Magnetic Activity of Pre-main Sequence Stars

We examine the spectrum of diffuse emission detected in the 17 ′ by 17 ′ field around Sgr A * during 625 ks of Chandra observations. The spectrum exhibits He-like and H-like lines from Si, S, Ar, Ca, and Fe, that are consistent with originating in a two-temperature plasma, as well as a prominent lowionization Fe K-α line. The cooler, kT ≈ 0.8 keV plasma differs in surface brightness across the image between (0.2 − 1.8) × 10 −13 erg cm −2 s −1 arcmin −2 (observed, 2-8 keV). This soft plasma is probably heated by supernovae, along with a small contribution from the winds of massive Wolf-Rayet and O stars. The radiative cooling rate of the soft plasma within the inner 20 pc of the Galaxy could be balanced by 1% of the kinetic energy of one supernova every 3 × 10 5 y. The hotter, kT ≈ 8 keV component is more spatially uniform, with a surface brightness of (1.5 − 2.6) × 10 −13 erg cm −2 s −1 arcmin −2 (observed; 2-8) keV. The intensity of the hard plasma is correlated with that of the soft, but they are probably only indirectly related, because neither supernova remnants nor WR/O stars are observed to produce thermal plasma hotter than kT ≈ 3 keV. Moreover, a kT ≈ 8 keV plasma would be too hot to be bound to the Galactic center, and therefore would form a slow wind or fountain of plasma. The energy required to sustain such a freely-expanding plasma within the inner 20 pc of the Galaxy is ∼ 10 40 erg s −1 . This corresponds to the entire kinetic energy of one supernova every 3000 y, which is unreasonably high. However, alternative explanations for the kT ≈ 8 keV diffuse emission are equally unsatisfying. The hard X-rays are unlikely to result from undetected point sources, because no known population of stellar object is numerous enough to the observed surface brightness. There is also no evidence that non-thermal mechanisms for producing the hard emission are operating, as the expected shifts in the line energies and ratios from their collisional equilibrium values are not observed. We are left to conclude that either there is a significant shortcoming in our understanding of the mechanisms that heat the interstellar medium, or that a population of faint (< 10 31 erg s −1 ), hard X-ray sources that are a factor of 10 more numerous than CVs remains to be discovered.

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Diffuse X‐Ray Emission in a DeepChandraImage of the Galactic Center

Muno

Baganoff

Bautz

et al. 2004

ApJ

224

334

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The Spectra and Variability of X‐Ray Sources in a DeepChandraObservation of the Galactic Center

Muno

Arabadjis

Baganoff

et al. 2004

ApJ

109

179

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We examine the X-ray spectra and variability of the sample of X-ray sources with L X ≈ 10 31 − 10 33 erg s −1 identified within the inner 9 ′ of the Galaxy by Muno et al. (2003a). Very few of the sources exhibit intra-day or inter-month variations. We find that the spectra of the point sources near the Galactic center are very hard between 2-8 keV, even after accounting for absorption. When modeled as power laws the median photon index is Γ = 0.7, while when modeled as thermal plasma we can only obtain lower limits to the temperature of kT > 8 keV. The combined spectra of the point sources is similarly hard, with a photon index of Γ = 0.8. Strong line emission is observed from low-ionization, He-like, and H-like Fe, both in the average spectra and in the brightest individual sources. The line ratios of the highly-ionized Fe in the average spectra are consistent with emission from a plasma in thermal equilibrium. This line emission is observed whether average spectra are examined as a function of the count rate from the source, or as a function of the hardness ratios of individual sources. This suggests that the hardness of the spectra may in fact be to due local absorption that partially-covers the X-ray emitting regions in the Galactic center systems. We suggest that most of these sources are intermediate polars, which (1) often exhibit hard spectra with prominent Fe lines, (2) rarely exhibit either flares on short time scales or changes in their mean X-ray flux on long time scales, and (3) are the most numerous hard X-ray sources with comparable luminosities in the Galaxy.

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Magnetic Activity of Pre-main Sequence Stars

Cited by 2 publications

References 55 publications

Diffuse X‐Ray Emission in a DeepChandraImage of the Galactic Center

Diffuse X‐Ray Emission in a DeepChandraImage of the Galactic Center

The Spectra and Variability of X‐Ray Sources in a DeepChandraObservation of the Galactic Center

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