High Optical State Research Articles

Discovery of the neutron star spin and a possible orbital period from the Be/X-ray binary IGR J05414-6858 in the LMC

The number of known Be/X-ray binaries in the Large Magellanic Cloud is small compared to the observed population of the Galaxy or the Small Magellanic Cloud. The discovery of a system in outburst provides the rare opportunity to measure its X-ray properties in detail. IGR J05414-6858 was discovered in 2010 by INTEGRAL and found in another outburst with the Swift satellite in 2011. In order to characterise the system, we analysed the data from a follow-up XMM-Newton target of opportunity observation of the 2011 outburst and investigate the stellar counterpart with photometry and spectroscopy. We modelled the X-ray spectra from the EPIC instruments on XMM-Newton and compared them with Swift archival data. In the X-ray and optical light curves, we searched for periodicities and variability. The optical counterpart was classified using spectroscopy obtained with ESO's Faint Object Spectrograph at NTT. The X-ray spectra as seen in 2011 are relatively hard with a photon index of ~0.3-0.4 and show only low absorption. They deviate significantly from earlier spectra of a probable type II outburst in 2010. The neutron star spin period of P_s = 4.4208 s was discovered with EPIC-pn. The I-band light curve revealed a transition from a low to a high state around MJD 54500. The optical counterpart is classified to B0-1IIIe and shows Halpha emission and a variable NIR excess, vanishing during the 2010 outburst. In the optical high state, we found a periodicity at 19.9 days, probably caused by binarity and indicating the orbital period.

Connection Between Optical and VHE Gamma-ray Emission in Blazar Jets

MAGIC has been performing optically triggered Target of Opportunity observations of flaring blazars since the beginning of its scientific operations. The alerts of flaring blazars originate from Tuorla Blazar Monitoring Programme, which started the optical monitoring of candidtate TeV blazars in 2002 and has now collected up to eight years of data on more than 60 blazars. These ToO observations have resulted in the discovery of five new VHE γ-ray emitting blazars (S5 0716+714, 1ES 1011+496, Mrk 180, ON 325 and B3 2247+381). In addition part of the discovery of BL Lac and the discovery of 3C 279 was made during a high optical state. In this contribution we present a detailed analysis of the optical light curves which are then compared to MAGIC observations of the same sources. We aim to answer the question: "Is there a connection between optical and VHE γ-ray high states in blazars or have we just been lucky?"

Recent results from MAGIC observations of AGN

MAGIC is a system of two 17m Cherenkov telescopes, sensitive to very high energy (VHE) gamma-rays between 50 GeV up to several tens of TeV. MAGIC observations of Active Galactic Nuclei (AGN) have resulted in the discovery of many previously unknown VHE gamma-ray emitting sources adding also new source types to the list of extragalactic gamma-ray emitters as well as new constraints for the emission models. Many of the discoveries have been triggered by the optical high state of the source as reported by the Tuorla blazar monitoring program, in several cases the targets were also listed as promising candidates by the Fermi collaboration based on their flux and spectrum. In this contribution I discuss the new discoveries as well as the new constraints for emission models of AGN derived from recent MAGIC observations.

Discovery of VHEγ-ray emission from the BL Lacertae object B3 2247+381 with the MAGIC telescopes

We study the non-thermal jet emission of the BL Lac object B3 2247+381 during a high optical state. The MAGIC telescopes observed the source during 13 nights between September 30th and October 30th 2010, collecting a total of 14.2 hours of good quality very high energy (VHE) $\gamma$-ray data. Simultaneous multiwavelength data was obtained with X-ray observations by the Swift satellite and optical R-band observations at the KVA-telescope. We also use high energy $\gamma$-ray (HE, 0.1 GeV-100 GeV) data from the Fermi satellite. The BL Lac object B3 2247+381 (z=0.119) was detected, for the first time, at VHE $\gamma$-rays at a statistical significance of 5.6 $\sigma$. A soft VHE spectrum with a photon index of -3.2 $\pm$ 0.6 was determined. No significant short term flux variations were found. We model the spectral energy distribution using a one-zone SSC-model, which can successfully describe our data.

