X-ray Source NGC Research Articles

Further probing of the X-ray source in NGC 4151: new constraints on the nuclear geometry

We report on the results of new Ginga observations of the archetypal Seyfert galaxy, NGC 4151, performed over an 18-month period between early 1990 and mid-1991. The present program complements earlier extensive monitoring of this source in the medium-energy X-ray band by both EXOSAT and Ginga to give a combined data set of some 36 separate observations made over a period of 8 years. The new observations bring to light a remarkable transient change in the low-energy extinction and soft X-ray excess apparent in the X-ray spectrum of NGC 4151. The spectrum became systematically softer below about 4 keV from mid-1988, with the spectral softness peaking in mid-1990 and then reverting to its former state by late 1990. Interpreting this in terms of the partial covering model, the covered fraction of the source, which normally takes a value of 80-90 percent, underwent a sharp decline to about 20 percent over a 2-year period and then returned to the canonical level over the following six months. This observation favors the presence of a normally stable 'bulk' structure such as an edge-on accretion disk. A transitory change of azimuthal structure in the disk could then account for the apparent dip in the covered fraction. If the central X-ray source radiates isotropically then the medium producing the bulk of the iron-line emission must deviate from a spherically symmetric distribution.

Correlated hard X-ray and ultraviolet variability in NGC 5548

view Abstract Citations (159) References (48) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Correlated Hard X-Ray and Ultraviolet Variability in NGC 5548 Clavel, J. ; Nandra, K. ; Makino, F. ; Pounds, K. A. ; Reichert, G. A. ; Urry, C. M. ; Wamsteker, W. ; Peracaula-Bosch, M. ; Stewart, G. C. ; Otani, C. Abstract We present the result of a series of contemporaneous (within one day) hard X-ray (Ginga) and ultraviolet (IUE) observations of the Seyfert 1 galaxy NGC 5548. Nine of these observations were carried out over a period of 51 days in 1990 May-July, and two in 1989. An additional simultaneous IUE-EXOSAT observation obtained on 1984 May 21 is also analyzed. In 1989-1990, the 1350 A continuum and 2-10 keV flux both varied by a factor 2.6 in a highly correlated (at better than the 99% confidence level) fashion. Adding the single 1984 data point significantly weakens the correlation, since the UV flux was 3.5 times larger then than its average 1990 value, while the X-ray flux was only 70% higher. In 1990 the UV and X-ray flux variations appear to be simultaneous to within a few (<= 6) days. These results, together with the simultaneity of the variations at 5000 and 1350 A, are incompatible with the standard geometrically thin accretion disk model. To salvage the disk model, one must postulate the existence of at least two continuum emission components in the UV-optical regime. The first one arises from thermal reprocessing of the X-ray impinging on and absorbed by the disk, while the other represents the thermal emission generated by viscous processes inside the disk itself. In this scheme, the reprocessed component varies in phase with the hard X-rays and accounts for the bulk of the UV emission when NGC 5548 is in a low to intermediate state of brightness, while larger amplitude variations are driven by a change in the amount of gravitational energy released locally inside the disk. The thermal reprocessing region extends to at least 200 Schwarzschild radii, much further out than the UV-emitting region in a standard disk without reprocessing. Since the relationship between the UV and the X-ray source in NGC 5548 is phenomenologically identical to that in NGC 4151, this scheme may be applicable to Seyfert galaxies as a class. Publication: The Astrophysical Journal Pub Date: July 1992 DOI: 10.1086/171490 Bibcode: 1992ApJ...393..113C Keywords: Galactic Clusters; Ginga Satellite; Iue; Seyfert Galaxies; X Ray Sources; Accretion Disks; Astronomical Models; Potassium; Thermal Emission; Ultraviolet Radiation; Astrophysics; GALAXIES: INDIVIDUAL NGC NUMBER: NGC 5548; GALAXIES: SEYFERT; ULTRAVIOLET: GALAXIES; X-RAYS: GALAXIES full text sources ADS | data products SIMBAD (5) NED (1) MAST (1) INES (1)

Einstein X-ray Observations of Planetary Nebulae and their Implications

Central stars of nineteen planetary nebulae were observed for X-ray emission using the Einstein Observatory and four of them were detected. High resolution observations with the Einstein Observatory indicates that the X-ray source in NGC 246 is a point source. These planetary nebulae with positive observations turn out to be the nearest, have the least extinction and also have the largest size of the nebulae around them. It is possible that X-ray emission is observed from these planetary nebulae with larger ages because of the smaller extinction by the nebulae and also due to the settling of heavy elements in the central star which otherwise prevents escape of X-rays by providing opacity.

Einstein X-ray observations of planetary nebulae and their implications

Central stars of 19 planetary nebulas were observed for X-ray emission using the Einstein Observatory and in four of them X-rays were detected. High-resolution observation with the Einstein Observatory indicates that the X-ray source in NGC 246 is a point source. These planetary nebulas with positive observation turn out to be the nearest, have the least extinction, and also have the largest size of the nebulas in their vicinity. It is possible that X-ray emission is observed from these planetary nebulas with larger ages because of the smaller extinction by the nebulas and also because of the settling of heavy elements in the central star, which would otherwise prevent escape of X-rays by providing opacity.

Ultraviolet spectroscopic measurements of globular clusters

Ultraviolet spectra of six globular clusters (M15, M92, NGC 1851, NGC 6624, 47 Tuc, and NGC 6752) have been taken with the International Ultraviolet Explorer satellite in order to investigate the surface brightness distributions in the central regions. The ultraviolet emission indicates composite stellar spectra. Short-wavelength emission (1200-1900 A) arises from blue horizontal-branch stars; long-wavelength emission (2100-3000 A) is characteristic of late-type horizontal-branch and giant stars. The surface brightness distribution is more compact at short wavelengths than at long wavelengths, suggesting a segregation of horizontal-branch stars. A dense core appears to be present in X-ray-emitting clusters and absent in those clusters without X-ray sources. The X-ray source in NGC 6624 may have been detected at short wavelengths (1300-1900 A).

Observations of variable X-ray sources in globular clusters

The discovery is reported of a variable X-ray source at midgalactic latitude in the globular cluster NGC 1851, and observations are reported which confirm the existence of variable X-ray sources in NGC 7078 and NGC 6441. The new source, designated MX 0513-40, was found to vary in intensity by a factor of at least 5 over a sixteen-day period and to have a hard spectrum as compared to that of the Crab Nebula in the energy range from 1 to 10 keV. The source in NGC 7078 (3U 2131+11) was found to vary by more than a factor of 2 over a 500-day period. Strong evidence of variability is presented for the source in NGC 6441 (3U 1746-37). It is concluded that both MX 0513-40 and 3U 2131+11 are probably stellar objects and are no more than about 160 kpc distant.