Outburst Maximum Research Articles

VLBA Observations of a Bursting Water Maser in Orion KL

We have performed multiepoch single-telescope and Very Long Baseline Interferometry (VLBI) observations of an anomalously strong outburst of a water vapor maser in the Orion KL region that occurred in 1998. This burst, dubbed a superburst, exhibited a flux density increase that started in 1997 December and reached a maximum of 4.6 × 106 Jy in 1998 September; it then decreased to 0.2 × 106 Jy in 1999 February. The spatial and the velocity structure revealed by our observations showed that the bursting feature consisted of at least two components having two different peak velocities. Moreover, these two components exhibited a relative motion over 287 days with the relative position of the two components changing across the period of the outburst maximum. This suggests that an overlapping of two maser clouds caused the superburst. The brightness temperature reached 1016 K, while the line width was as narrow as 0.48 km s-1 at maximum phase. This extremely high brightness and narrow line width can be understood as the radiation from the maser being highly beamed.

Bipolar jet growth and decline in Hen 3-1341: a direct link to fast wind and outburst evolution★

We report on and investigate the evolution and disappearance in the symbiotic star Hen 3-1341 of collimated bipolar jets, which take the form of symmetrically displaced components of emission lines. From modelling of the emission-line spectrum it turns out that the accreting white dwarf (WD) in quiescence has T WD ∼ 1.2 x 10 5 K and R WD ∼ 0.14 R ○. , for a luminosity of 3.8 x 10 3 L ○. , and it is stably burning hydrogen on the surface at a rate of M H ∼ 5 x 10 -8 M ○. yr -1 , feeding ionizing photons to a radiation bounded circumstellar nebula extending for ∼17 au. The WD underwent a multimaxima outburst lasting from 1998 to 2004 during which its H-burning envelope reacted to a probable small increase in the mass accretion by expanding and cooling to T eff ∼ 1 x 10 4 K and R ∼ 20 R ○. , mimicking an A-type giant that radiated a total of ∼6 x 10 44 erg, at an average rate of ∼1 × 10 3 L ○. . Bipolar jets developed at the time of outburst maximum and their strength declined in parallel with the demise of the fast wind from the inflated WD, finally disappearing when the wind stopped halfway to quiescence, marking a 1:1 correspondence between jets presence and feeding action of the fast wind. The total mass in the jets was M jet ∼ 2.5 x 10 -7 M ○. for a kinetic energy of E kin jet ∼ 1.7 x 10 42 (sin i) -1 erg, corresponding to ∼0.3(sin i) -1 per cent of the energy radiated during the whole outburst. We suggest that the spectroscopic search for jets in symbiotic stars could pay higher dividends if focused on the outburst phases characterized by maximum wind intensity.

The distances to Galactic low-mass X-ray binaries: consequences for black hole luminosities and kicks

We investigated the reported distances of Galactic black hole (BH) and neutron star low‐mass X‐ray binaries (LMXBs). Comparing the distances derived for the neutron stars Cyg X–2 and XTE J2123–058 using the observed Eddington limited photospheric radius expansion bursts with the distances derived using the observed radius and effective temperature of the companion star, we find that the latter are smaller by approximately a factor of 1.5–2. The latter method is often employed to determine the distance to BH LMXBs. A possible explanation for this discrepancy is that the stellar absorption lines in fast rotating companion stars are different from those in the slowly rotating template stars as was found before for early‐type stars. This could lead to a systematic mis‐classification of the spectral type of the companion star, which in turn would yield a systematic error in the distance. Further, we derive a distance of 4.0+2.0−1.2 kpc for V404 Cyg, using parameters available in the literature. The interstellar extinction seems to have been overestimated for XTE J1550–564 and possibly for two other BH sources (H 1705–25 and GS 2000+25) as well. As a result of this, the distance to XTE J1550–564 may have been underestimated by as much as a factor three. We find that, using the new distances for XTE J1550–564 and V404 Cyg, the maximum outburst luminosity for at least five, but perhaps even seven, of the 15 BH soft X‐ray transients exceed the Eddington luminosity for a 10‐M⊙ BH – showing that these systems would be classified as ultra‐luminous X‐ray sources had we observed them in other Galaxies. This renders support for the idea that many ultra‐luminous X‐ray sources are stellar‐mass rather than intermediate‐mass BHs. We find that the rms‐value of the distance to the Galactic plane for BHs is consistent with that of neutron star LMXBs. This suggests that BHs could also receive a kick‐velocity during their formation, although this has to be investigated in more detail. We find that the Galactic neutron star and BH l‐ and b‐distributions are consistent with being the same. The neutron star and BH distribution is asymmetric in l with an excess of systems between −30° < l < 0° over systems with 0° < l < 30°.