The multifrequency campaign on 3C 279 in January 2006

We present the results of a multifrequency campaign from radio to hard X-ray energies on the blazar 3C 279 during an optical high-state in January 2006. We give the observational results (multifrequency light curves and spectra) and compile an SED. This complements an SED from an optical low-state in June 2003. Surprisingly the two SEDs differ only in their high-energy synchrotron emission (near-IR - UV), while the low-energy inverse-Compton emission (X- to Gamma-rays) remained unchanged. By interpreting with a steady-state leptonic emission model, the variability among the SED can be reproduced by a change solely of the low-energy cutoff of the relativistic electron distribution. In an internal shock model for blazar emission, such a change could e.g. achieved through a varying relative Lorentz factor of colliding shells producing internal shocks in the jet.

DISCOVERY OF VERY HIGH ENERGY γ-RAYS FROM THE BLAZAR S5 0716+714

The MAGIC Collaboration reports the detection of the blazar S5 0716+714 (z = 0.31 ± 0.08) in very high energy gamma rays. The observations were performed in 2007 November and in 2008 April, and were triggered by the Kungliga Vetenskapliga Akademi telescope due to the high optical state of the object. An overall significance of the signal accounts to S = 5.8σ for 13.1 hr of data. Most of the signal (S = 6.9σ) comes from the 2008 April data sample during a higher optical state of the object suggesting a possible correlation between the Very High Energy γ-ray and optical emissions. The differential energy spectrum of the 2008 data sample follows a power law with a photon index of Γ = 3.45 ± 0.54stat ± 0.2syst, and the integral flux above 400 GeV is at the level of (7.5 ± 2.2stat ± 2.3syst) × 10−12 cm−2 s−1, corresponding to a 9% Crab Nebula flux. Modeling of the broadband spectral energy distribution indicates that a structured jet model appears to be more promising in describing the available data than a simple one-zone synchrotron self-Compton model.

PHYSICS INSIGHTS FROM RECENT MAGIC AGN OBSERVATIONS

The total set of the 14 active galactic nuclei detected by MAGIC so far includes well-studied bright blazars like Mkn 501, the giant radio galaxy M 87, but also the distant flat-spectrum radio quasar 3C 279, and an intriguing gamma-ray source in the 3C 66A/B region, whose energy spectrum is not compatible with the expectations from 3C 66A. Besides scheduled observations, so far MAGIC succeeded in discovering TeV gamma rays from three blazars following triggers from high optical states. I report selected highlights from recent MAGIC observations of extragalactic TeV gamma-ray sources, emphasizing and discussing the new physics insights the MAGIC observations were able to contribute.

Hubble Space Telescope STIS Spectroscopy of the Peculiar Nova-Like Variables BK Lyn, V751 Cygni, and V380 Oph

ABSTRACTWe obtained Hubble STIS spectra of three nova-like variables: V751 Cygni, V380 Oph, and—the only confirmed nova-like variable known to be below the period gap—BK Lyn. In all three systems, the spectra were taken during high optical brightness state, and a luminous accretion disk dominates their far-ultraviolet (FUV) light. We assessed a lower limit of the distances by applying the infrared photometric method of Knigge. Within the limitations imposed by the poorly known system parameters (such as the inclination, white dwarf mass, and the applicability of steady state accretion disks) we obtained satisfactory fits to BK Lyn using optically thick accretion disk models with an accretion rate of for a white dwarf mass of Mwd = 1.2M⊙ and for Mwd = 0.4M⊙. However, for the VY Scl-type nova-like variable V751 Cygni and for the SW Sex star V380 Oph, we are unable to obtain satisfactory synthetic spectral fits to the high state FUV spectra using optically thick steady state accretion disk models. The lack of FUV spectra information down to the Lyman limit hinders the extraction of information about the accreting white dwarf during the high states of these nova-like systems.

THE WHOLE EARTH BLAZAR TELESCOPE CAMPAIGN ON THE INTERMEDIATE BL LAC OBJECT 3C 66A IN 2007-2008

Prompted by a high optical state in September 2007, the Whole Earth Blazar Telescope (WEBT) consortium organized an intensive optical, near-IR (JHK) and radio observing campaign on the intermediate BL Lac object 3C 66A throughout the fall and winter of 2007 -- 2008. The source remained in a high optical state throughout the observing period and exhibited several bright flares on time scales of ~ 10 days. This included an exceptional outburst around September 15 - 20, 2007, reaching a peak brightness at R ~ 13.4. Our campaign revealed microvariability with flux changes up to |dR/dt| ~ 0.02 mag/hr. Our observations do not reveal evidence for systematic spectral variability or spectral lags. We infer a value of the magnetic field in the emission region of B ~ 19 e_B^{2/7} \tau_h^{-6/7} D_1^{13/7} G. From the lack of systematic spectral variability, we can derive an upper limit on the Doppler factor, D <= 28 \tau_h^{-1/8} e_B^{3/16}. This is in agreement with superluminal motion measurements of \beta_{app} \le 27 and argues against models with very high Lorentz factors of \Gamma > 50, required for a one-zone SSC interpretation of some high-frequency-peaked BL Lac objects detected at TeV gamma-ray energies.

