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Register a new userHigh-Resolution Ultraviolet Spectra of the Dwarf Seyfert 1 Galaxy NGC 4395: Evidence for Intrinsic Absorption
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Added by
D. Michael Crenshaw
Publication date
2004
fields
Physics
and research's language is
English
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We present ultraviolet spectra of the dwarf Seyfert 1 nucleus of NGC 4395, obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE) and the Hubble Space Telescopes Space Telescope Imaging Spectrograph at velocity resolutions of 7 to 15 km/sec. We confirm our earlier claim of C IV absorption in low-resolution UV spectra and detect a number of other absorption lines with lower ionization potentials. In addition to the Galactic lines, we identify two kinematic components of absorption that are likely to be intrinsic to NGC 4395. We consider possible origins of the absorption, including the interstellar medium (ISM) of NGC 4395, the narrow-line region (NLR), outflowing UV absorbers, and X-ray ``warm absorbers. Component 1, at a radial velocity of -770 km/sec with respect to the nucleus, is only identified in the C IV 1548.2 line. It most likely represents an outflowing UV absorber, similar to those seen in a majority of Seyfert 1 galaxies, although additional observations are needed to confirm the reality of this feature. Component 2, at -114 km/sec, most likely arises in the ISM of NGC 4395; its ionic column densities cannot be matched by photoionization models with a power-law continuum. Our models of the highly ionized X-ray absorbers claimed for this active galactic nucleus indicate that they would have undetectable C IV absorption, but large O VI and H I columns should be present. We attribute our lack of detection of the O VI and Ly-beta absorption from the X-ray absorbers to a combination of noise and dilution of the nuclear spectrum by hot stars in the large FUSE aperture.
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We report on two Chandra observations, and a simultaneous Hubble Space Telescope ultraviolet observation, of the dwarf Seyfert 1 galaxy NGC 4395. Each Chandra observation had a duration of ~30 ks, with a separation of ~50 ks. The spectrum was observed to harden between these observations via a scaling down of the soft-band flux. The inter-observation variability is in a different sense to the observed variability within each observation and is most likely the result of increased absorption. Spectral variations were seen during the first observation suggesting that the X-ray emission is produced in more than one disconnected region. We have also re-analyzed a ~17 ks Chandra observation conducted in 2000. During the three Chandra observations the 2-10 keV flux is about a factor of 2 lower than seen during an XMM-Newton observation conducted in 2003. Moreover, the fractional variability amplitude exhibited during the XMM-Newton observation is significantly softer than seen during the Chandra observations. A power-spectral analysis of the first of the two new Chandra observations revealed a peak at 341s with a formal detection significance of 99%. A similar peak was seen previously in the 2000 Chandra data. However, the detection of this feature is tentative given that it was found in neither the second of our two new Chandra observations nor the XMM-Newton data, and it is much narrower than expected. The Hubble Space Telescope observation was conducted during part of the second Chandra visit. A zero-lag correlation between the ultraviolet and X-ray fluxes was detected with a significance of about 99.5%, consistent with the predictions of the two-phase model for the X-ray emission from active galactic nuclei.
We present a flux-resolved X-ray analysis of the dwarf Seyfert 1.8 galaxy NGC 4395, based on three archival $XMM-Newton$ and one archival $NuSTAR$ observations. The source is known to harbor a low mass black hole ($sim 10^4- {rm a~ few~}times 10^{5}~rm M_odot$) and shows strong variability in the full X-ray range during these observations. We model the flux-resolved spectra of the source assuming three absorbing layers: neutral, mildly ionized, and highly ionized ($N_{rm H} sim 1.6times 10^{22}-3.4 times 10^{23}~rm cm^{-2}$, $sim 0.8-7.8 times 10^{22}~rm cm^{-2}$, and $ 3.8 times 10^{22}~rm cm^{-2}$, respectively. The source also shows intrinsic variability by a factor of $sim 3$, on short timescales, due to changes in the nuclear flux, assumed to be a power law ($Gamma = 1.6-1.67$). Our results show a positive correlation between the intrinsic flux and the absorbers ionization parameter. The covering fraction of the neutral absorber varies during the first $XMM-Newton$ observation, which could explain the pronounced soft X-ray variability. However, the source remains fully covered by this layer during the other two observations, largely suppressing the soft X-ray variability. This suggests an inhomogeneous and layered structure in the broad line region. We also find a difference in the characteristic timescale of the power spectra between different energy ranges and observations. We finally show simulated spectra with $XRISM$, $Athena$, and $eXTP$, which will allow us to characterize the different absorbers, study their dynamics, and will help us identify their locations and sizes.
