R. Mark Wagner

Optical Continuum and Emission-Line Variability of Seyfert 1 Galaxies

We present the light curves obtained during an 8 yr program of optical spectroscopic monitoring of nine Seyfert 1 galaxies: 3C 120, Akn 120, Mrk 79, Mrk 110, Mrk 335, Mrk 509, Mrk 590, Mrk 704, and Mrk 817. All objects show significant variability in both the continuum and emission-line fluxes. We use cross-correlation analysis to derive the sizes of the broad Hβ-emitting regions based on emission-line time delays, or lags. We successfully measure time delays for eight of the nine sources and find values ranging from about 2 weeks to a little over 2 months. Combining the measured lags and widths of the variable parts of the emission lines allows us to make virial mass estimates for the active nucleus in each galaxy. The virial masses are in the range 107-108 M☉.

An energetic stellar outburst accompanied by circumstellar light echoes

Some classes of stars, including novae and supernovae, undergo explosive outbursts that eject stellar material into space. In 2002, the previously unknown variable star V838 Monocerotis brightened suddenly by a factor of ∼104. Unlike a supernova or nova, it did not explosively eject its outer layers; rather, it simply expanded to become a cool supergiant with a moderate-velocity stellar wind. Superluminal light echoes were discovered as light from the outburst propagated into the surrounding, pre-existing circumstellar dust. Here we report high-resolution imaging and polarimetry of those light echoes, which allow us to set direct geometric distance limits to the object. At a distance of >6 kpc, V838 Mon at its maximum brightness was temporarily the brightest star in the Milky Way. The presence of the circumstellar dust implies that previous eruptions have occurred, and spectra show it to be a binary system. When combined with the high luminosity and unusual outburst behaviour, these characteristics indicate that V838 Mon represents a hitherto unknown type of stellar outburst, for which we have no completely satisfactory physical explanation.

The Halo Black Hole X-Ray Transient XTE J1118+480*

Optical spectra were obtained of the optical counterpart of the high-latitude (b 62°) soft X-ray transient XTE J1118+480 near its quiescent state (R 18.3) with the new 6.5 m Multiple Mirror Telescope and the 4.2 m William Herschel Telescope. The spectrum exhibits broad, double-peaked emission lines of hydrogen (FWHM 2400 km s-1) arising from an accretion disk superposed with absorption lines of a late-type secondary star. Cross-correlation of the 27 individual spectra with late-type stellar template spectra reveals a sinusoidal variation in radial velocity with amplitude K = 701 ± 10 km s-1 and orbital period P = 0.169930 ± 0.000004 days. The mass function, 6.1 ± 0.3 M☉, is a firm lower limit on the mass of the compact object and strongly implies that it is a black hole. We estimate the spectral type of the secondary to be K7 V-M0 V, and that it contributes 28% ± 2% of the light in the 5800-6400 A region on 2000 November 20, increasing to 36% ± 2% by 2001 January 4 as the disk faded. Photometric observations (R-band) with the Instituto de Astrofisica de Canarias 0.8 m telescope reveal ellipsoidal light variations of full amplitude 0.2 mag. Modeling of the light curve gives a large mass ratio (M1/M2 ~ 20) and a high orbital inclination (i = 81° ± 2°). Our combined fits yield a mass of the black hole in the range M1 = 6.0-7.7 M☉ (90% confidence) for plausible secondary star masses of M2 = 0.09-0.5 M☉. The photometric period measured during the outburst is 0.5% longer than our orbital period and probably reflects superhump modulations, as observed in some other soft X-ray transients. The estimated distance is d = 1.9 ± 0.4 kpc, corresponding to a height of 1.7 ± 0.4 kpc above the Galactic plane. The spectroscopic, photometric, and dynamical results indicate that XTE J1118+480 is the first firmly identified black hole X-ray system in the Galactic halo.

