Context: The colliding-wind binary Eta Car exhibits soft X-ray thermal emission that varies strongly around periastron, and non-thermal emission seen in hard X-rays and gamma-rays.
Aims: To definitively identify Eta Car as the source of the hard X-ray emission, to examine how changes in the 2-10 keV band influence changes in the hard X-ray band, and to understand more clearly the mechanisms producing the non-thermal emission using new INTEGRAL observations obtained close to periastron.
Methods: A Chandra observation encompassing the ISGRI error circle was analysed, and all other soft X-ray sources (including the outer shell of Eta Car itself) were discarded as likely counter-parts. New hard X-ray images of Eta Car were studied close to periastron, and compared to previous observations far from periastron.
Results: The INTEGRAL component, when represented by a power law (with a photon index of 1.8), would produce more emission in the Chandra band than observed from any point source in the ISGRI error circle apart from Eta Car, as long as the hydrogen column density to the ISGRI source is lower than 1E24 cm^{-2}. Such sources are rare, thus the ISGRI emission is very likely to be associated with Eta Car. The eventual contribution of the outer shell to the non-thermal component also remains fairly limited. Close to periastron, a 3-sigma detection is achieved for the hard X-ray emission of Eta Car, with a flux similar to the average value far from periastron.
Conclusions: Assuming a single absorption component for both the thermal and non-thermal sources, this detection can be explained with a hydrogen column density that does not exceed 6E23 cm^{-2} without resorting to an intrinsic increase in the hard X-ray emission. The energy injected in hard X-rays (averaged over a month) appears rather constant as close as a few stellar radii, well within the acceleration region of the wind.
Context. Colliding-wind binaries are massive stellar systems featuring strong, interacting winds. These binaries may be actual particle accelerators, making them variable gamma-ray sources due to changes in the wind collision region along the orbit. However, only two of these massive stellar binary systems have been identified as high-energy sources. The first and archetypical system of this class is Eta Carinae, a bright gamma-ray source with orbital variability peaking around its periastron passage. Aims. The origin of the high energy emission in Eta Carinae is still unclear, with both lepto…
The historical light curve of the 19th century "Great Eruption" of etaCar provides a striking record of violent instabilies encountered by the most massive stars. We report and analyze newly uncovered historical estimates of the visual brightness of etaCar during its eruption, and we correct some mistakes in the original record. The revised light curve looks substantially different from previous accounts: it shows two brief eruptions in 1838 and 1843 that resemble modern supernova impostors, while the final brightening in December 1844 marks the time when etaCar reached its peak brightness. We…
The η Carinae binary system hosts the most massive stars with the highest known mass-loss rate. Its dense wind encounters the faster wind expelled by the companion, dissipating mechanical energy in the shock, accelerating particles up to relativistic energies and producing high-energy (HE) γ-rays. We used the first 7-year data of the Fermi LAT which span two passages of η Carinae at periastron. We extracted low and HE light curves and spectra in different orbital phase bins using the new PASS8 pipeline. We used particle acceleration in hydrodynamic simulations of the system in a multi-cell geo…
X-ray emission from the supermassive binary system Eta Carinae declines sharply around periastron. This X-ray minimum has two distinct phases - the lowest flux phase in the first ~3 weeks and a brighter phase thereafter. In 2009, the Chandra X-ray Observatory monitored the first phase five times and found the lowest observed flux at ~1.9e-12 ergs cm-2 s-1 (3-8 keV). The spectral shape changed such that the hard band above ~4 keV dropped quickly at the beginning and the soft band flux gradually decreased to its lowest observed value in ~2 weeks. The hard band spectrum had begun to recover by th…
We report high-precision X-ray monitoring observations in the 0.4-10 keV band of the luminous, long-period colliding-wind binary Eta Carinae up to and through its most recent X-ray minimum/periastron passage in February 2020. Eta Carinae reached its observed maximum X-ray flux on 7 January 2020, at a flux level of $3.30 \times 10^{-10}$ ergs s$^{-1}$ cm$^{-2}$, followed by a rapid plunge to its observed minimum flux, $0.03 \times 10^{-10}$ ergs s$^{-1}$ cm$^{-2}$ near 17 February 2020. The NICER observations show an X-ray recovery from minimum of only $\sim$16 days, the shortest X-ray minimum …