Systematic Search for VHE Gamma‐Ray Emission from X‐Ray–bright High‐Frequency BL Lac Objects

All but three (M87, BL Lac and 3C 279) extragalactic sources detected so far at very high energy (VHE) gamma-rays belong to the class of high-frequency peaked BL Lac (HBL) objects. This suggested to us a systematic scan of candidate sources with the MAGIC telescope, based on the compilation of X-ray blazars by Donato et al. (2001). The observations took place from December 2004 to March 2006 and cover sources on the northern sky visible under small zenith distances zd < 30 degrees at culmination. The sensitivity of the search was planned for detecting X-ray bright F(1 keV) > 2 uJy) sources emitting at least the same energy flux at 200 GeV as at 1 keV. In order to avoid strong gamma-ray attenuation close to the energy threshold, the redshift of the sources was constrained to values z<0.3. Of the fourteen sources observed, 1ES 1218+304 and 1ES 2344+514 have been detected in addition to the known bright TeV blazars Mrk 421 and Mrk 501. A marginal excess of 3.5 sigma from the position of 1ES 1011+496 was observed and has been confirmed as a source of VHE gamma-rays by a second MAGIC observation triggered by a high optical state (Albert et al. 2007). For the remaining sources, we present here the 99% confidence level upper limits on the integral flux above ~200 GeV. We characterize the sample of HBLs (including all HBLs detected at VHE so far) by looking for correlations between their multi-frequency spectral indices determined from simultaneous optical, archival X-ray, and radio luminosities, finding that the VHE emitting HBLs do not seem to constitute a unique subclass. The absorption corrected gamma-ray luminosities at 200 GeV of the HBLs are generally not higher than their X-ray luminosities at 1 keV.

A Limit Cycle Model For Long‐Term Optical Variations of V Sagittae: The Second Example of Accretion Wind Evolution

V Sagittae shows quasi-periodic optical high (soft X-ray-off) and low (soft X-ray-on) states with the total period of ~300 days. A binary model is presented to explain orbital light curves for both the high and low states, as well as the transition mechanism between them. The binary model consists of a white dwarf (WD), a disk around the WD, and a lobe-filling main-sequence companion. In the optical low state, the mass-transfer rate to the WD is small and the size of the disk is as small as its Roche lobe size. In the optical high state, the mass-transfer rate to the WD exceeds the critical rate of ~1 × 10-6 M☉ yr-1 and the WD blows an optically thick, massive wind. Surface layers of the disk are blown in the wind, and the disk surface extends to the companion or over. As a result, optical luminosity of the disk increases by a magnitude because of its large irradiation effect. The massive wind completely obscures soft X-rays. This corresponds to the optical high/soft X-ray off state. The transition between optical high and low states is driven by an attenuation of the mass transfer from the secondary. During the optical high state, the wind from the WD hits the companion and strips off its surface layer. The mass transfer from the companion gradually reduces and stops. As the mass supply stops, the WD wind weakens and eventually stops. The disk shrinks to a Roche lobe size, and the optical magnitude drops. This phase corresponds to the optical low/soft X-ray on state. Then a rapid mass transfer resumes. The mass of the WD envelope increases and blows a wind again. This cycle is repeated like a limit cycle. During this intermittent wind phase, the WD can grow in mass at the critical rate and eventually reach the Chandrasekhar mass limit. This process is called wind evolution, which is a key evolutionary process in a recently developed evolutionary scenario of Type Ia supernovae. This evolutionary process was first confirmed in the LMC supersoft X-ray source RX J0513.9-6951, although it commonly occurs in the supersoft X-ray sources when the mass-transfer rate exceeds the critical rate. Thus, V Sge is the second example of accretion wind evolution.

