Context. The neutrino event IceCube−170922A appears to originate from the BL Lac object TXS 0506+056. To understand the neutrino creation process and to localize the emission site, we studied the ...radio images of the jet at 15 GHz. Aims. Other BL Lac objects show properties similar to those of TXS 0506+056, such as multiwavelength variability or a curved jet. However, to date only TXS 0506+056 has been identified as neutrino emitter. The aim of this paper is to determine what makes the parsec-scale jet of TXS 0506+056 specific in this respect. Methods. We reanalyzed and remodeled 16 VLBA 15 GHz observations between 2009 and 2018. We thoroughly examined the jet kinematics and flux-density evolution of individual jet components during the time of enhanced neutrino activity between September 2014 and March 2015, and in particular before and after the neutrino event. Results. Our results suggest that the jet is very strongly curved and most likely observable under a special viewing angle of close to zero. We may observe the interaction between jet features that cross each other’s paths. We find subsequent flux-density flaring of six components passing the likely collision site. In addition, we find a strong indication for precession of the inner jet, and model a precession period of about 10 yr via the Lense-Thirring effect. We discuss an alternative scenario, which is the interpretation of observing the signature of two jets within TXS 0506+056, again hinting toward a collision of jetted material. We essentially suggest that the neutrino emission may result from the interaction of jetted material in combination with a special viewing angle and jet precession. Conclusions. We propose that the enhanced neutrino activity during the neutrino flare in 2014–2015 and the single EHE neutrino IceCube-170922A could have been generated by a cosmic collision within TXS 0506+056. Our findings seem capable of explaining the neutrino generation at the time of a low gamma-ray flux and also indicate that TXS 0506+056 might be an atypical blazar. It seems to be the first time that a potential collision of two jets on parsec scales has been reported and that the detection of a cosmic neutrino might be traced back to a cosmic jet-collision.
On 2019/07/30.86853 UT, IceCube detected a high-energy astrophysical neutrino can-didate. The Flat Spectrum Radio Quasar PKS 1502+106 is located within the 50 percent uncertainty region of the event. ...Our analysis of 15 GHz Very Long Baseline Ar-ray (VLBA) and astrometric 8 GHz VLBA data, in a time span prior and after theIceCube event, reveals evidence for a radio ring structure which develops with time.Several arc-structures evolve perpendicular to the jet ridge line. We find evidence forprecession of a curved jet based on kinematic modelling and a periodicity analysis.An outflowing broad line region (BLR) based on the C IV line emission (Sloan Dig-ital Sky Survey, SDSS) is found. We attribute the atypical ring to an interaction ofthe precessing jet with the outflowing material.We discuss our findings in thecontext of a spine-sheath scenario where the ring reveals the sheath andits interaction with the surroundings (NLR clouds).We find that the radioemission is correlated with theγ-ray emission, with radio lagging theγ-rays. Basedon theγ-ray variability timescale, we constrain theγ-ray emission zone to the BLR(30-200rg) and within the jet launching region. We discuss that the outflowing BLRprovides the external radiation field forγ-ray production via external Compton scat-tering.The neutrino is most likely produced by proton-proton interactionin the blazar zone (beyond the BLR), enabled by episodic encounters ofthe jet with dense clouds, i.e. some molecular cloud in the NLR.
Context. In the binary system LS I +61°303 the peak flux density of the radio outburst, which is related to the orbital period of 26.4960 ± 0.0028d, exibits a modulation of 1667 ± 8 d. The radio ...emission at high spatial resolution appears structured in a precessing jet with a precessional period of 27−28 d. Aims. How close is the precessional period of the radio jet to the orbital period? Any periodicity in the radio emission should be revealed by timing analysis. The aim of this work is to establish the accurate value of the precessional period. Methods. We analyzed 6.7 years of the Green Bank Interferometer database at 2.2 GHz and 8.3 GHz with the Lomb-Scargle and phase dispersion minimization methods and performed simulations. Results. The periodograms show two periodicities, P1 = 26.49 ± 0.07d (ν1 = 0.03775 d-1) and P2 = 26.92 ± 0.07d (ν2 = 0.03715 d-1). Whereas radio outbursts have been known to have nearly orbital occurrence P1 with timing residuals exhibiting a puzzling sawtooth pattern, we probe in this paper that they are actually periodical outbursts and that their period is Paverage = 2/(ν1 + ν2) = 26.70 ± 0.05 d. The period Paverage as well as the long-term modulation Pbeat = 1/(ν1 − ν2) = 1667 ± 393 d result from the beat of the two close periods, the orbital P1 and the precessional P2 periods. Conclusions. The precessional period, indicated by the astrometry to be of 27–28 d, is P2 = 26.92 d. The system LS I +61°303 seems to be one more case in astronomy of beat, i.e., a phenomenon occurring when two physical processes create stable variations of nearly equal frequencies. The very small difference in frequency creates a long-term variation of period 1/(ν1 − ν2). The long-term modulation of 1667 d results from the beat of the two close orbital and precessional rates.