We present optical observations of the low-luminosity Seyfert 1 nucleus of NGC 4395, as part of a multiwavelength reverberation-mapping program. Observations were carried out over two nights in 2004 April at Lick, Wise, and Kitt Peak Observatories. We obtained V-band and B-band photometry, and spectra over the range 3500-6800 Angstroms. Simultaneous Hubble Space Telescope UV and Chandra X-ray observations are presented in companion papers. Even though NGC 4395 was in an extremely low state of activity, we detect significant continuum variability of 2-10%, increasing toward shorter wavelengths. The continuum light curves, both spectroscopic and photometric, are qualitatively similar to the simultaneous UV and X-ray light curves. Inter-band cross-correlations suggest that the optical continuum emission lags behind the UV continuum emission by 24 +7/-9 min, and that the optical continuum emission lags behind the X-ray continuum emission by 44 +/- 13 min, consistent with a reprocessing model for active galactic nucleus emission. There are also hints of Balmer emission lines lagging behind the optical continuum by an amount slightly larger than the emission-line lag detected in the UV. These results are all similar to those of other Seyfert 1 nuclei. The emission-line lag yields a mass measurement of the central black hole, which although not very significant, is consistent with the value derived from the simultaneous UV data.
A reverberation-mapping program on NGC 4395, the least-luminous known Seyfert 1 galaxy, undertaken with the Space Telescope Imaging Spectrograph on the Hubble Space Telescope, yields a measurement of the mass of the central black hole of 360,000 solar masses. The observations consist of two visits of 5 orbits each, in 2004 April and July. During each of these visits, the UV continuum varied by at least 10% (rms) and only C IV 1549 showed corresponding variations large enough to reliably determine the emission-line lag, which was measured to be of order one hour for both visits. The size of the C IV-emitting region is about a factor of three smaller than expected if the slope of the broad-line region radius-luminosity relationship is identical to that for the H-beta emission line. NGC 4395 is underluminous even for its small black hole mass; the Eddington ratio of 0.0012 is lower than that of any other active galactic nucleus for which a black hole mass measurement has been made by emission-line reverberation.
We present far-ultraviolet spectra of the Seyfert 1 galaxy Mrk 509 obtained in 1999 November with the Far Ultraviolet Spectroscopic Explorer (FUSE). Our data span the observed wavelength range 915-1185 A at a resolution of ~20 km/s. The spectrum shows a blue continuum, broad OVI 1032,1038 emission, and a broad CIII 977 emission line. Superposed on these emission components, we resolve associated absorption lines of OVI 1032,1038, CIII 977, and Lyman lines through Lzeta. Seven distinct kinematic components are present, spanning a velocity range of -440 to +170 km/s relative to the systemic velocity. The absorption is clustered in two groups, one centered at -370m km/s and another at the systemic velocity. The blue-shifted cluster may be associated with the extended line emission visible in deep images of Mrk 509 obtained by Phillips et al. Although several components appear to be saturated, they are not black at their centers. Partial covering or scattering permits ~7% of the broad-line or continuum flux to be unaffected by absorption. Of the multiple components, only one has the same ionization state and column density as highly ionized gas that produces the OVII and OVIII ionization edges in X-ray spectra of Mrk 509. This paper will appear in a special issue of Astrophysical Journal Letters devoted to the first scientific results from the FUSE mission.
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