Nova Sagittarii 1994 1 (V4332 Sagittarii): The Discovery and Evolution of an Unusual Luminous Red Variable Star

We report photometry and spectroscopy of the evolution of Nova Sagittarii 1994 1 (V4332 Sagittarii) during outburst. We compare the photometric and spectral evolution of this outburst with known classes of outbursts—including classical novae and outbursts occurring on symbiotic stars—and find this object does not conform to any known class of outburst. The closest match to the behavior of this unusual object is M31 RV, an extremely luminous and red variable object discovered in the bulge of M31 in 1988. However, the temporal behavior and maximum luminosity of the two events differ by several orders of magnitude, requiring substantial intrinsic variation if these two events are members the same type of outburst. Our model of the spectroscopic evolution of this outburst shows that the effective temperature cooled from 4400 to 2300 K over the 3 month span of our observations. In combination with line diagnostics in our later spectra, including [O I] λ5577 and the dramatic increase in the Hα-to-Hβ ratio, we infer the existence of a cool, dense (Ne ~ 108–109 cm-3) envelope that is optically thick in the hydrogen Balmer recombination lines (case C). We suggest that a nuclear event in a single star, in which a slow shock drove the photosphere outward, can power the observed luminosity evolution and the emission spectrum.

V838 MONOCEROTIS: A GEOMETRIC DISTANCE FROM HUBBLE SPACE TELESCOPE POLARIMETRIC IMAGING OF ITS LIGHT ECHO*

Following the outburst of the unusual variable star V838 Monocerotis in 2002, a spectacular light echo appeared. A light echo provides the possibility of direct geometric distance determination, because it should contain a ring of highly linearly polarized light at a linear radius of ct, where t is the time since the outburst. We present imaging polarimetry of the V838 Mon light echo, obtained in 2002 and 2005 with the Advanced Camera for Surveys on board the Hubble Space Telescope, which confirms the presence of the highly polarized ring. Based on detailed modeling that takes into account the outburst light curve, the paraboloidal echo geometry, and the physics of dust scattering and polarization, we find a distance of 6.1 ± 0.6 kpc. The error is dominated by the systematic uncertainty in the scattering angle of maximum linear polarization, taken to be θmax = 90° ± 5°. The polarimetric distance agrees remarkably well with a distance of 6.2 ± 1.2 kpc obtained from the entirely independent method of main-sequence fitting to a sparse star cluster associated with V838 Mon. At this distance, V838 Mon at maximum light had MV −9.8, making it temporarily one of the most luminous stars in the Local Group. Our validation of the polarimetric method offers promise for measurement of extragalactic distances using supernova light echoes.

The early ultraviolet, optical, and radio evolution of the soft X-ray transient GRO J0422+32

We have monitored the evolution of the transient X-ray source GRO J0422+32 from approximately 2 weeks postdiscovery into its early decline phase at ultraviolet, optical, and radio wavelengths. Optical and ultraviolet spectra exhibit numerous, but relatively weak, high-excitation emission lines such as those arising from He II, N III, N V, and C IV superposed on an intrinsically blue continuum. High-resolution optical spectroscopy reveals line profiles which are double peaked, and in the case of the higher order Balmer lines, superposed on a broad absorption profile. The early outburst optical-ultraviolet continuum energy distribution is well represented by a two power-law fit with a break at approximately equal 4000 A. Radio observations with the Very Large Array (VLA) reveal a flat-spectrum source, slowly increasing in intensity at the earliest epochs observed, followed by an approximate power-law decay light curve with an index of -1. Light curves for each wavelength domain are presented and discussed. Notable are the multiple secondary outbursts seen in the optical more than 1 year postdiscovery, and spectral changes associated with secondary rises seen in the radio and UV. We find that the ultraviolet and optical characteristics of GRO J0422+32 as well as its radio evolution, are similar to other recent well-observed soft X-ray transients (also called X-ray novae) such as Cen X-4, A0620-00 (V616 Mon), and Nova Muscae 1991 (GS 1124-683), suggesting that GRO J0422+32 is also a member of that subclass of low-mass X-ray binaries. We present definitive astrometric determination of the source position, and place an upper limit of R approximately equals 20 from our analysis of the Palomar Observatory Sky Survey (POSS). Additionally, we derive distinct values for color excess from analysis of the optical (E(B-V) = 0.23) and ultraviolet (E(B-V) = 0.4) data, suggesting an intrinsic magnitude of 19-19.5 for the progenitor if it is mid-K dwarf. This leads to a likely range of 2.4-3.0 kpc for the source distance, which is consistent with our separate estimate of 2.4 +/- 0.4 kpc based on measurement of the NaD interstellar line profile. Adopting 2.4 kpc and E(B-V) = 0.23, the outburst absolute magnitude was M approximately equals 0.0, which is a typical value for this class of objects.