Aims. The aim of this paper is to analyse the previously discovered discontinuity of the periodicity of the GeV γ-ray emission of the radio-loud X-ray binary LS I + 61°303 and to determine its ...physical origin. Methods. We used a wavelet analysis to explore the temporal development of periodic signals. The wavelet analysis was first applied to the whole data set of available Fermi-LAT data and then to the two subsets of orbital phase intervals Φ = 0.0−0.5 and Φ = 0.5−1.0. We also performed a Lomb-Scargle timing analysis. We investigated the similarities between GeV γ-ray emission and radio emission by comparing the folded curves of the Fermi-LAT data and the Green Bank Interferometer radio data. Results. During the epochs when the timing analysis fails to determine the orbital periodicity, the periodicity is present in the two orbital phase intervals Φ = 0.0−0.5 and Φ = 0.5−1.0. That is, there are two periodical signals, one towards periastron (i.e., Φ = 0.0−0.5) and another one towards apoastron (Φ = 0.5−1.0). The apoastron peak seems to be affected by the same orbital shift as the radio outbursts and, in addition, reveals the same two periods P1 and P2 that are present in the radio data. Conclusions. The γ-ray emission of the apoastron peak normally just broadens the emission of the peak around periastron. Only when it appears at Φ ≈ 0.8−1.0 because of the orbital shift, it is enough detached from the first peak to become recognisable as a second orbital peak, which is the reason why the timing analysis fails. Two γ-ray peaks along the orbit are predicted by the two-peak accretion model for an eccentric orbit that was proposed by several authors for LS I + 61°303.
ABSTRACT
The gamma-ray binary LS I +61°303 has been widely monitored at different wavelengths since its discovery more than 60 yr ago. However, the nature of the compact object and the peculiar ...behaviour of the system are still largely debated. Aimed at investigating the rapid X-ray variability of LS I +61°303, we have analysed all the archival Rossi X-ray Timing Explorer/Proportional Counter Array (RXTE/PCA) data of the source, taken between 1996 and 2011. The timing analysis yields a periodicity of P ∼ 26.6 ± 0.3 d, which is statistically compatible with several periodicities reported in the literature for LS I +61°303. Using this period, we performed a data phase-resolved analysis to produce a set of phase-bin-averaged energy spectra and power density spectra. These power density spectra are dominated by weak red noise below 0.1 Hz, and show no signal above this frequency. The amplitude of the red noise varies mildly with the phase, and shows a maximum that coincides with a dip of the X-ray flux and a softer photon index. Aside from low-frequency noise, this analysis does not provide any statistically significant periodic or quasi-periodic timing feature in the RXTE/PCA data of LS I +61°303.
Context. MWC 656 is the recently discovered first binary system case composed of a Be-type star and an accreting black hole. Its low X-ray luminosity indicates that the system is in a quiescent X-ray ...state. Aims. The aim of our investigation is to establish if the MWC 656 system has detectable radio emission and if the radio characteristics are consistent with those of quiescent black hole systems. Methods. We used three archived VLA data sets, one hour each at 3 GHz, and seven new VLA observations, two hours each at 10 GHz, to produce very high sensitivity images down to ~1μJy. Results. We detected the source twice in the new observations: in the first VLA run, at apastron passage, with a flux density of 14.2 ± 2.9μJy and by combining all together the other six VLA runs, with a flux density of 3.7 ± 1.4μJy. The resulting combined map of the archived observations has the sensitivity of 1σ = 6.6 μJy, but no radio emission is detected there. Conclusions. The radio and X-ray luminosities agree with the behaviour of accreting binary black holes in the hard and quiescent state. In particular, MWC 656 in the LX, LR plane occupies the same region as A0620−00 and XTE J1118+480, the faintest known black holes up to now.
