes around superior conjunction of the pulsar. The very low companion mass, the observation of radio eclipses, and the detection of time variations of the orbital period establish PSR J2055+3829 as a “black widow” (BW) pulsar. Inspection of the radio signal from the pulsar during ingress and egress phases shows that the eclipses in PSR J2055+3829 are asymmetric and variable, as is commonly observed in other similar systems. More generally, the orbital properties of the new pulsar are found to be very similar to those of other known eclipsing BW pulsars. No gamma-ray source has been detected at the location of the pulsar in recent Fermi-LAT source catalogs. We used the timing ephemeris to search ten years of Fermi Large Area Telescope (LAT) data for gamma-ray pulsations, but were unable to detect any. This non-detection could be a consequence of the large distance of the pulsar compared to those of known gamma-ray millisecond pulsars outside of globular clusters. We finally compared the mass functions of eclipsing and non-eclipsing BW pulsars and confirmed previous findings that eclipsing BWs have higher mass functions than their non-eclipsing counterparts. Larger inclinations could explain the higher mass functions of eclipsing BWs. On the other hand, the mass function distributions of Galactic disk and globular cluster BWs appear to be consistent, suggesting, despite the very different environments, the existence of common mechanisms taking place in the last stages of evolution of BWs.

BER and channel capacity of a deep space FSO communication system using L-PPM-MSK-SIM scheme during superior solar conjunction.

Although free space optical (FSO) communication is a promising technique for deep space communication and it can help in the rapid development of space exploration missions, it encounters coronal turbulence during superior solar conjunction. To improve the bit error rate (BER) performance of FSO communication system under the influence of coronal turbulence, a hybrid modulation scheme, L-PPM-MSK-SIM-which is a combination of pulse position modulation (PPM), minimum shift keying (MSK), and sub-carrier intensity modulation (SIM) techniques-is proposed in this study. Considering various noise sources, both the BER and channel capacity of the communication system are evaluated under the lognormal (LN) turbulence channel. Our simulation results demonstrate that the BER performance with the L-PPM-MSK-SIM scheme is superior to that with L-PPM and BPSK-SIM schemes. In addition, the parameters of the coronal turbulence and FSO communication system have a tremendous influence on the link BER and channel capacity. Moreover, our results also revel that thermal noise is more predominant than the short noise and background noise for the BER performance of deep space FSO communication.

Solar Scintillation Effect for Optical Waves Propagating Through Gamma–Gamma Coronal Turbulence Channels

Optical communication is a promising strategy for deep space exploration but it is susceptible to coronal solar wind turbulence impairments during superior solar conjunction. The variance of amplitude fluctuations caused by coronal turbulence on optical waves propagation is proposed in this paper. Both the generalized non-Kolmogorov coronal turbulence spectrum and the aperture averaging effect are taken into account. The analytic expression of the bit error rate (BER) for free-space optical (FSO) link is then derived based on Gamma–Gamma distribution model under the weak-to-moderate coronal turbulence channels. Numerical evaluations results demonstrate that coronal turbulence with small non-Kolmogorov spectral index, large outer scale, and inner scale has more potential to deduce amplitude fluctuations. With the increment of antenna radius, the amplitude fluctuations decrease obviously. In addition, the variation tendencies of these parameters can further result in small BER. Therefore, the link performance will be improved by decreasing the optical wavelength and enlarging the antenna radius. Our proposed amplitude fluctuations model with aperture averaging effect has potential application for the future deep space FSO communication.