ROSAT observations of the black hole candidate V404 Cygni in quiescence

V404 Cyg was detected with the ROSAT PSPC in a 16 ks observation 1265 days after the 1989 May outburst. This is the first unambiguous detection of a soft X-ray transient or X-ray nova black hole candidate in quiescence. The observed X-ray spectrum is extremely soft and can be described equally well by either a blackbody, power-law, or a thermal bremsstrahlung continuum. The best-fit blackbody spectrum is characterized by a temperature of 0.2 keV. A significant amount of absorption, both along the line of sight and intrinsic to the V404 Cyg system, is required to understand the observations. The 0.2-2.4 keV light curve of V404 Cyg exhibits substantial variability on timescales of less than a day. Assuming a distance of 3.5 kpc, the quiescent X-ray luminosity of V404 Cyg is 8 x 10(exp 33) ergs/s, nearly two orders of magnitude larger than the upper such as Cen X-4. The lack of a significant hard X-ray luminosity in quiescence, the presence of a large and cold neutral region in the accretion disk, and a low-mass accretion rate suggests that an accretion disk instability might account for the outburst of V404 Cyg.

Broad Emission-line Variability in Markarian 335

We present the results of 6 years of spectroscopic monitoring of the Seyfert 1 galaxy Mrk 335. Each year, the observations span 5¨7 months at a nominal sampling rate of one observation per week. Analysis of the spectroscopic observations shows clear coordinated variability in the continuum and in the broad Hb and He II j4686 emission lines. The principal goal of this monitoring program is to measure the time delay between the continuum variations and the corresponding responses of the broad Hb and He II emission lines. We derive time delays of D17 days for Hb and D2 days for He II. From the spectral time series we construct average, root mean square, and diUerence spectra, which show that the broad Hb variations are con—ned to the core of the broad emission lines, whereas the He II varia- tions take place across the full width of the pro—le. The He II emission line is about twice as broad as the Hb line. Comparison of the He II and Hb lags and line widths indicates that the BLR has a radially strati—ed ionization structure and that the highest velocity gas is closest to the continuum source. We also —nd no evidence for correlation between the line pro—le variations and the continuum and emission- line —ux variations, as has been found in other cases. Rather, the line pro—le variations seem to imply dynamical changes in the BLR. Subject headings: galaxies: activegalaxies: individual (Markarian 335) ¨ galaxies: Seyfert

M33's variable a: a hypergiant star more than 35 years in eruption

Variable A in M33 is a member of a rare class of highly luminous, evolved stars near the upper luminosity boundary that show sudden and dramatic shifts in apparent temperature due to the formation of optically thick winds in high mass loss episodes. Recent optical and infrared spectroscopy and imaging reveal that its eruption, begun in ~1950, has ended, having lasted ≈45 yr. Our current observations show major changes in its wind from a cool, dense envelope to a much warmer state surrounded by low-density gas with rare emission lines of Ca II, [Ca II], and K I. Its spectral energy distribution has unexpectedly changed, especially at the long wavelengths, with a significant decrease in its apparent flux, while the star remains optically obscured. We conclude that much of its radiation is now escaping out of our line of sight. We attribute this to the changing structure and distribution of its circumstellar ejecta, corresponding to the altered state of its wind as the star recovers from a high mass loss event.

ASCA, RXTE, EUVE, and optical observations of the high magnetic field cataclysmic variable AR Ursae Majoris

The highest field polar AR UMa was observed with ASCA during a low state, and with simultaneous EUVE, RXTE, and ground-based optical photometry during a high state. The marginal detection at the low state places a limit on the hard X-ray emission, which is a factor of 5 below the high-state flux limit. The high-state EUV light curves are highly modulated with peak flux at phase 0.9, but the flux never entirely disappears, implying some view of a heated area at all times of the orbit. The spectra during bright phases suggest a temperature of 265,000 K while the fainter phases are cooler, ~215,000 K, and the spectrum is hottest (320,000 K) during the stream dip phases. However, neither a blackbody nor a standard stellar atmosphere model provides a good fit to the data. In order for the EUV modulation to be consistent with geometrical parameters derived from polarimetry and spectroscopy, the primary heated area on the white dwarf must be geometrically extended from the southern magnetic pole while the northern pole is constantly in view. The temperatures of the heated regions are typical of polars, but the projected areas are small, which could be due to the lack of a good view of the main pole at the low inclination of the system. The RXTE light curve shows no modulation over the orbit and only a marginal detection in hard X-rays, implying a weak bremsstrahlung component that is typical for the highest magnetic field polars. In the optical, a low-amplitude sinusoidal modulation peaking at phase 0.5 is consistent with an origin from the irradiated secondary.