Context. One possible scenario to explain the emission from the stellar binary system LS I + 61°303 is that the observed flux is emitted by precessing jets powered by accretion. Accretion models ...predict two ejections along the eccentric orbit of LS I + 61°303: one major ejection at periastron and a second, lower ejection towards apastron. Our GeV gamma-ray observations show two peaks along the orbit (orbital period P1) but reveal that at apastron the emission is also affected by a second periodicity, P2. Strong radio outbursts also occur at apastron, which are affected by both periodicities (i.e. P1 and P2), and radio observations show that P2 is the precession of the radio jet. Consistently, a long-term modulation, equal to the beating of P1 and P2, affects both radio and gamma-ray emission at apastron but it does not affect gamma-ray emission at periastron. Aims. If there are two ejections, why does the one at periastron not produce a radio outburst there? Is the lack of a periastron radio outburst somehow related to the lack of P2 from the periastron gamma-ray emission? Methods. We develop a physical model in which relativistic electrons are ejected twice along the orbit. The ejecta form a conical jet that is precessing with P2. The jet radiates in the radio band by the synchrotron process and the jet radiates in the GeV energy band by the external inverse Compton and synchrotron self-Compton processes. We compare the output fluxes of our physical model with two available large archives: Owens Valley Radio Observatory (OVRO) radio and Fermi Large Area Telescope (LAT) GeV observations, the two databases overlapping for five years. Results. The larger ejection around periastron passage results in a slower jet, and severe inverse Compton losses result in the jet also being short. While large gamma-ray emission is produced, there is only negligible radio emission. Our results are that the periastron jet has a length of 3.0 × 106rs and a velocity β ~ 0.006, whereas the jet at apastron has a length of 6.3 × 107rs and β ~ 0.5. Conclusions. In the accretion scenario the observed periodicities can be explained if the observed flux is the intrinsic flux, which is a function of P1, times the Doppler factor, a function of βcos(f(P2)). At periastron, the Doppler factor is scarcely influenced by P2 because of the low β. At apastron the larger β gives rise to a significant Doppler factor with noticeable variations induced by jet precession.
Context. The gamma-ray binary LS I +61°303 shows multiple periodicities. The timing analysis of 6.7 yr of GBI radio data and of 6 yr of Fermi-LAT GeV gamma-ray data both have found two close ...periodicities P1,GBI = 26.49 ± 0.07 d, P2,GBI = 26.92 ± 0.07 d and P1,γ = 26.48 ± 0.08 d, P2,γ = 26.99 ± 0.08 d. Aims. The system LS I +61°303 is the object of several continuous monitoring programs at low and high energies. The frequency difference between ν1 and ν2 of only 0.0006 d-1 requires long-term monitoring because the frequency resolution in timing analysis is related to the inverse of the overall time interval. The Owens Valley Radio Observatory (OVRO) 40 m telescope has been monitoring the source at 15 GHz for five years and overlaps with Fermi-LAT monitoring. The aim of this work is to establish whether the two frequencies are also resolved in the OVRO monitoring. Methods. We analysed OVRO data with the Lomb-Scargle method. We also updated the timing analysis of Fermi-LAT observations. Results. The periodograms of OVRO data confirm the two periodicities and . Conclusions. The three independent measurements of P1 and P2 with GBI, OVRO, and Fermi-LAT observations confirm that the periodicities are permanent features of the system LS I +61°303. The similar behaviours of the emission at high (GeV) and low (radio) energy when the compact object in LS I +61°303 is toward apastron suggest that the emission is caused by the same periodically (P1) ejected population of electrons in a precessing (P2) jet.
Context. In the gamma-ray binary LS I + 61°303, radio outbursts occur every 26.70 days and are modulated by a long-term periodicity of 1667 days. Aims. Until now the prediction of the periodical ...radio outbursts has been made using the orbital period P1 = 26.4960 ± 0.0028days. This procedure implies timing residuals up to ~7 days affected by a systematic error with a sawtooth pattern. On the other hand, the direct use of the known periodicity of the radio outbursts, that is of Poutburst = Paverage = 26.70 ± 0.05d, is prevented because of a time variable phase term. Our aim is to analyze this phase term and determine its exact value at each given epoch. Methods. First, we modeled the systematic sawtooth pattern affecting the timing residuals between the observed outbursts and those predicted by P1. Then, we removed this pattern from 6.7 yr of 8.3 GHz Green Bank Inferferometer radio data to generate noise-limited residuals. Finally, we determined a criterion to determine the phase term based on the number of elapsed cycles of the long-term modulation at a given epoch. Results. The prediction of the outburst with P = Paverage is now straightforward and produces noise-limited timing residuals.
ABSTRACT
The stellar binary system LS I +61°303, composed of a compact object in an eccentric orbit around a B0 Ve star, emits from radio up to γ-ray energies. The orbital modulation of radio ...spectral index, X-ray, and GeV γ-ray data suggests the presence of two peaks. This two-peaked profile is in line with the accretion theory predicting two accretion–ejection events for LS I +61°303 along the 26.5 d orbit. However, the existing multiwavelength data are not simultaneous. In this paper, we report the results of a campaign covering radio, X-ray, and γ-ray observations of the system along one single orbit. Our results confirm the two predicted events along the orbit and in addition show that the positions of radio and γ-ray peaks are coincident with X-ray dips as expected for radio and γ-ray emitting ejections depleting the X-ray emitting accretion flow. We discuss future observing strategies for a systematic study of the accretion–ejection physical processes in LS I +61°303.
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