The binary millisecond pulsar PSR J1023+0038 – II. Optical spectroscopy

ABSTRACT We present time-resolved optical spectroscopy of the ‘redback’ binary millisecond pulsar system PSR J1023+0038 during both its radio pulsar (2009) and accretion disc states (2014 and 2016). We provide observational evidence for the companion star being heated during the disc state. We observe a spectral type change along the orbit, from ∼G5 to ∼F6 at the secondary star’s superior and inferior conjunction, respectively, and find that the corresponding irradiating luminosity can be powered by the high-energy accretion luminosity or the spin-down luminosity of the neutron star. We determine the secondary star’s radial velocity semi-amplitude from the metallic (primarily Fe and Ca) and Hα absorption lines during these different states. The metallic and Hα radial velocity semi-amplitude determined from the 2009 pulsar-state observations allows us to constrain the secondary star’s true radial velocity K2 = 276.3 ± 5.6 km s−1 and the binary mass ratio q = 0.137 ± 0.003. By comparing the observed metallic and Hα absorption-line radial velocity semi-amplitudes with model predictions, we can explain the observed semi-amplitude changes during the pulsar state and during the pulsar/disc-state transition as being due to different amounts of heating and the presence of an accretion disc, respectively.

Error performance of deep space optical communication with M-ary pulse position modulation over coronal turbulence channels.

Free space optical (FSO) communication is a promising technology for future deep space exploration, but it encounters coronal turbulence during superior solar conjunction. In this paper, the bit-error rate (BER) performance of deep space FSO communication systems is evaluated for optical waves propagating in the non-Kolmogorov coronal turbulence. By virtue of its high energy efficiency, the pulse position modulation (PPM) technique is adopted to mitigate the influence of turbulence under lognormal distribution channels. The effects of the parameters of the coronal turbulence and of the FSO system, such as the turbulence outer scale, spectral index, symbol number, data bit rate, equivalent load resistor, and average gain, on the BER are investigated and discussed in this paper. In addition, the performance improvement from M-ary PPM and binary phase-shift keying subcarrier intensity modulation is studied. The results of a numerical evaluation illustrate that a deep space FSO communication system with PPM scheme can well be used to mitigate the impact of coronal turbulence.

Effects of Solar Scintillation on Deep Space Communications: Challenges and Prediction Techniques

A reliable deep space communications system plays a key role in extending the range of space communications and has become an indispensable part of space information networks. With the emergence of many deep space exploration projects, it can be envisioned that a new era of research on deep space communications has arrived. However, the performance of deep space communications is severely degraded by solar scintillation, which occurs frequently during superior solar conjunction. In this article, we first review the state of the art of solar scintillation research, including the definition of solar scintillation, influencing factors, assessment methods, and scintillation models. Then an effective solar scintillation prediction model is proposed to overcome the challenges related to solar scintillation for deep space communications. After that, a coronal turbulence channel model is investigated and discussed. Based on this coronal channel model and our proposed solar scintillation model, the link bit error rate performance is further analyzed. Finally, we identify some open problems and research directions that could be investigated for a deep space communications system during superior solar conjunctions, which will support space information networks.

Spacecraft Radio Frequency Fluctuations in the Solar Corona: A MESSENGER–HELIOS Composite Study

Abstract Fluctuations in plasma electron density may play a role in solar coronal energy transport and the dissipation of wave energy. Transcoronal spacecraft radio sounding observations reveal frequency fluctuations (FFs) that encode the electron number density disturbances, allowing an exploration of the coronal compressive wave and advected inhomogeneity models. Primary FF observations from MESSENGER 2009 and published FF residuals from HELIOS 1975–1976 superior conjunctions were combined to produce a composite view of equatorial region FF near solar minimum over solar offset range 1.4–25R ⊙. Methods to estimate the electron number density fluctuation variance from the observed FF were developed. We created a simple stacked, magnetically structured slab model that incorporated both propagating slow density waves and advected spatial density variations to explain the observed FF. Slow density waves accounted for most of the FF at low solar offset, while spatial density inhomogeneities advected at solar wind speed dominated above the sonic point at 6R ⊙. Corresponding spatial scales ranged 1–38 Mm, with scales above 10 Mm contributing most to FF variance. Magnetic structuring of the model introduced radial elongation anistropy at lower solar offsets, but geometric conditions for isotropy were achieved as the slab correlation scales increased further out in the corona. The model produced agreement with the FF observations up to 12R ⊙. FF analysis provides information on electron density fluctuations in the solar corona, and should take into account the background compressive slow waves and solar wind-related advection of quasi-static spatial density variations.