The massive binary system Eta Carinae is characterized by intense colliding winds that form shocks and emit X-rays. The system is highly eccentric ($e\simeq0.9$), resulting in modulated X-ray emission during its 5.54 year orbit. The X-ray flux increases in the months prior to periastron passage, exhibiting strong flares, then rapidly declines to a flat minimum lasting a few weeks, followed by a gradual recovery. We present Neutron Star Interior Composition Explorer (NICER) telescope spectra obtained before, during, and after the 2020 X-ray minimum, and perform spectral analysis to establish th…
We used data from the Fermi Large Area Telescope obtained during the last 7 years and spanning two passages of $η$ Carinae at periastron and compared them with the predictions of particle acceleration in hydrodynamic simulations. 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 differently during the two periastrons. The variabilities observed at low-energy, includ…
This paper discusses a model where a violent periastron collision of stars in an eccentric binary system induces an eruption or explosion seen as a brief transient source, attributed to LBVs, SN impostors, or other transients. The key ingredient is that an evolved primary increases its photospheric radius on relatively short timescales, to a point where the radius is comparable to or larger than the periastron separation in an eccentric binary. In such a configuration, a violent and sudden collision would ensue, possibly leading to substantial mass ejection instead of a binary merger. Repeated…
The binary system eta Carinae has completed its first 5.54y orbit since the beginning of science operation of the Fermi Large Area Telescope (LAT). We are now able to investigate the high-energy gamma-ray source at the position of eta Carinae over its full orbital period. By this, we can address and confirm earlier predictions for temporal and spectral variability. Newer versions of the LAT datasets, instrument response functions and background models allow for a more accurate analysis. Therefore it is important to re-evaluate the previously analyzed time period along with the new data to furt…
Eta Carinae shows broad peaks in near-infrared (IR) JHKL photometry, roughly correlated with times of periastron passage in the eccentric binary system. After correcting for secular changes attributed to reduced extinction from the thinning Homunculus Nebula, these peaks have IR spectral energy distributions (SEDs) consistent with emission from hot dust at 1400-1700 K. The excess SEDs are clearly inconsistent, however, with the excess being entirely due to free-free wind or photospheric emission. One must conclude, therefore, that the broad near-IR peaks associated with Eta Carinae's 5.5 yr va…
$η$ Carinae is a colliding wind binary hosting two of the most massive stars and featuring the strongest wind collision mechanical luminosity. The wind collision region of this system is detected in X-rays and $γ$-rays and offers a unique laboratory for the study of particle acceleration and wind magneto-hydrodynamics. Our main goal is to use X-ray observations of $η$ Carinae around periastron to constrain the wind collision zone geometry and understand the reasons for its variability. We analysed 10 Nuclear Spectroscopic Telescope Array (NuSTAR) observations, which were obtained around the 20…
The Suzaku X-ray observatory monitored the supermassive binary system Eta Carinae 10 times during the whole 5.5 year orbital cycle between 2005-2011. This series of observations presents the first long-term monitoring of this enigmatic system in the extremely hard X-ray band between 15-40 keV. During most of the orbit, the 15-25 keV emission varied similarly to the 2-10 keV emission, indicating an origin in the hard energy tail of the kT ~4 keV wind-wind collision (WWC) plasma. However, the 15-25 keV emission declined only by a factor of 3 around periastron when the 2-10 keV emission dropped b…
The X-ray emission of $η$ Carinae shows multiple features at various spatial and temporal scales. The central constant emission (CCE) component is centred on the binary and arises from spatial scales much smaller than the bipolar Homunculus nebula, but likely larger than the central wind--wind collision region between the stars as it does not vary over the $\sim$2-3 month X-ray minimum when it can be observed. Using large-scale 3D smoothed particle hydrodynamics (SPH) simulations, we model both the colliding-wind region between the stars, and the region where the secondary wind collides with p…
