3 research outputs found
NICER X-ray Observations of Eta Carinae During its Most Recent Periastron Passage
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 ergs s cm, followed by a rapid
plunge to its observed minimum flux, ergs s
cm near 17 February 2020. The NICER observations show an X-ray recovery
from minimum of only 16 days, the shortest X-ray minimum observed so far.
We provide new constraints of the "deep" and "shallow" minimum intervals.
Variations in the characteristic X-ray temperature of the hottest observed
X-ray emission indicate that the apex of the wind-wind "bow shock" enters the
companion's wind acceleration zone about 81 days before the start of the X-ray
minimum. There is a step-like increase in column density just before the X-ray
minimum, probably associated with the presence of dense clumps near the shock
apex. During recovery and after, the column density shows a smooth decline,
which agrees with previous measurements made by SWIFT at the same
orbital phase, indicating that changes in mass-loss rate are only a few percent
over the two cycles. Finally, we use the variations in the X-ray flux of the
outer ejecta seen by NICER to derive a kinetic X-ray luminosity of the ejecta
of ergs s near the time of the "Great Eruption'
Eta Carinae: an evolving view of the central binary, its interacting winds and its foreground ejecta
FUV spectra of Eta Car, recorded across two decades with HST/STIS, document
multiple changes in resonant lines caused by dissipating extinction in our line
of sight. The FUV flux has increased nearly ten-fold which has led to increased
ionization of the multiple shells within the Homunculus and photo-destruction
of molecular hydrogen. Comparison of observed resonant line profiles with
CMFGEN model profiles allows separation of wind-wind collision and shell
absorptions from the primary wind, P Cygni profiles.The dissipating occulter
preferentially obscured the central binary and interacting winds relative to
the very extended primary wind. We are now able to monitor changes in the
colliding winds with orbital phase. High velocity transient absorptions
occurred across the most recent periastron passage, indicating acceleration of
the primary wind by the secondary wind which leads to a downstream, high
velocity bowshock that is newly generated every orbital period. There is no
evidence of changes in the properties of the binary winds.Comment: 36 pages, 22 figures, accepted Astrophysical Journa
The Long-term Spectral Changes of Eta Carinae: Are they Caused by a Dissipating Occulter as Indicated by cmfgen Models?
Eta Carinae ( η Car) exhibits a unique set of P Cygni profiles with both broad and narrow components. Over many decades, the spectrum has changed—there has been an increase in observed continuum fluxes and a decrease in Fe ii and H i emission-line equivalent widths. The spectrum is evolving toward that of a P Cygni star such as P Cygni itself and HDE 316285. The spectral evolution has been attributed to intrinsic variations such as a decrease in the mass-loss rate of the primary star or differential evolution in a latitudinal-dependent stellar wind. However, intrinsic wind changes conflict with three observational results: the steady long-term bolometric luminosity; the repeating X-ray light curve over the binary period; and the constancy of the dust-scattered spectrum from the Homunculus. We extend previous work that showed a secular strengthening of P Cygni absorptions by adding more orbital cycles to overcome temporary instabilities and by examining more atomic transitions. cmfgen modeling of the primary wind shows that a time-decreasing mass-loss rate is not the best explanation for the observations. However, models with a small dissipating absorber in our line of sight can explain both the increase in brightness and changes in the emission and P Cygni absorption profiles. If the spectral evolution is caused by the dissipating circumstellar medium, and not by intrinsic changes in the binary, the dynamical timescale to recover from the Great Eruption is much less than a century, different from previous suggestions