T. Ohashi

X-ray evidence for Seyfert activity buried in the infrared galaxy NGC 4945

We have observed the infrared galaxy NGC 4945 using the Japanese X-ray astronomy satellite, Ginga. The X-ray spectrum is found to be composed of three components: hard X-ray emission which is heavily absorbed by cold material with column density of about 10 24.7 cm −2 ; a soft X-ray component seen in the 2-10 keV band; and an iron emission line. The hard component exhibits a power-law spectrum with photon index of about 1.7. The mean X-ray luminosity in the 2-20 keV band, after correction for absorption is 3×10 42 ergs s −1 with significant intensity variation on a time scale of several hours. These results are thought to be evidence for an AGN (active galactic nucleus) in the galaxy

The X-ray spectra of high-luminosity active galactic nuclei observed by Ginga

Results are presented on the X-ray emission from 13 objects, observed by Ginga as part of a spectral survey of bright quasars in the energy range 2-20 keV. The distribution of the power-law energy index for this sample has a mean of 0.81 and shows significant intrinsic dispersion, σ = 0.31, values which are broadly compatible with those from samples of lower luminosity active galactic nuclei. The mean spectral slope is clearly too sleep to fit the spectrum of the cosmic diffuse X-ray background (CXB) at energies less than 20 keV, confirming the spectral paradox in the discrete-source explanation of the CXB

The X-ray spectral variability of the BL Lacertae type object PKS 2155-304

We present a detailed study of the hard X-ray properties of the BL Lacertae object PKS 2155-304 based on measurements made in 1988 and 1989 with the Large Area Counter (LAC) on board the Ginga satellite. The source exhibited a high degree of variability with a dynamic range of a factor 7 in the 2-6 keV band. The fastest amplitude variation was a factor 2 decline in the intensity in this band within 4 hours. The spectrum is characterized by a break which occurs at about 4 keV. Spectral fits to the data integrated in 6400 s time bins reveal that, in common with previous observations of BL Lacertae objects, the spectral slope is generally anticorrelated with intensity in the sense that the spectrum hardens as the intensity increases. However, the tracks of sequential points in the index-intensity plane are occasionally seen to differ during the rise and decay stages of individual flares. Furthermore, during one, or possibly two, flaring episodes the spectral index is observed to correlate with intensity variations.

Discovery of a prominent cyclotron absorption feature from the transient X-ray pulsar X0331+53

A remarkable absorption feature at 28.5 keV, attributable to electron cyclotron resonance, has been discovered in the 1.9-60-keV X-ray spectrum of the recurrent transient X-ray pulsar X0331 + 53. The observed resonance energy implies a neutron star surface magnetic field of 2.5(1 + z) x 10 to the 12th G, where z is the gravitational redshift. The detection was made with the Ginga observatory in October 1989, during an outburst of this transient with a flux level of about 0.3 Crab. The feature is very deep and has been resolved with excellent statistics. This is the fourth unambiguous detection of cyclotron resonant scattering features from X-ray pulsars, suggesting that these features are a common phenomenon among these objects. An empirical relation found between the cyclotron resonance energy and the spectral cutoff energy suggests that the magnetic field strengths of the known X-ray pulsars are clustered in a range (1-4) x 10 to the 12th G. 30 refs.

GINGA observations of three X-ray-luminous early-type galaxies : NGC 4472, NGC 4636, and NGC 3998

Observations of three X-ray luminous early-type galaxies, NGC 4472, NGC 4636 and NGC 3998, with the satellite Ginga, are presented. The X-ray spectra of NGC 4472 and NGC 4636 can be described as thermal emission from an optically thin plasma. The temperatures of the hot gas are determined. The X-ray spectrum of NGC 3998 is well fitted by a power-law model having a photon index of 2.0, the same sort of spectrum typical of AGN.