The binary system $η$ Carinae is a unique laboratory in which to study particle acceleration to high energies under a wide range of conditions, including extremely high densities around periastron. To date, no consensus has emerged as to the origin of the GeV $γ$-ray emission in this important system. With a re-analysis of the full Fermi-LAT dataset for $η$ Carinae we show that the spectrum is consistent with a pion decay origin. A single population leptonic model connecting the X-ray to $γ$-ray emission can be ruled out. Here, we revisit the physical model of Ohm et al. (2015), based on two a…
It is believed that eta Carinae is actually a massive binary system, with the wind-wind interaction responsible for the strong X-ray emission. Although the overall shape of the X-ray light curve can be explained by the high eccentricity of the binary orbit, other features like the asymmetry near periastron passage and the short quasi-periodic oscillations seen at those epochs, have not yet been accounted for. In this paper we explain these features assuming that the rotation axis of eta Carinae is not perpendicular to the orbital plane of the binary system. As a consequence, the companion star…
Colliding wind binaries (CWBs) are unique laboratories for X-ray astrophysics. The massive stars in these systems possess powerful stellar winds with speeds up to $\sim$3000 km s$^{-1}$, and their collision leads to hot plasma (up to $\sim10^8$K) that emit thermal X-rays (up to $\sim$10 keV). Many X-ray telescopes have observed CWBs, including Suzaku, and our work aims to model these X-ray observations. We use 3D smoothed particle hydrodynamics (SPH) to model the wind-wind interaction, and then perform 3D radiative transfer to compute the emergent X-ray flux, which is folded through X-ray tele…
$η$ 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 accelerate particles up to relativistic energies, producing non thermal X- 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 diagnostic on the physics involved and on the emission mechanisms. The low-energy component, which cuts off below 10 GeV and varies by a fact…
Three dimensional (3D) adaptive-mesh refinement (AMR) hydrodynamical simulations of the wind-wind collision between the enigmatic super-massive star \etacar and its mysterious companion star are presented which include radiative driving of the stellar winds, gravity, optically-thin radiative cooling, and orbital motion. Simulations with static stars with a periastron passage separation reveal that the preshock companion star's wind speed is sufficiently reduced that radiative cooling in the postshock gas becomes important, permitting the runaway growth of non-linear thin shell (NTSI) instabili…
The stellar binary $η$ Carinae has been observed during its full orbital period in gamma rays by the Fermi-Large Area Telescope (LAT). The shock-accelerated electrons in the colliding winds of the two stars radiate synchrotron photons in the magnetic field of the shocked region and inverse Compton photons, where the target photons are from the thermal emissions by the more massive and luminous of the two stars. The inverse Compton emission dominates the gamma-ray flux data from the $η$ Carinae, however the spectral energy distribution shows signature of a hadronic component in the $\sim 10$-30…
Eta Carinae (or Eta Car) is a colliding-wind binary that shows non-thermal emission from hard X-rays to high-energy $γ$-rays. The $γ$-ray spectrum exhibits two spectral components, where the high-energy component extends up to 300 GeV. Previous observations of Eta Carinae with the High Energy Stereoscopic System (H.E.S.S.) resulted in upper limits. With the addition of the large 28-m central telescope to the H.E.S.S. array in 2012 the lower bound on the energy range has significantly been reduced. This lowers the energy threshold of the analysis compared to the published results due to the imp…
While SN impostors resemble the Great Eruption of eta Car in the sense that their spectra show narrow H lines and they have typical peak absolute magnitudes of -13 to -14 mag, most extragalactic events observed so far are quite different from eta Car in duration. Their bright phases typically last for 100~d or less, rather than persisting for several years. The transient object UGC2773-OT had a similar peak absolute magnitude to other SN impostors, but with a gradual 5-yr prediscovery rise. In the 6 yr since discovery, it has faded very slowly (0.26 mag/yr). Overall, we suggest that its decade…
The evolved, massive highly eccentric binary system, eta Carinae, underwent a periastron passage in the summer of 2014. We obtained two coordinated X-ray observations with XMM-Newton and NuSTAR during the elevated X-ray flux state and just before the X-ray minimum flux state around this passage. These NuSTAR observations clearly detected X-ray emission associated with eta Car extending up to ~50 keV for the first time. The NuSTAR spectrum above 10 keV can be fit with the bremsstrahlung tail from a kT ~6 keV plasma. This temperature is Delta kT ~2 keV higher than those measured from the iron K …