Spectroscopic Diagnostics of Precessing Discs: The Stream-Disc Impact in XTE J1118+480 (= KV UMA) in Outburst

AbstractWe present orbital trailed spectrograms of XTE J1118+480 taken at the rise to outburst maximum in 2000. These show an evolving S-wave feature arising from the stream impact on the edge of the eccentric precessing disc. By comparing the observed S-waves with those generated in an SPH simulation, we deduce the disc precession phases corresponding to our observations. Hence we infer changes in the disc precession period around the outburst maximum: taken at face value our spectroscopy suggests a precession period temporarily as short as 20 days.

Optical Variations of XTE J1859+226 during the 1999 Outburst

Abstract We report on our optical photometric observations of a black hole X-ray transient, XTE J1859$+$226 ($=$V406 Vul). We detected an optical reflare associated with an X-ray reflare about 50 d after the outburst maximum. The X-ray delay of the reflare was shorter than 2.3 d. We detected coherent modulations having a period of $0.38385 \pm 0.00073 \,\mathrm{d}$. The modulation appeared within 7 d after the outburst maximum. We propose two obvious interpretations for the nature of the modulations, that is, superhumps or orbital-period variations caused by an irradiated secondary star or an outer disk. The asymmetric profile of the modulations suggests the superhump scenario. The early appearance of superhumps indicates that the tidal dissipation had already grown before the reflare. The growing tidal dissipation is hence less likely to induce the reflare. In conjunction with the short X-ray delay of the reflare, the reflare may be triggered by the X-ray irradiation onto an outer cold disk. The appearance of humps may be earlier than the eccentricity growth time expected from the tidal instability model. It may imply a mass ratio larger than what could be inferred from the reported orbital period.

Dramatic Spectral Evolution of WZ Sge during the 2001 Superoutburst

We spectroscopically observed WZ SGE in 11 nights during the 2001 superoutburst. The feature of the spectra dramatically varied from at the outburst maximum through during the rebrightening phase.WZ SGE is the prototypical star of WZ SGE-type stars, a small class of SU UMa-type dwarf novae. WZ SGE stars have unusual characteristics of quite large outburst amplitudes up to ~8 mag, extraordinary long recurrence cycles of the outbursts (≥10 years), no (or only a few) normal outbursts.

Grazing Eclipsing Dwarf Nova CW Monocerotis: Dwarf Nova-Type Outburst in a Possible Intermediate Polar?

Abstract We observed the 2002 October–November outburst of the dwarf nova CW Mon. The outburst showed a clear signature of a premaximum halt, and a more rapid decline after reaching the outburst maximum. On two separate occasions, during the premaximum stage and near the outburst maximum, shallow eclipses were recorded. This finding confirms the previously suggested possibility of the grazing eclipsing nature of this system. The separate occurrence of the eclipses and the premaximum halt can be understood as being the result of a combination of a two-step ignition of an outburst and the inside-out propagation of the heating wave. We detected a coherent short-period (0.02549 d) signal on two subsequent nights around the optical maximum. This signal was likely present during the maximum phase of the 2000 January outburst. We interpret this signal as being a signature of the intermediate polar (IP) type pulses. The rather strange outburst properties, strong and hard X-ray emission, and the low luminosity of the outburst maximum might be understood as a consequence of the supposed IP nature. The ratio between the suggested spin period and the orbital period, however, is rather unusual for a system having an orbital period of $\sim 0.176 \,\mathrm{d}$.