The orbital parameters of the gamma-ray binary LMC P3†

LMC P3 is the most luminous gamma-ray binary discovered to date and the first detected outside of the Galaxy, with an orbital period of 10.301 d. We report on optical spectroscopic observations undertaken with the Southern African Large Telescope (SALT) using the High Resolution spectrograph (HRS). We find the binary is slightly eccentric, $e = 0.40\pm0.07$, and place the time of periastron at HJD $2457412.13 \pm 0.29$. Stellar model fitting finds an effective temperature of $T_{\rm eff} = 36351 \pm 53$ K. The mass function, $f = 0.0010 \pm 0.0004$ M$_{\odot}$, favours a neutron star compact object. The phases of superior and inferior conjunctions are $0.98$ and $0.24$, respectively (where phase 0 is at the Fermi-LAT maximum), close to the reported maxima in the GeV and TeV light curves.

Signatures of Intra-binary Shock Emission in the Black Widow Pulsar Binary PSR J2241−5236

Abstract We report on high-energy properties of the black widow pulsar PSR J2241−5236 in the X-ray and the Fermi-Large Area Telescope (LAT; GeV gamma-ray) bands. In the LAT band, the phase-averaged gamma-ray light curve shows orbital modulation below ∼1 GeV with a chance probability (p) monotonically decreasing with time to p ∼ 10−5. The peak of the light curve is near the superior conjunction of the pulsar (binary phase ϕ B ≈ 0.25). We attribute the modulation to the intra-binary shock (IBS) emission and search for IBS signatures in the archival X-ray data. We find that the X-ray spectral fit requires a nonthermal component, which implies a possible IBS origin of the X-rays. We discuss our observations in the context of IBS scenarios.

Evidence on the Orbital Modulated Gamma-Ray Emissions from the Redback Candidate 3FGL J2039.6–5618

Abstract Using 9.5 yr of Fermi Large Area Telescope data, we report the evidence on the orbital modulated gamma-ray emissions from the redback candidate 3FGL J2039.6–5618. We produced the folded light curve with the orbital period of ∼5.47 hr at a ∼4σ level. We also computed the gamma-ray spectra in two orbital phases corresponding to the inferior conjunction and the superior conjunction. We found that the <3 GeV excess in the spectrum of inferior conjunction can be modeled by the inverse Compton scattering between a relativistic pulsar wind and background soft photons of the companion star. The orbital modulation can also be explained by the evolving collision angle between the particles and photons in the same model. Through period searches by the Rayleigh test and the flux variability, we speculate that the orbital modulation is not detected after MJD ∼57,000. We propose a possible explanation in which the intrabinary shock is located closer to the pulsar so that the pulsar wind carries a smaller Lorentz factor. We estimated that the resultant inverse Compton component will be too soft and too weak to be observed.

Test of general relativity during the BepiColombo interplanetary cruise to Mercury

The theory of general relativity first introduced the concept that space-time is curved by the presence of masses. As a consequence, any signal between two sources passing by a massive body is deflected and therefore experiences a transmission time delay with respect to the straight-line propagation. In the parametrized Post-Newtonian formalism---which makes use of a set of post-Newtonian parameters to describe all metric theories of gravitation that assume Einstein equivalence principle---the curvature of space-time is accounted for by the parameter $\ensuremath{\gamma}$. In general relativity $\ensuremath{\gamma}$ is equal to unity. The measure of $\ensuremath{\gamma}$ is one of the most popular general relativity tests that have been undertaken since 1919. The current state-of-the-art measure of $\ensuremath{\gamma}$ is $\ensuremath{\gamma}=1+(2.1\ifmmode\pm\else\textpm\fi{}2.3)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$, obtained by radio tracking of the NASA/ESA/ASI Cassini-Huygens satellite near solar superior conjunction (Superior Conjunction Experiment---SCE). The ESA/JAXA BepiColombo mission to Mercury will perform a SCE during its cruise-phase. Thanks to a more advanced telecommunication system, made up of a X/X, Ka/Ka, X/Ka multifrequency link, BepiColombo will allow range measurements as accurate as 15 cm at 300 s integration time and range-rate (or Doppler) measurements at the level of $1.5\text{ }\text{ }\ensuremath{\mu}\mathrm{m}/\mathrm{s}$ at 1000 s integration time, one-way. In this work we present the results of the numerical simulations of the BepiColombo SCE with the ORBIT14 orbit determination and parameter estimation software, developed at the University of Pisa. Our software is able to simulate measurements of range and range-rate of the BepiColombo probe during solar superior conjunction and obtain an estimate of $\ensuremath{\gamma}$ based on a non-linear least squares fit. Through a thorough and complete analysis, which includes the study of the impact of possible systematic errors in the range observables, we show that BepiColombo has the capability to improve the result yielded by Cassini up to a factor 10.