X-ray outburst of the quasar 3C 279

A large X-ray flare-up of the quasar 3C 279, coinciding with an unusual multifrequency outburst, was detected with X-ray astronomy satellite Ginga. Observations carried out on two occasions indicate that the 2-20 keV flux has increased by a factor of more than 4, from 1.2 to 5.3 micro-Jy at 2 keV, within about a year. During the outburst, the X-ray spectrum became slightly harder than that in the quiet period, the photon index changing from Gamma = 1.70 + or - 0.06 to 1.58 + or -0.03. The X-ray flux varied by 20 percent during the outburst within a period as short as 45 minutes, which indicates Delta L/Delta t greater than 2 x 10 to the 42nd ergs/sq s for H(0) = 100 km/s mpc and q(0) = 0.5, the largest value ever observed for a quasar. The observed Delta L/Delta t suggests that the X-ray-emitting region is moving with a relativistic velocity toward us. These results suggest that a relativistic jet is being formed. 33 refs.

H0538 + 608 (= BY Camelopardalis) - An asynchronously rotating AM Herculis binary?

Evidence from a concurrent X-ray/optical observing program is presented which indicates that the orbital period of the anomalous AM Her-type cataclysmic variable H0538 + 608 (BY Cam) differs from the rotational period by 1.3 +/- 0.1 percent. This finding would explain much of the unusual behavior previously exhibited in optical photometry and polarimetry. The model proposed by Ishida et al. (1991) is developed further using observations of accretion and the primary characteristics of optical and X-ray light curves and radial velocity measures of H-alpha. An optical QPO with a period of about 30 min was seen on two nights and possible causes of it are discussed.

Discovery of a cyclotron resonance feature at 30 keV from the transient X-ray pulsar Cepheus X-4

From Ginga observations of the transient X-ray pulsar Cep X-4, a spectral line feature attributable to electron cyclotron resonance was discovered at about 30 keV in the 1.2−37 keV X-ray spectrum. The detection, a fifth firm example of cyclotron resonance from X-ray pulsars, implies a surface magnetic field of about 2.6×10 12 (1+z) G for this pulsar, where z is the gravitational redshift. Throughout the 66.25 s pulses, the cyclotron feature appears in absorption, with at most ±5% variation in the resonance center energy around the mean value of 30.5±0.4 keV

Simultaneous U, B, V, and X-ray measurements of a burst from 4U/MXB 1636-53

Data are presented on the first simultaneous X-ray and optical burst to be measured in more than one optical color. Various analyses agree that, to a first approximation, the optical burst is produced through blackbody reprocessing of the X-ray burst, with a short delay. Depending on the technique used, the value of the delay is 2 or 3 s. The smearing of the optical signal is determined to be less than 3 s. The temperature of the optical reprocessor ranges from approximately 25,000 K at quiescence to approximately 50,000 K at burst maximum. An extinction toward the source is derived from the color-color diagram, suggesting a distance greater than or approximately equal to 2 kpc. The projected effective area of the blackbody reprocessor is approximately 5 x 10 to the 21st (D/5 kpc)-squared sq cm. The fraction of the total X-ray burst energy converted into optical energy at all wavelengths is, within an order of magnitude, approximately 3 percent. These parameters are discussed in relation to the 4 hr orbital periodicity in the system reported by Pedersen et al. (1981).

X-RAY OBSERVATIONS OF NGC-253 AND M83 WITH GINGA

X-ray spectra of two starburst galaxies NGC 253 and M83 in 2-20 keV have been obtained with the X-ray astronomy satellite Ginga. The observed spectra can be described by thermal bremsstrahlung models with temperatures of 6-7 keV for both galaxies, with no significant absorption. The estimated luminosities in 2-10 keV are about 1 x 10 to the 40th ergs/s and about 8 x 10 to the 39th ergs/s for NGC 253 and M83, respectively, consistent with IPC measurements in a lower energy band. No significant iron K-emission line was detected from either galaxy with 90 percent upper limit on the equivalent width about 400 eV, suggesting a significant depletion of iron if most of the continuum emission is of thermal origin. The spectral shape and the enhanced X-ray luminosity of these two galaxies, compared with M31 or the Galaxy, suggest that a major fraction of the X-ray emission may originate from hot gas. However, the lack of an iron emission line remains a puzzle. 37 refs.