Paul J. Wiita

Accretion disk models for optical and ultraviolet microvariability in active galactic nuclei

We describe scenarios for rapid, low-amplitude UV and optical variability in AGNs through phenomenologically modeled asymmetries on accretion disks, which can represent perturbations arising from magnetic, gravitational, and other instabilities. The simulations were done for both the more familiar α-disks, where the viscous shear, t rφ , is proportional to the total pressure, as well as for the β-disk geometry, where the shear is proportional to only the gas pressure; a range of parameters for flare properties and disk structures were considered

Spiral shocks in accretion disks as a contributor to variability in active galactic nuclei

We simulate the variations in emission from accretion disks subjected to nonaxisymmetric tidal perturbations generated by masses within or passing through the disk, using a two-dimensional hydrodynamics code. Spiral shocks can form, fragment, and reform on relatively short time scales, producing variations in the optical band for accretion disks around supermassive black holes. These fluctuations may be adequate to explain much of the recently observed microvariability in BL Lacertae objects and some quasars. For short-lived encounters, substantial flaring can occur

Periodic Oscillations in the Intra-Day Optical Light Curves of the Blazar S5 0716+714

We present the results of a periodicity search of 20 intra-day variable optical light curves of the blazar S5 0716+714, selected from a database of 102 light curves spanning over three years. We use a wavelet analysis technique along with a randomization test and find strong candidates for nearly periodic variations in eight light curves, with probabilities ranging from 95% to >99%. This is the first good evidence for periodic, or more precisely, quasi-periodic, components in the optical intra-day variable light curves of any blazar. Such periodic flux changes support the idea that some active galactic nuclei variability, even in blazars, is based on accretion disk fluctuations or oscillations. These intra-day variability timescales are used to estimate that the central black hole of the blazar S5 0716+714 has a mass >2.5 × 106 M ☉. As we did not find any correlations between the flux levels and intra-day variability timescales, it appears that more than one emission mechanism is at work in this blazar.

The N Enrichment and Supernova Ejection of the Runaway Microquasar LS 5039

We present an investigation of new optical and ultraviolet spectra of the mass donor star in the massive X-ray binary LS 5039. The optical band spectral line strengths indicate that the atmosphere is N-rich and C-poor, and we classify the stellar spectrum as type ON6.5 V((f)). The N-strong and C-weak pattern is also found in the stellar wind P Cygni lines of N V lambda1240 and C IV lambda1550 (narrow absorption components in the former indicate that the wind terminal velocity is V∞=2440+/-190 km s-1). We suggest that the N enrichment may result from internal mixing if the O star was born as a rapid rotator or the O star may have accreted N-rich gas prior to a common envelope interaction with the progenitor of the supernova. We reevaluated the orbital elements to find an orbital period of P=4.4267+/-0.0005 days. We compared the spectral line profiles with new non-LTE line-blanketed model spectra from Lanz and Hubeny, from which we derive an effective temperature Teff=37.5+/-1.7 kK, gravity logg=4.0+/-0.1, and projected rotational velocity Vsini=140+/-8 km s-1. We fitted the UV, optical, and IR flux distribution by using a model spectrum and extinction law with parameters E(B-V)=1.28+/-0.02 and R=3.18+/-0.07. We confirm the covariability of the observed X-ray flux and stellar wind mass-loss rate derived from the Halpha profile, which supports the wind accretion scenario for the X-ray production in LS 5039. Wind accretion models indicate that the compact companion has mass MX/Msolar=1.4+/-0.4, consistent with its identification as a neutron star. We argue that the O star has mass in the range 20-35 Msolar (based on a lower limit for the distance and the lack of eclipses). The observed eccentricity and runaway velocity of the binary can be reconciled only if the neutron star received a modest kick velocity due to a slight asymmetry in the supernova explosion (during which more than 5 Msolar was ejected).

The Spectral Components of SS 433

We present results from new optical and UV spectroscopy of the unusual binary system SS 433, and we discuss the relationship of the particular spectral components that we observe to the properties of the binary. These spectral components include

Wind Accretion and State Transitions in Cygnus X-1

We present the results of a spectroscopic monitoring program (from 1998 to 2002) of the Hα emission strength in HDE 226868, the optical counterpart of the black hole binary Cyg X-1. The feature provides an important probe of the mass-loss rate in the base of the stellar wind of the supergiant star. We derive an updated ephemeris for the orbit based on radial velocities measured from He I λ6678. We list net equivalent widths for the entire Hα emission/absorption complex, and we find that there are large variations in emission strength over both long (years) and short (hours to days) time spans. There are coherent orbital phase-related variations in the profiles when the spectra are grouped by Hα equivalent width. The profiles consist of (1) a P Cygni component associated with the wind of the supergiant, (2) emission components that attain high velocity at the conjunctions and that probably form in enhanced outflows both toward and away from the black hole, and (3) an emission component that moves in antiphase with the supergiants motion. We argue that the third component forms in accreted gas near the black hole and that the radial velocity curve of the emission is consistent with a mass ratio of MX/Mopt ≈ 0.36 ± 0.05. We find that there is a general anticorrelation between the Hα emission strength and X-ray flux (from the Rossi X-Ray Timing Explorer All Sky Monitor) in the sense that when the Hα emission is strong (Wλ < -0.5 A) the X-ray flux is weaker and the spectrum harder. On the other hand, there is no correlation between Hα emission strength and X-ray flux when Hα is weak. We argue that this relationship is not caused by wind X-ray absorption nor by the reduction in Hα emissivity by X-ray heating. Instead, we suggest that the Hα variations track changes in wind density and strength near the photosphere. The density of the wind determines the size of X-ray ionization zones surrounding the black hole, and these in turn control the acceleration of the wind in the direction of the black hole. During the low/hard X-ray state, the strong wind is fast and the accretion rate is relatively low, while during the high/soft state, the weaker, highly ionized wind attains only a moderate velocity and the accretion rate increases. We argue that the X-ray transitions from the normal low/hard to the rare high/soft state are triggered by episodes of decreased mass-loss rate in the supergiant donor star.