Probing f(T) gravity with gravitational time advancement

The effects of a quadratic gravity on astronomical observations and solar system experiments were recently considered in previous works. Its deviation from Einstein’s general relativity is characterized by a model parameter α and the cosmological constant Λ whose constraints were respectively found as m2 and m−2 in the solar system. In this paper, a new test of the gravity by measuring the gravitational time advancement is presented and studied. The gravitational time advancement arises in a two-way light propagation between an observer and a distant spacecraft, where the light traveling time is recorded by the observer’s proper time. It is found that (1) relying on their signs, α and Λ can make the gravitational time advancement smaller or larger than the one of general relativity; (2) the configuration of the inferior conjunction between the observer and the spacecraft is more suitable for detecting the advancement, because its effect is almost 3.5 times larger than the one in the superior conjunction; (3) the time advancement could be effectively complementary to Shapiro time delay for gravitational experiments in practice; and (4) the implement of planetary laser ranging and optical clocks in the future will provide much more insight into the gravity.

Solar Scintillation Effects on the Deep Space Communication Performance for Radio Wave Propagation Through Non-Kolmogorov Turbulence

Solar scintillation refers to the rapid amplitude fluctuations of the radio signal passing through large-scale coronal irregularities. In this letter, the scintillation index model is first derived to characterize radio wave propagation in coronal turbulence during superior solar conjunction. The proposed model takes into account the non-Kolmogorov turbulence model with the Rytov approximation method. After that, the bit error rate (BER) performance is analyzed based on the proposed scintillation index model, which has instructional significance for deep space communication but still remains largely unexplored. Simulation results demonstrate that the coronal parameters have extensive effects on the scintillation index, and further affect the link BER performance. Our proposed scintillation index model also achieves better agreement with the observation data.

Effects of a brane world on gravitational time advancement

Solar System tests of a brane world, which is called DMPR model, were studied in recent works. The correction of DMPR model to the general relativity (GR) in the four-dimensional curved spacetime can be parametrized by a “tidal charge” parameter [Formula: see text]. The parameter [Formula: see text] in this model was obtained and improved as [Formula: see text] by the Earth–Mercury ranging. A new test of the DMPR model based on gravitational time advancement is proposed and investigated in this work. The advancement is a gravitational consequence on round-trip proper time duration of a photon. For ranging a distant spacecraft, it is shown that (1) the “tidal charge” parameter can make the advancement larger or smaller than the one of GR, depending on the sign of [Formula: see text]; (2) the superior conjunction (SC) and the inferior conjunction (IC) are all suitable for detecting the advancement; (3) the advancement can be complementary to the classical test of Shapiro time delay for detecting the brane world; and (4) the implementation of optical clocks and planetary laser ranging will provide more insights on the brane world model in the future.

The First Continuous Optical Monitoring of the Transitional Millisecond Pulsar PSR J1023+0038 with Kepler

Abstract We report on the first continuous, 80-day optical monitoring of the transitional millisecond pulsar PSR J1023+0038 carried out in mid 2017 with Kepler in the K2 configuration, when an X-ray subluminous accretion disk was present in the binary. Flares lasting from minutes to 14 hr were observed for 15.6% of the time, which is a larger fraction than previously reported on the basis of X-ray and past optical observations, and more frequently when the companion was at superior conjunction of the orbit. A sinusoidal modulation at the binary orbital period was also present with an amplitude of ≃16%, which varied by a few percent over timescales of days, and with a maximum that took place 890 ± 85 s earlier than the superior conjunction of the donor. We interpret this phenomena in terms of reprocessing of the X-ray emission by an asymmetrically heated companion star surface and/or a non-axisymmetric outflow possibly launched close to the inner Lagrangian point. Furthermore, the non-flaring average emission varied by up to ≈40% over a timescale of days in the absence of correspondingly large variations of the irradiating X-ray flux. The latter suggests that the observed changes in the average optical luminosity might be due to variations of the geometry, size, and/or mass accretion rate in the outer regions of the accretion disk.