A New Clue to the Transition Mechanism between Optical High and Low States of the Supersoft X‐Ray Source RX J0513.9−6951, Implied By the Recurrent Nova CI Aquilae 2000 Outburst Model

We have found a new clue to the transition mechanism between optical high/X-ray off and optical low/X-ray on states of the LMC supersoft X-ray source RX J0513.9-6951. A sharp ~1 mag drop is common to the CI Aql 2000 outburst. These drops are naturally attributed to the cessation of optically thick winds on white dwarfs. A detailed light-curve analysis of CI Aql indicates that the size of a disk drastically shrinks when the wind stops. This causes a ~1-2 mag drop in the optical light curve. In RX J0513.9-6951, the same mechanism reproduces sharp ~1 mag drop from optical high to low states. We predict that this mechanism also works on the transition from low to high states. Interaction between the wind and the companion star attenuates the mass transfer and drives full cycles of low and high states.

A New Orbital Ephemeris and Reinterpretation of Spectroscopic Data for the Supersoft X-Ray Binary RX J0513.9−6951

We have analyzed nearly eight years of MACHO optical photometry of the supersoft X-ray binary RX J0513.9-6951 and derived a revised orbital period and ephemeris. Previously published velocities are reinterpreted using the new ephemeris. We show that the spectroscopic characteristics of the system depend strongly on whether the system is in a high or low optical state. We also discuss the properties of the source's high and low optical states and its long-term light curve. Evidence for an 83.3 day periodicity in the photometry is presented.

X-ray off states and optical variability in CAL 83

CAL 83 was one of the rst supersoft X-ray binaries (SSBs) to be discovered and is considered to be the prototype of its class. In 15 X-ray observations between 1983{1997 it was observed to have nearly constant X-ray luminosity and temperature, with the exception of one o-state in 1996 (Kahabka et al. 1996). We report on a second X-ray o-state, discovered with a Chandra ACIS-S observation in November 1999. We consider the long-term X-ray and MACHO optical light curves. We nd that, during more than 7 years of monitoring by the MACHO team, CAL 83 has exhibited distinct and well-dened low, intermediate, and high optical states. Transitions between states are not accompanied by color variations. We also nd that both X-ray o states were observed during optical high states and were followed by optical low states within50 days. We discuss possible explanations for the observed optical and X-ray variations. While photospheric adjustments might account for the variations in soft X-ray flux, optical variations can be explained only by invoking changes in the accretion disk, which is the primary source of optical radiation.

The X-Ray Spectra of VY Sculptoris Stars Are Not Blackbodies

Using ASCA data, we find, contrary to other researchers using ROSAT data, that the X-ray spectra of the VY Sculptoris stars TT Arietis and KR Aurigae are poorly fitted by an absorbed blackbody model but are well fitted by an absorbed thermal plasma model. The different conclusions about the nature of the X-ray spectrum of KR Aur may be due to differences in the accretion rate since this star was in a high optical state during the ROSAT observation but in an intermediate optical state during the ASCA observation. TT Ari, on the other hand, was in a high optical state during both observations and so directly contradicts the hypothesis that the X-ray spectra of VY Scl stars in their high optical states are blackbodies. Instead, based on theoretical expectations and the ASCA, Chandra, and XMM spectra of other nonmagnetic cataclysmic variables, we believe that the X-ray spectra of VY Scl stars in their low and high optical states are due to hot thermal plasma in the boundary layer between the accretion disk and the surface of the white dwarf, and we appeal to the acquisition of Chandra and XMM grating spectra to test this prediction.

A model for the optical high state light curve of AM Herculis

We present a simple quantitative model that can describe the photometric B and V band light curves of AM Herculis obtained during a high state. The double-humped shape of the V band light curve is dominated by cyclotron emission from a region at the main accreting pole with an area of ~$5\times10^{16} \mathrm{cm^2}$ and sustaining an inflow of ~0.06 $\rm g cm^{-2} s^{-1}$. The almost unmodulated B band is dominated by emission from the accretion stream. The contribution of the heated white dwarf to the optical emission is small in the B band, but comparable to that of the accretion stream in the V band. The emission of the secondary star is negligible both in B and in V .

