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Register a new userA diagnostic on the orbital spectrum of LS 5039 with Fermi-LAT
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LS 5039 is a well-known $gamma$-ray binary system which consists of an unknown compact object and a massive companion O star. It shows rather stable emissions at high energies over years and hence serves as an ideal laboratory to investigate the emission mechanism for such peculiar systems which emit prominent $gamma$-rays. To this end, we take the orbital phase resolved energy spectrum as observed by fermi over 10 years. We divide the orbit into four orbital phases, each with an orbital phase range of 0.25, centered at 0.00, 0.25, 0.50 and 0.75 respectively, where the phase 0.0 is the periastron and phase 0.5 is the apastron. The phases around 0.25 and 0.75 are symmetric and hence are supposed to have identical local acceleration environment. The spectral analysis shows that, the fermi spectra are largely different from these two symmetric orbital phases: the emission from orbital phase 0.25 turns out to be significantly stronger than that from 0.75. This result does not fit a scenario that $gamma$-rays are Doppler boosted emission from bow shock tails if LS 5039 has a shock configuration similar to PSR B1259-63, and indicates that the inverse Compton scatterings between the shock accelerated plasma and the stellar particle environment is the underline procedure. We also find that the previous report for a disappearance of the orbital modulation at 3--20 GeV is due to the similar spectral turn-over energies of the different orbital phases. The spectral properties of periastron and apastron regions are addressed in the context of the measurements in phase regions around 0.25 and 0.75.
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LS 5039 is a high-mass gamma-ray binary hosting a compact object of unknown type. NuSTAR observed LS 5039 during its entire 3.9 day binary period. We performed a periodic signal search up to 1000 Hz which did not produce credible period candidates. We do see the 9.05 s period candidate, originally reported by Yoneda et al. 2020 using the same data, in the Fourier power spectrum, but we find that the statistical significance of this feature is too low to claim it as a real detection. We also did not find significant bursts or quasi-periodic variability. The modulation with the orbital period is clearly seen and remains unchanged over a decade long timescale when compared to the earlier Suzaku light curve. The joint analysis of the NuSTAR and Suzaku XIS data shows that the 0.7-70 keV spectrum can be satisfactory described by a single absorbed power-law model with no evidence of cutoff at higher energies. The slope of the spectrum anti-correlates with the flux during the binary orbit. Therefore, if LS 5039 hosts a young neutron star, its X-ray pulsations appear to be outshined by the intrabinary shock emission. The lack of spectral lines and/or an exponential cutoff at higher energies suggests that the putative neutron star is not actively accreting. Although a black hole scenario still remains a possibility, the lack of variability or Fe K$alpha$ lines, which typically accompany accretion, makes it less likely.
Context. The gamma-ray binary LS 5039 and the isolated pulsar PSR J1825-1446 were proposed to have been formed in the supernova remnant (SNR) G016.8-01.1. Aims. We aim to obtain the Galactic trajectory of LS 5039 and PSR J1825-1446 to find their origin in the Galaxy, and in particular to check their association with SNR G016.8-01.1 to restrict their age. Methods. By means of radio and optical observations we obtained the proper motion and the space velocity of the sources. Results. The proper motion of PSR J1825-1446 corresponds to a transverse space velocity of 690 km/s at a distance of 5 kpc. Its Galactic velocity at different distances is not compatible with the expected Galactic rotation. The velocity and characteristic age of PSR J1825-1446 make it incompatible with SNR G016.8-01.1. There are no clear OB associations or SNRs crossing the past trajectory of PSR J1825-1446. We estimate the age of the pulsar to be 80-245 kyr, which is compatible with its characteristic age. The proper motion of LS 5039 is 7.09 and -8.82 mas/yr in right ascension and declination, respectively. The association of LS 5039 with SNR G016.8-01.1 is unlikely, although we cannot to discard it. The system would have had to be formed in the association Ser OB2 (at 2.0 kpc) if the age of the system is 1.0-1.2 Myr, or in the association Sct OB3 (distance 1.5-2 kpc) for an age of 0.1-0.2 Myr. If the system were not formed close to Ser OB2, the pseudo-synchronization of the orbit would be unlikely. Conclusions. PSR J1825-1446 is a high-velocity isolated pulsar ejected from the Galaxy. The distance to LS 5039, which needs to be constrained by future astrometric missions such as Gaia, is a key parameter for restricting its origin and age.
