Extreme X-ray behavior of the low-luminosity active nucleus in NGC 4395

被引:67
作者
Moran, EC [1 ]
Eracleous, M
Leighly, KM
Chartas, G
Filippenko, AV
Ho, LC
Blanco, PR
机构
[1] Wesleyan Univ, Dept Astron, Middletown, CT 06459 USA
[2] Penn State Univ, Dept Astron & Astrophys, Davey Lab 525, University Pk, PA 16802 USA
[3] Univ Oklahoma, Dept Phys & Astron, Norman, OK 73019 USA
[4] Univ Calif Berkeley, Dept Astron, Berkeley, CA 94720 USA
[5] Observ Carnegie Inst Washington, Pasadena, CA 91101 USA
[6] Univ Calif San Diego, Ctr Astrophys & Space Sci, La Jolla, CA 92093 USA
关键词
galaxies : active; galaxies : individual (NGC 4395); galaxies : Seyfert; X-rays : galaxies;
D O I
10.1086/429522
中图分类号
P1 [天文学];
学科分类号
0704 ;
摘要
We present the results of a 17 ks Chandra ACIS-S observation of the nearby dwarf spiral galaxy NGC 4395. Chandra affords the first high-quality, broadband X-ray detection of the active nucleus of this object that is uncontaminated by nearby sources in the field. We find the nuclear X-ray source to be unresolved and confirm the rapid, large-amplitude variability reported in previous studies. The light curve appears to show similar to 11 cycles of a quasi-periodic oscillation with a period of similar to 400 s. If associated with an orbital feature near the innermost stable orbit of the accretion disk, this period would constrain the black hole mass to be M < 9 x 10(5) M-.. The X-ray spectrum indicates absorption by an ionized medium, and the spectral shape appears to vary over the course of our observation. Contrary to prior reports, however, the spectral variations are uncorrelated with changes in the hard X-ray flux. It is possible that the short-term spectral variability we observe results from column density fluctuations in the ionized absorber. A power-law fit to the spectrum above 1 keV yields a photon index of Gamma approximate to 0.6, much flatter than that typically observed in the spectra of Seyfert 1 galaxies. We have ruled out photon pile-up as the cause of the flat spectrum. Even when complex spectral features are considered, the photon index is constrained to be Gamma < 1.25 (90% confidence). Comparing our results with previous determinations of the photon index (Gamma = 1.46 and 1.72), we conclude that the slope of the primary continuum varies significantly on timescales of 1 yr or less. The extreme flatness and dramatic long-term variability of the X-ray spectrum are unprecedented among active galactic nuclei.
引用
收藏
页码:2108 / 2118
页数:11
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