Context. The η Carinae binary system hosts one of the most massive stars, which features the highest known mass-loss rate. This dense wind encounters the much faster wind expelled by the stellar ...companion, dissipating mechanical energy in the shock, where particles can be accelerated up to relativistic energies and subsequently produce very-high-energy γ-rays. Aims. We aim at comparing the variability of the γ-ray emission of η Carinae along the binary orbit with the predictions of simulations to establish the nature of the emission and of the seed particles. Methods. We have used data from the Fermi Large Area Telescope obtained during its first seven years of observations and spanning two passages of η Carinae at periastron. We performed the analysis using the new PASS8 pipeline and its improved instrument response function, extracting low and high-energy light curves as well as spectra in different orbital phase bins. We also introduced particle acceleration in hydrodynamic simulations of the system, assuming a dipolar magnetic field generated by the most massive star, and compared the γ-ray observations with the predictions of diffuse shock acceleration in a multi-cell geometry. Results. The main source of the γ-ray emission originates from a position compatible with η Carinae and located within the Homunculus Nebula. Two emission components can be distinguished. The low-energy component cuts off below 10 GeV and its flux, modulated by the orbital motion, varies by a factor less than 2. Short-term variability occurs at periastron. The flux of the high-energy component varies by a factor 3–4 but is different during the two periastrons. The variabilities observed at low energy, including some details of them, and those observed at high energy during the first half of the observations, match the prediction of the simulation, assuming a surface magnetic field of 500 G. The high-energy component and the thermal X-ray emission were weaker than expected around the second periastron suggesting a modification of the wind density in the inner wind collision zone. Conclusions. Diffuse shock acceleration in the complex geometry of the wind collision zone of η Carinae provides a convincing match to the observations and new diagnostic tools to probe the geometry and energetics of the system. This demonstrates that Fermi acceleration is at work in the wind collisions and that a few percent of the shock mechanical energy are converted into particle acceleration. Further observations are required to understand the periastron-to-periastron variability of the high-energy component and to associate it firmly with hadronic origin. We estimate that η Carinae is a pevatron at periastron and is bright enough to be detected by IceCube after many years of observations. Orbital modulations of the high-energy component can be distinguished from those of photo absorption by the four large size telescopes of the Cherenkov Telescope Array to be placed in the southern hemisphere.
Context.
The origin of the
γ
-ray emission of the blazar
Mrk 421
is still a matter of debate.
Aims.
We used 5.5 years of unbiased observing campaign data, obtained using the FACT telescope and the
...Fermi
-LAT detector at TeV and GeV energies, the longest and densest so far, together with contemporaneous multi-wavelength observations, to characterise the variability of
Mrk 421
and to constrain the underlying physical mechanisms.
Methods.
We studied and correlated light curves obtained by ten different instruments and found two significant results.
Results.
The TeV and X-ray light curves are very well correlated with a lag of < 0.6 days. The GeV and radio (15 Ghz band) light curves are widely and strongly correlated. Variations of the GeV light curve lead those in the radio.
Conclusions.
Lepto-hadronic and purely hadronic models in the frame of shock acceleration predict proton acceleration or cooling timescales that are ruled out by the short variability timescales and delays observed in Mrk 421. Instead the observations match the predictions of leptonic models.
Context:
Insufficient sleep is associated with increased cardiometabolic risk. Alterations in hypothalamic-pituitary-adrenal axis may underlie this link.
Objective:
Our objective was to examine the ...impact of restricted sleep on daytime profiles of ACTH and cortisol concentrations.
Methods:
Thirteen subjects participated in 2 laboratory sessions (2 nights of 10 hours in bed versus 2 nights of 4 hours in bed) in a randomized crossover design. Sleep was polygraphically recorded. After the second night of each session, blood was sampled at 20-minute intervals from 9:00 am to midnight to measure ACTH and total cortisol. Saliva was collected every 20 minutes from 2:00 pm to midnight to measure free cortisol. Perceived stress, hunger, and appetite were assessed at hourly intervals by validated scales.
Results:
Sleep restriction was associated with a 19% increase in overall ACTH levels (P < .03) that was correlated with the individual amount of sleep loss (rSp = 0.63, P < .02). Overall total cortisol levels were also elevated (+21%; P = .10). Pulse frequency was unchanged for both ACTH and cortisol. Morning levels of ACTH were higher after sleep restriction (P < .04) without concomitant elevation of cortisol. In contrast, evening ACTH levels were unchanged while total and free cortisol increased by, respectively, 30% (P < .03) and 200% (P < .04). Thus, the amplitude of the circadian cortisol decline was dampened by sleep restriction (−21%; P < .05). Sleep restriction was not associated with higher perceived stress but resulted in an increase in appetite that was correlated with the increase in total cortisol.
Conclusion:
The impact of sleep loss on hypothalamic-pituitary-adrenal activity is dependent on time of day. Insufficient sleep dampens the circadian rhythm of cortisol, a major internal synchronizer of central and peripheral clocks.
Context.
Multi-wavelength light curves in long-term campaigns show that, for several blazars, the radio emission occurs with a significant delay with respect to the
γ
-ray band, with timescales ...ranging from weeks to years. Such observational evidence has long been a matter of debate, and is usually interpreted as a signature of the
γ
-ray emission originating upstream in the jet, with the emitting region becoming radio transparent at larger scales.