QUASI-PERIODIC OSCILLATIONS OF ∼15 MINUTES IN THE OPTICAL LIGHT CURVE OF THE BL LAC S5 0716+714

Over the course of 3 hr on 2008 December 27, we obtained optical (R band) observations of the blazar S5 0716+714 at a very fast cadence of 10 s. Using several different techniques, we find fluctuations with an approximately 15 minute quasi-period to be present in the first portion of these data at a >3σ confidence level. This is the fastest quasi-periodic oscillation that has been claimed to be observed in any blazar at any wavelength. While these data are insufficient to strongly constrain models for such fluctuations, the presence of such a short timescale when the source is not in a very low state seems to favor the action of turbulence behind a shock in the blazars relativistic jet.

Neutrino emission by the pair, plasma, and photo processes in the Weinberg-Salam model

The results of numerical integrations of the rates and emissivities of the photo, pair, and plasma neutrino emission mechanisms in the Weinberg-Salam theory of the weak interaction are presented. The range of densities 10 gm cm/sup -3/ less than or equal to rho < 10/sup 14/ gm cm/sup -3/ and the temperature range 10/sup 8/K less than or equal to T less than or equal to 10/sup 11/K are considered. Fitting formulae, similar to those provided by Beaudet, Petrosian, and Salpeter, which reproduce the numerical result for the total emissivity to within 20% in the temperature range 10/sup 8.2/K less than or equal to T less than or equal to 10/sup 11/K are presented. 24 refs., 21 figs., 1 tab.

Intranight optical variability of radio-quiet and radio lobe dominated quasars

We present results of a programme of multi-epoch, intranight optical monitoring of a sample of non-blazar-type active galactic nuclei (AGN), which includes seven radio-quiet quasars (RQQs) and an equal number of radio-loud, lobe-dominated quasars (LDQs), covering a redshift range from about 0.2 to 2.0. These two sets of optically bright and intrinsically luminous quasi-stellar objects (QSOs) are well matched in the redshift-optical luminosity (z-M B ) plane. Our CCD monitoring covered a total of 61 nights with an average of 6.1 hours of densely sampled monitoring of just a single QSO per night, thereby achieving a typical detection threshold of ∼1 per cent variation over the night. Unambiguous detection of intranight optical variability (INOV) amplitudes in the range 1-3 per cent on day-like or shorter time-scales were thus made for both RQQs and LDQs. Based on these clear detections of INOV, we estimate duty cycles of 17 and 9 per cent for RQQs and LDQs, respectively; inclusion of the two cases of probable variations of LDQs would raise the duty cycle to 15 per cent for LDQs. The similarity in the duty cycle and amplitude of INOV for the RQQs and LDQs suggests, first, that the radio loudness alone does not guarantee an enhanced INOV in QSOs and, secondly, that as in LDQs, relativistic jets may also be present in RQQs. We argue that, as compared to BL Lacs, the conspicuously milder, rarer and possibly slower INOV of RQQs and LDQs can in fact be readily understood in terms of their having optical synchrotron jets which are modestly misaligned from us, but are otherwise intrinsically as relativistic and active as the jets in BL Lacs. This points toward an orientation-based unifying scheme for the INOV of radio-loud and radio-quiet quasars. Variability of up to ∼0.3 mag on month- to year-like time-scales is seen for nearly all those RQQs and LDQs in our sample for which sufficient temporal coverage is available. These data have revealed an interesting event that seems most likely explained as an occultation, lasting less than six months, of much of the nuclear optical continuum source in an RQQ. The observations reported here form part of a larger ongoing project to study the intranight optical variability of four major classes of powerful AGN, including blazars.

Intranight optical variability of BL Lacs, radio-quiet quasars and radio-loud quasars

We report monitoring observations of 20 high-luminosity active galactic nuclei (AGN), 12 of which are radio-quiet quasars (RQQs). Intranight optical variability (INOV) was detected for 13 of the 20 objects, including 5 RQQs. The variations are distinctly stronger and more frequent for blazars than for the other AGN classes. By combining these data with results obtained earlier in our programme, we have formed an enlarged sample consisting of 9 BL Lacs, 19 RQQs and 11 lobe-dominated radio-loud quasars (RLQs). The moderate level of rapid optical variability found for both RQQs and radio lobe-dominated quasars (LDQs) argues against a direct link between INOV and radio loudness. We supplemented the present observations of 3 BL Lacs with additional data from the literature. In this extended sample of 12 well observed BL Lacs, stronger INOV is found for the EGRET detected subset.