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Register a new userNuSTAR observation of LS 5039
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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.
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With 8 hours 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 $8.8sigma$. The measured flux above 1 TeV is $(2.5 pm 0.4) times 10^{-12} rm , cm^{-2} , s^{-1}$ near inferior conjunction and $(7.8 pm 2.8) times 10^{-13} rm , cm^{-2} , s^{-1}$ near superior conjunction. The pulsar wind nebula HESS J1825-137 is detected with a statistical significance of $6.7sigma$ and a measured flux above 1 TeV of $(3.9 pm 0.8) times 10^{-12} rm , cm^{-2} , s^{-1}$.
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.
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.
Recent observations of the binary system LS5039 with the High Energy Stereoscopic System (H.E.S.S.) revealed that its Very High Energy (VHE) gamma-ray emission is modulated at the 3.9 days orbital period of the system. The bulk of the emission is largely confined to half of the orbit, peaking around the inferior conjunction epoch of the compact object. The flux modulation provides the first indication of gamma-ray absorption by pair production on the intense stellar photon field. This implies that the production region size must be not significantly greater than the gamma-gamma photosphere size (~1 AU), thus excluding the large scale collimated outflows or jets (extending out to ~1000 AU). A hardening of the spectrum is also observed at the same epoch between 0.2 and a few TeV which is unexpected under a pure absorption scenario and could rather arise from variation with phase in the maximum electron energy and/or the dominant VHE gamma-ray production mechanism. This first-time observation of modulated gamma-ray emission allows precise tests of the acceleration and emission models in binary systems.
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.
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