Time-Resolved Photometry of GK Persei during the 1996 Outburst

GK Per is a unique cataclysmic variable star which has shown a nova explosion as well as dwarf nova-type outbursts, and has an intermediate-polar nature. We carried out $V$-band time-resolved photometry and $B$-band monitoring during the 1996 outburst. This outburst lasted for about 60 d, and was divided into three parts: a slow-rise branch for 35 d, a gradual-decay branch with a decay rate of $20.0 \,\mathrm{d} \,\mathrm{mag}^{-1}$ for $\sim 16 \,\mathrm{d}$, and a rapid-decline branch with a rate of $5.6 \,\mathrm{d} \,\mathrm{mag}^{-1}$ for $\sim 10 \,\mathrm{d}$. The $B-V$ color became bluest ($B-V \sim 0.18$) at about 10 d before the outburst maximum, which supports an idea that the outburst in GK Per is of the inside-out type. The spin pulse, 440-s quasi periodic oscillations (QPOs), and $\sim 5000$-s QPOs were detected in our light curve, as previously seen in X-ray and optical observations. In addition, we report on the discovery of an $\sim 300$-s periodicity, which is shorter than the spin period.

Blue Variable Stars from the MACHO Database. I. Photometry and Spectroscopy of the Large Magellanic Cloud Sample

We present the photometric properties of 1279 blue variable stars within the Large Megallanic Cloud. Photometry is derived from the MACHO database. The light curves of the sample exhibit a variety of quasi-periodic and aperiodic outburst behavior. A characteristic feature of the photometric variation is that the objects are reddest when at maximum outburst. A subset of 102 objects were examined spectroscopically. Within this subset, 91% exhibited Balmer emission in at least one epoch, in some cases with spectacular spectral variability. The variability observed in the sample is consistent with the establishment and maintenance of the Be phenomenon.

Spectral mapping of the spiral structures in IP Pegasi on the decline from an outburst

(abridged) We report eclipse mapping of time resolved spectroscopy of the dwarf nova IP Pegasi on the late decline from the May 1993 outburst. The spiral arms are still visible some 8 days after the onset of the outburst. Their fractional contribution to the continuum emission is similar to that measured close to outburst maximum, whereas their orientation is rotated by 58 degrees with respect to the spirals seen in the eclipse map at outburst maximum. Velocity-resolved light curves across the Halpha and the HeI lines show the classical rotational disturbance, with the blue side of the line being eclipsed earlier than the red side. The differences between the Halpha and the HeI maps are significant. The spiral arms are clearly seen in the HeI maps, with the receding arm being stronger in the red side while the approaching arm is stronger in the blue side of the line. The analysis of the Halpha maps suggests that this emission arises mainly from a large and vertically-extended region which we interpret as an outflowing (and spiraling) disc wind. The spectrum of the uneclipsed light is dominated by a strong, blueshifted and narrow Halpha emission line superimposed on a red continuum and can be understood as a combination of emission from an M5V secondary star plus optically thin emission from the outer parts of the vertically-extended disc wind. The inner disc regions show an emission line spectrum with a strong and broad Halpha component superimposed on a flat continuum. This is in marked contrast with the results from the spectral mapping of nova-like variables of comparable inclination and mass ratio and suggests that intrinsically different physical conditions hold in the inner disc regions of outbursting dwarf novae and nova-like systems.

A photometric and spectroscopic study of the B[e]/X-ray transient CI Cam

We combined the results of UBVR photometry of CI Cam taken at Sternberg Astronomical Institute in 1998--2001, and moderate resolution spectroscopy taken at Special Astrophysical Observatory during the same time period. Photometry as well as fluxes of Balmer emissions and of some Fe II emission lines of CI Cam in quiet state reveal a cyclic variation with the period of $1100\pm50^d$. The variation like this may be due to an orbital motion in a wide pair with a giant star companion that exhibits the reflection effect on its side faced to a compact companion. The V-band photometry also confirms the pre-outburst 11.7 day period found by Miroshnichenko earlier, but with a lower amplitude of 3 per cent. The possibility of identity of this photometric period with the period of jet's rotation in the VLA radio map of the object CI Cam was investigated. The radio map modelling reveals the inclination of the jet rotation axis to the line of sight, $i = 35-40^o$, the angle between the rotation axis and the direction of ejection of the jet, $\theta = 7-10^o$, and jet's spatial velocity of 0.23--0.26c. Equivalent widths and fluxes of various spectral lines show different amplitudes of changes during the outburst, and essentially distinct behaviour in quiescence. Five types of such behaviour were revealed, that indicates the strong stratification of a gas and dust envelope round the system . The time lag of strengthening of 50--250$^d$ in the forbidden line of nitrogen [N II] was found relatively to the X-ray outburst maximum.