Aims.
In this paper, we show, by means of self-consistent numerical modelling, that the adiabatic expansion of a relativistic blob can explain these delays, reproducing lags compatible with the observed timescales.
Methods.
We use the
JetSeT
framework to reproduce the numerical modelling of the radiative and accelerative processes, reproducing the temporal evolution of a single blob, from the initial flaring activity and the subsequent expansion. We follow the spectral evolution and the corresponding light curves, investigating the relations among the observed parameters, rise time, delay, and decay time, and we identify the link with physical parameters.
Results.
We find that, when adiabatic expansion is active, lags due to the shift of the synchrotron frequency occur. The corresponding time lags have an offset equal to the distance in time between the flaring onset and the beginning of the expansion, whilst the rising and decaying timescales depend on the velocity of the expansion and on the time required for the source to exhibit a synchrotron self-absorption frequency below the relevant radio spectral window. We derive an inter-band response function, embedding the aforementioned parameters, and we investigate the effects of the competitions between radiative and adiabatic cooling timescales on the response. We apply the response function to long-term radio and
γ
-ray light curves of Mrk 421, Mrk 501, and 3C 273, finding satisfactory agreement on the log-term behaviour, and we use a Monte Carlo Markov chain approach to estimate some relevant physical parameters. We discuss applications of the presented analysis to polarization measurements and to jet collimation profile kinematics. The collimation profiles observed in radio images are in agreement with the prediction from our model.
Context. Jets from rotation-powered pulsars so far have only been observed in systems moving subsonically through their ambient medium and/or embedded in their progenitor supernova remnant (SNR). ...Supersonic runaway pulsars are also expected to produce jets, but they have not been confirmed to so far. Aims. We investigated the nature of the jet-like structure associated with the INTEGRAL source IGR J11014-6103 (the “Lighthouse nebula”). The source is a neutron star escaping its parent SNR MSH 11-61A supersonically at a velocity exceeding 1000 km s-1. Methods. We observed the Lighthouse nebula and its jet-like X-ray structure through dedicated high spatial resolution observations in X-rays (with Chandra) and in the radio band (with ATCA). Results. Our results show that the feature is a true pulsar’s jet. It extends highly collimated over ≳11pc, displays a clear precession-like modulation, and propagates nearly perpendicular to the system direction of motion, implying that the neutron star’s spin axis in IGR J11014-6103 is almost perpendicular to the direction of the kick received during the supernova explosion. Conclusions. Our findings suggest that jets are common to rotation-powered pulsars, and demonstrate that supernovae can impart high kick velocities to misaligned spinning neutron stars, possibly through distinct, exotic, core-collapse mechanisms.
Aims. Despite recent progress in the field, there are still many open questions regarding γ-ray binaries. In this paper we provide an overview of non-transient γ-ray binaries and discuss how ...observations with the Cherenkov Telescope Array (CTA) will contribute to their study. Methods. We simulated the spectral behaviour of the non-transient γ-ray binaries using archival observations as a reference. With this we tested the CTA capability to measure the spectral parameters of the sources and detect variability on various timescales. Results. We review the known properties of γ-ray binaries and the theoretical models that have been used to describe their spectral and timing characteristics. We show that the CTA is capable of studying these sources on timescales comparable to their characteristic variability timescales. For most of the binaries, the unprecedented sensitivity of the CTA will allow studying the spectral evolution on a timescale as short as 30 min. This will enable a direct comparison of the TeV and lower energy (radio to GeV) properties of these sources from simultaneous observations. We also review the source-specific questions that can be addressed with these high-accuracy CTA measurements.
η Carinae is composed of two very massive stars orbiting each other in 5.5 years. The primary star features the densest known stellar wind, colliding with that expelled by its companion. The wind ...collision region dissipates energy and accelerates particles up to relativistic energies, producing nonthermal X‐ray and γ‐ray emission detected by Beppo‐SAX, INTEGRAL, Swift, Suzaku, Agile, Fermi, and H.E.S.S. The orbital variability of the system provides key diagnosis of the physics involved and of the emission mechanisms. The low‐energy component, which cuts off below 10 GeV and varies by a factor <2 along the orbit, is likely of the inverse Compton origin. The high‐energy component varies by larger factors and differently during the two periastrons observed by Fermi. These variations match the predictions of simulations assuming a magnetic field in the range 0.4–1 kG at the surface of the primary star. The high‐energy component and the thermal X‐ray emission were weaker than expected around the 2014 periastron suggesting a modification of the inner wind density. Diffuse shock acceleration in the complex geometry of the wind collision zone provides a convincing match to the observations and new diagnostic tools to probe the geometry and energetics of the system. A future instrument sensitive in the MeV energy range could discriminate between lepto‐hadronic and hadronic models for the gamma‐ray emission. At higher energies, the Cherenkov Telescope Array will distinguish orbital modulations of the high‐energy component from those of ultraviolet‐TeV photoabsorption providing a wealth of information constraining acceleration physics under more extreme conditions than found in supernova remnants (SNR).
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