Observations of high energy gamma rays recently revealed a persistent source in spatial coincidence with the binary system Eta Carinae. Since modulation of the observed gamma-ray flux on orbital time scales has not been reported so far, an unambiguous identification was hitherto not possible. Particularly the observations made by the Fermi Large Area Telescope (LAT) posed additional questions regarding the actual emission scenario owing to the existence of two energetically distinct components in the gamma-ray spectrum of this source, best described by an exponentially cutoff power-law functio…
We observed the massive binary stellar system of Eta Carinae in the 0.3-10 keV energy range with the X-ray Telescope onboard the Swift satellite during the period 15 December 2008 - 11 March 2009, i.e. 1 month before to 2 months after the X-ray drop from maximum to minimum, thought to be associated with the periastron encounter of the primary star by the hot companion. Beginning a few months before eclipse, the interaction between the winds of the two stars intensifies and the X-ray flux reaches maximum. The flux drops dramatically thereafter, subsiding in about 20 days to a level that is at l…
WR 140 is a long-period, highly eccentric Wolf-Rayet star binary system with exceptionally well-determined orbital and stellar parameters. Bright, variable X-ray emission is generated in shocks produced by the collision of the winds of the WC7pd+O5.5fc component stars. We discuss the variations in the context of the colliding-wind model using broad-band spectrometry from the RXTE, SWIFT, and NICER observatories obtained over 20 years and nearly 1000 observations through 3 consecutive 7.94-year orbits including 3 periastron passages. The X-ray luminosity varies as expected with the inverse of t…
We present the results of extensive observations by the gamma-ray AGILE satellite of the Galactic region hosting the Carina nebula and the remarkable colliding wind binary Eta Carinae (Eta Car) during the period 2007 July to 2009 January. We detect a gamma-ray source (1AGL J1043-5931) consistent with the position of Eta Car. If 1AGL J1043-5931 is associated with the Eta Car system our data provide the long sought first detection above 100 MeV of a colliding wind binary. The average gamma-ray flux above 100 MeV and integrated over the pre-periastron period 2007 July to 2008 October is F = (37 +…
We present 2D radiative transfer modeling of the Eta Carinae binary system accounting for the presence of a wind-wind collision (WWC) cavity carved in the optically-thick wind of the primary star. By comparing synthetic line profiles with HST/STIS spectra obtained near apastron, we show that the WWC cavity has a strong influence on multi-wavelength diagnostics. This influence is regulated by the modification of the optical depth in the continuum and spectral lines. We find that H-alpha, H-beta, and Fe II lines are the most affected by the WWC cavity, since they form over a large volume of the …
We quantify the rapid variations in X-ray brightness ("flares") from the extremely massive colliding wind binary Eta Carinae seen during the past three orbital cycles by RXTE. The observed flares tend to be shorter in duration and more frequent as periastron is approached, although the largest ones tend to be roughly constant in strength at all phases. Plausible scenarios include (1) the largest of multi-scale stochastic wind clumps from the LBV component entering and compressing the hard X-ray emitting wind-wind collision (WWC) zone, (2) large-scale corotating interacting regions in the LBV w…
High Spectral Resolution X-ray Observations of the Evolved Supermassive Stellar Binary System $η$ Carinae - Iron K$α$ Band Profile Revealed with XRISM
University of Geneva（日内瓦大学）；Ehime University（爱媛大学）；University of Maryland, College Park（马里兰大学帕克分校）；NASA Goddard Space Flight Center（美国宇航局戈达德太空飞行中心）；Center for Research and Exploration in Space Science and Technology (CRESST II)（空间科学与技术研究探索中心（CRESST II））；University of Tokyo（东京大学）；Technion（以色列理工学院）；Center for Space Sciences and Technology, University of Maryland, Baltimore County（马里兰大学巴尔的摩县空间科学与技术中心）；Center for Astrophysics — Harvard-Smithsonian（哈佛-史密森天体物理中心）
Recent reports claiming tentative association of the massive star binary system gamma^2 Velorum (WR 11) with a high-energy gamma-ray source observed by Fermi-LAT contrast the so-far exclusive role of Eta Carinae as the hitherto only detected gamma-ray emitter in the source class of particle-accelerating colliding-wind binary systems. We aim to shed light on this claim of association by providing dedicated model predictions for the nonthermal photon emission spectrum of WR 11. We use three-dimensional magneto-hydrodynamic modeling to trace the structure and conditions of the wind-collision regi…