Spiral Structure in WZ Sagittae around the 2001 Outburst Maximum

Abstract Intermediate-resolution phase-resolved spectra of WZ Sge were obtained on five consecutive nights (July 23–27) covering the initial stage of the 2001 superoutburst. Double-peaked emission lines of He II at 4686 Å, which were absent on July 23, emerged on July 24 together with emission lines of C III$/$ N III Bowen blend. Analyses of the He II emission lines using the Doppler tomography revealed an asymmetric spiral structure on the accretion disk. This finding demonstrates that spiral shocks with a very short orbital period can arise during the initial stage of an outburst and may be present in all SU UMa stars.

Multicolor Photometry of the SU Majoris Dwarf Nova V1504 Cygni

On the basis of multicolor observations of the dwarf nova V1504 Cyg, made from 1988 to 2001 at the Crimean Astrophysical Observatory and at the Crimean Laboratory of the P. K. Shternberg State Astronomical Institute in bands of the standard BVR photometric system, the brightness characteristics and color indices were obtained for a superoutburst and normal outbursts of the star. At an outburst maximum V1504 Cyg is bluer than at a minimum, with the ascending branch of an outburst being characterized by “redder” color indices than the descending branch. A detailed analysis of the behavior of two outbursts is consistent with Smak's model A, in which an outburst is caused by temperature instability developing first in the outer parts of the accretion disk around the compact component of the binary system and propagating into the inner parts. The characteristics of the other outburst are consistent with Smak's model B, in which instability develops first in the inner parts of the disk. It is suggested that type A outbursts are typical of V1504 Cyg, while type B outbursts are rare.

Photometric and spectrophotometric observations of the classical symbiotic star YY Her during its return to quiescence

We present new photometric UBVRI and spectroscopic observations of the symbiotic star YY Her during its return to quiescence after a strong outburst in 1993. High-resolution spectra of YY Her at similar phases at outburst maximum ϕ=0.48) and in quiescence ϕ=0.37) are presented for the first time. The ephemeris of YY Her has been refined P=586d). The last two observed minima (in 1999 and 2000) differed radically in shape from the 1997 minimum described previously. Both were sharp and deep $$(\Delta U \sim 1\mathop .\limits^m 6, \Delta V \sim 0\mathop .\limits^m 9)$$ . To explain this shape of the V light curve, which is only slightly affected by nebular emission, it should be assumed that the cool component of YY Her fills much of its Roche lobe and has a hot spot on the hemisphere facing the hot component. The emission spectrum rich in Fe II lines, which is characteristic of symbiotic stars, was observed during the outburst, but high-ionization lines (He II λ4686) were also observed. The He I λλ5876, 7065 lines exhibit distinct P Cyg profiles; the centers of the absorption components are shifted from the emission ones by V r≈100 km s−1, suggesting moderate outflow velocities.

Changes in the structure of the accretion disc of EX Draconis through the outburst cycle

ABSTRA C T We report on the analysis of high-speed photometry of the dwarf nova EX Draconis through its outburst cycle with eclipse mapping techniques. The eclipse maps show evidence of the formation of a one-armed spiral structure in the disc at the early stages of the outburst, and reveal how the disc expands during the rise phase until it fills most of the primary Roche lobe at maximum light. During the decline phase the disc becomes progressively fainter until only a small bright region around the white dwarf is left at minimum light. The eclipse maps also suggest the presence of an inward- and an outward-moving heating wave during the rise and an inward-moving cooling wave in the decline. The inferred speed of the outward-moving heating wave is of the order of 1 km s 21 , while the speed of the cooling wave is a fraction of that. Our results suggest a systematic deceleration of both the heating and the cooling waves as they travel across the disc, in agreement with predictions of the disc instability model. The analysis of the brightness temperature profiles indicates that most of the disc appears to be in steady state during quiescence and at outburst maximum, but not during the intermediate stages. As a general trend, the mass accretion rate in the outer regions is larger than in the inner disc on the rising branch, while the opposite holds during the declining branch. We estimate a mass accretion rate of _ Ma 10 28 M( yr 21 at outburst maximum and _ Ma 10 29:1 M( yr 21 in quiescence. The brightness temperature profile in quiescence also suggests that the viscosity parameter is high at this stage, acool * 0:25, which favours the masstransfer instability model. The uneclipsed light has a steady component, understood in terms of emission from the red secondary star, and a variable component that is proportional to the out-of-eclipse flux and corresponds to about 3 per cent of the total brightness of the system. The variable component is interpreted as arising in a disc wind.