Amplitude fluctuations for optical waves propagation through non-Kolmogorov coronal solar wind turbulence channels.

Optical communication has a great potential for the future deep space communication, while the amplitude fluctuations caused by the coronal solar wind irregularities has been a challenging topic during superior solar conjunction. In this paper, a closed-form amplitude fluctuations expression for optical waves propagation through non-Kolmogorov solar wind turbulence is derived by establishing a generalized coronal turbulence spectrum model. The profound impact of the coronal parameters on the bit error rate (BER) performance of the free space optical system is also investigated based on the derived amplitude fluctuations model. The derived expression allows easy analysis of the evolution of the amplitude fluctuations and, in particular, an understanding of the imposed effects caused by the parameters during the waves propagation. The combined effect of the optical wavelength, non-Kolmogorov spectral index, turbulence outer scale, relative solar wind density fluctuation factor, and link distance on amplitude fluctuations are evaluated. Numerical calculations show that these parameters produce obvious effects on the amplitude fluctuations and the BER. The large optical wavelength can mitigate the influence of the coronal turbulence. Our results have potential applications for evaluating the link performance of the future deep space communication.

A Non-thermal Pulsed X-Ray Emission of AR Scorpii

Abstract We report the analysis result of UV/X-ray emission from AR Scorpii, which is an intermediate polar (IP) composed of a magnetic white dwarf and an M-type star, with the XMM-Newton data. The X-ray/UV emission clearly shows a large variation over the orbit, and their intensity maximum (or minimum) is located at the superior conjunction (or inferior conjunction) of the M star orbit. The hardness ratio of the X-ray emission shows a small variation over the orbital phase and shows no indication of the absorption by an accretion column. These properties are naturally explained by the emission from the M star surface rather than that from the accretion column on the white dwarf’s (WD) star, which is similar to usual IPs. Additionally, the observed X-ray emission also modulates with the WD’s spin with a pulse fraction of ∼14%. The peak position is aligned in the optical/UV/X-ray band. This supports the hypothesis that the electrons in AR Scorpii are accelerated to a relativistic speed and emit non-thermal photons via the synchrotron radiation. In the X-ray bands, evidence of the power-law spectrum is found in the pulsed component, although the observed emission is dominated by the optically thin thermal plasma emissions with several different temperatures. It is considered that the magnetic dissipation/reconnection process on the M star surface heats up the plasma to a temperature of several keV and also accelerates the electrons to the relativistic speed. The relativistic electrons are trapped in the WD’s closed magnetic field lines by the magnetic mirror effect. In this model, the observed pulsed component is explained by the emissions from the first magnetic mirror point.

Theoretical analysis of the angle-of-arrival fluctuations for optical wavext propagation through solar corona turbulence

Optical communication is promising in deep space exploration, but there is the problem of angle-of-arrival fluctuations due to solar scintillation during superior solar conjunction. In this paper, an analytic expression of the angle-of-arrival fluctuations was derived for optical waves propagating through coronal environment by establishing a solar corona non-Kolmogorov turbulence spectrum model. The combined effects of the non-Kolmogorov spectral index, turbulent outer scale, wavelength, and antenna radius are evaluated. Results demonstrate that the spectral index and the turbulent outer scale obviously affect the angle-of-arrival fluctuations. Moreover, minimizing the wavelength and enlarging the receiver aperture can effectively mitigate the angle-of-arrival fluctuations caused by coronal turbulence. It is expected that this work will be beneficial for the investigation of coronal turbulence and deep space optical communication.