The Fermi Large Area Telescope (LAT) has made the first definitive GeV detections of the binaries LS I +61^{circ}303 and LS5039 in the first year after its launch in August 2008. These detections were unambiguous because, apart from a reduced positional uncertainty, the gamma-ray emission in each case was orbitally modulated with the corresponding orbital period. The LAT results posed new questions about the nature of these objects, after the unexpected observation of an exponential cutoff in the GeV gamma-ray spectra of both LS I +61^{circ}303 and LS5039, at least along part of their orbital motion. We present here the analysis of new data from the LAT, comprising 2.5 years of observations through which LS I +61^{circ}303 continues to provide some surprises. We find an increase in flux in March 2009 and a steady decrease in the flux fraction modulation. The LAT now detects emission up to 30 GeV, where prior datasets led to upper limits only. At the same time, contemporaneous TeV observations either no longer detected the source, or found it -at least in some orbits- close to periastron, far from the usual phases in which the source usually appeared at TeV energies. The on-source exposure of LS 5039 has also drastically increased along the last years, and whilst our analysis shows no new behavior in comparison with our earlier report, the higher statistics of the current dataset allows for a deeper investigation of its orbital and spectral evolution.
LS I +61 303 and LS 5039 are exceptionally rare examples of HMXBs with MeV-TeV emission, making them two of only five known or proposed gamma-ray binaries. There has been disagreement within the literature over whether these systems are microquasars, with stellar winds accreting onto a compact object to produce high energy emission and relativistic jets, or whether their emission properties might be better explained by a relativistic pulsar wind colliding with the stellar wind. Here we present an attempt to detect radio pulsars in both systems with the Green Bank Telescope. The upper limits of flux density are between 4.1-14.5 uJy, and we discuss the null results of the search. Our spherically symmetric model of the wind of LS 5039 demonstrates that any pulsar emission will be strongly absorbed by the dense wind unless there is an evacuated region formed by a relativistic colliding wind shock. LS I +61 303 contains a rapidly rotating Be star whose wind is concentrated near the stellar equator. As long as the pulsar is not eclipsed by the circumstellar disk or viewed through the densest wind regions, detecting pulsed emission may be possible during part of the orbit.
LS~5039 is a powerful gamma-ray binary that probably hosts a non-accreting pulsar. Despite the wealth of data available, the power source of the non-thermal emitter is still unknown. We use a dynamical-radiative numerical model and multiwavelength data to constrain the properties of a pulsar wind that may power the non-thermal emitter in LS~5039. We ran simulations of an ultrarelativistic (low-$B$) cold $e^pm$-wind that Compton scatters stellar photons and that dynamically interacts with the stellar wind. The effects of energy losses on the unshocked $e^pm$-wind dynamics, and the geometry of the two-wind contact discontinuity, are computed for different wind models. The predicted unshocked $e^pm$-wind radiation at periastron, when expected to be highest, is compared to LS~5039 data. The minimum possible radiation from an isotropic cold $e^pm$-wind overpredicts the X-ray to gamma-ray fluxes at periastron by a factor of $sim 3$. In the anisotropic wind case X-ray and $gtrsim 100$ MeV data are not violated by wind radiation if the wind axis is at $lesssim 20-40^circ$ from the line of sight (probability of $lesssim 6-24$%), depending on the anisotropic wind model, or if the wind Lorentz factor $in 10^2-10^3$, in which case the wind power can be higher, but it requires $e^pm$-multiplicities of $sim 10^6$ and $10^9$ for a $10^{-2}$~s and 10~s pulsar period, respectively. The studied model predicts that a low-$B$ cold pulsar $e^pm$-wind in LS~5039 should be strongly anisotropic, with either a wind Lorentz factor $in 10^2-10^3$ and very high multiplicities or with a fine-tuned wind orientation. A low-$B$, cold baryon-dominated wind would be possible, but then the multiplicities should be rather low, while the baryon-to-$e^pm$ energy transfer should be very efficient at wind termination. A strongly magnetized cold wind seems to be the most favorable (least constrained) option.
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