The first outburst of SAX J1808.4-3658 revisited

Data of the 1996 outburst of the single-known accreting millisecond pulsar SAX J1808.4-3658, taken with the Wide Field Cameras (WFCs) on BeppoSAX , are revisited with more complete data coverage and more comprehensive analysis techniques than in a previous report. An additional type-I X-ray burst was identified which occurred at a time when the persistent emission is below the detection limit, roughly 30 days after outburst maximum. This burst is three times longer than the first two bursts, and 50% brighter. It is the brightest burst within the ~1700 type-I bursts detected so far with the WFCs. A spectral analysis of the data reveals a distance to SAX J1808.4-3658 of ~2.5 kpc. This is an update from a previously reported value of 4 kpc. We present the evidence that we have for the presence of oscillations at the pulsar frequency during part of the newly found burst. Such an oscillation would lend support to the idea that the frequency of millisecond burst oscillations in other objects is very close to the neutron star rotation frequency.

Z Cam stars: A particular response to a general phenomenon

We show that the disc instability model can reproduce all the observed properties of Z Cam stars if the energy equation includes heating of the outer disc by the mass-transfer stream impact and by tidal torques and if the mass-transfer rate from the secondary varies by about 30% around the value critical for stability. In particular the magnitude difference between outburst maxima and standstills corresponds to observations, all outbursts are of the inside-out type and can be divided into two classes: long (wide) and short (narrow) outbursts, as observed. Mass transfer rate fluctuations should occur in other dwarf novae but one can exclude variations similar to those observed in magnetic systems (AM Her's and some DQ Her's) and some nova-like systems (VY Scl's), in which \dot{M} become very small during low states; these would produce mini-outburst which, although detectable, have never been observed.

A Spiral Structure in the Disk of EX Draconis on the Rise to Outburst Maximum

We report on the R-band eclipse mapping analysis of high-speed photometry of the dwarf nova EX Dra on the rise to the maximum of the 1995 November outburst. The eclipse map shows a one-armed spiral structure of ~180° in azimuth, extending in radius from R 0.2RL1 to 0.43RL1 (where RL1 is the distance from the disk center to the inner Lagrangian point), that contributes about 22% of the total flux of the eclipse map. The spiral structure is stationary in a reference frame corotating with the binary and is stable for a timescale of at least five binary orbits. The comparison of the eclipse maps on the rise and in quiescence suggests that the outbursts of EX Dra may be driven by episodes of enhanced mass transfer from the secondary star. Possible explanations for the nature of the spiral structure are discussed.

The 1999 Outburst of the Recurrent Nova U Scorpii

The optical spectra of the recurrent nova U Sco obtained in five epochs 0.45–42.35 days after the 1999 outburst maximum are presented here. For the first time, an outburst spectrum of U Sco has been obtained as early as 0.45 days after maximum. The spectral evolution is broadly similar to the 1979 and 1987 outbursts. The mass of the ejected shell is estimated to be ~10-7 M⊙. Based on the Mg I b 5174 A and the Fe + Ca I 5270 A absorption indices in the late-decline phase spectrum of day 42.35, we estimate the secondary to be a K2 subgiant star.

Spiral shocks in the accretion disc of IP Peg during outburst maximum

In response to our recent discovery of spiral arms in the accretion disc of IP Peg during rise to outburst, we have obtained time-resolved spectrophotometry of IP Peg during outburst maximum. In particular, indirect imaging of HeII 4686, using Doppler tomography, shows a two-arm spiral pattern on the disc image, which confirms repeatability over different outbursts. The jump in HeII intensity (a factor of more than two) and in velocity (~200--300 km/s) clarifies the shock nature of the spiral structure. The HeII shocks show an azimuthal extent of ~90 degrees, a shallow power-law emissivity ~V^{-1}, an upper limit of 30 degrees in opening angle, and a flux contribution of 15 per cent of the total disc emission. We discuss the results in view of recent simulations of accretion discs which show that spiral shocks can be raised in the accretion disc by the secondary star.