Coordinated Optical and X‐Ray Observations of the AM Herculis Objects VV Puppis, V834 Centauri, and EF Eridani

ABSTRACTWe obtained coordinated broadband optical photometry and hard X‐ray data on the AM Her objects VV Pup, V834 Cen, and EF Eri. The X‐ray data were obtained using the Proportional Counter Array detectors of the Rossi X‐Ray Timing Explorer, and the optical data were obtained using the 1 m telescope of the Cerro Tololo Inter‐American Observatory. During our observations, VV Pup and V834 Cen were in their high optical luminosity states with average mV∼17 and 15.1, respectively. EF Eri was in a very low state at the time of our observation: mV>17.7. VV Pup and V834 Cen had average 2–10 keV X‐ray fluxes of 1.7 × 10-12 and 2.13 × 10-11 ergs cm−2 s−1, respectively. EF Eri was not detected in X‐rays. The X‐ray spectrum of V834 Cen was adequately modeled using an absorbed bremsstrahlung continuum with a Gaussian emission line. The best‐fit model had continuum temperature kTX = 11.8 ± 1.0 keV and absorption column nH = (1.03 ± 0.39) × 1022 cm−2. The Gaussian emission line had line center energy E = 6.7 ± 0.1 keV and equivalent width EW = 0.64 keV. VV Pup was too faint to produce reliable spectral fits. The 2–10 keV flux for VV Pup quoted above was based on the best‐fit spectral model. Holding kTX fixed in the fitting process yielded roughly the same flux for kTX = 0.5–30 keV.We detected ∼1 Hz optical quasi‐periodic oscillations (QPOs) with amplitude ∼2.2% from VV Pup and ∼1% from V834 Cen. We did not detect optical QPOs over the range 0.2–1 Hz from EF Eri at a 90% confidence upper limit of 4.5%. The QPO properties are consistent with those found in the literature. None of the targets showed X‐ray QPO activity. However, only V834 Cen yielded a meaningful upper limit: ∼14% at the 90% confidence level over 0.2–1.2 Hz.

UBV photometry, UV spectroscopy and radio observations of the peculiar binary V Sagittae

We have performed high-speed UBV photometric observations on the peculiar binary V Sagittae. Using three new eclipse timings we update the orbital ephemeris and convert it to a dynamical time-scale (TDB). We also searched for quasi-periodic oscillations but did not detect them. Using the Wilson–Devinney algorithm we have modelled the light curve to find the stellar parameters of V Sge. We find that the system is a detached binary but that the primary star is very close to filling its Roche lobe, while the secondary star fills 90 per cent of its Roche lobe volume. We find temperatures of the primary and the secondary star to be T1=41 000 K and T2=22 000 K. We find i=72° and masses of 0.8 M⊙ and 3.3 M⊙ for the primary and secondary stars respectively. De-archived Hubble Space Telescope (HST) spectroscopy of V Sge shows evidence of mass loss via a wind or winds. In addition we report radio observations of V Sge during an optical high state at 2 cm, 3.6 cm and 6 cm wavelengths. The 3.6 cm emission is increased by a factor of more than six compared with an earlier detection in a previous optical high state.

ORFEUS IIFar‐Ultraviolet Spectroscopy of AM Herculis

Six high-resolution (λ/Δλ ≈ 3000) far-UV (λλ = 910-1210 A) spectra of the magnetic cataclysmic variable AM Herculis were acquired in 1996 November during the flight of the ORFEUS-SPAS II mission. AM Her was in a high optical state at the time of the observations, and the spectra reveal emission lines of O VI λλ1032, 1038, C III λλ977, 1176, and He II λ1085 superposed on a nearly flat continuum. Continuum flux variations can be described as per Gansicke et al. by a ≈ 20 kK white dwarf with a ≈ 37 kK hot spot covering a fraction f ~ 0.15 of the surface of the white dwarf, but we caution that the expected Lyman absorption lines are not detected. The O VI emission lines have a broad and narrow component structure similar to that of the optical emission lines, the C III and He II emission lines are dominated by the broad component, and the radial velocities of these lines are consistent with an origin of the narrow and broad components in the irradiated face of the secondary and the accretion funnel, respectively. The density of the narrow- and broad-line regions is nnlr ~ 3 × 1010 cm-3 and nblr ~ 1 × 1012 cm-3, respectively, yet the narrow-line region is optically thick in the O VI line and the broad-line region is optically thin; apparently, the velocity shear in the broad-line region allows the O VI photons to escape, rendering the gas effectively optically thin. The orbital phase variations of the emission-line fluxes are unexplained.

The equatorial disc of the Be star X Persei

We study the long-term behaviour of the equatorial disc of the Be/X-ray binary X Persei (X Per), combining new low-resolution IUE spectra and IR photometry with UV, optical and IR observations collected from the literature. We find that the near-UV continuum level of X Per varies along with the optical brightness. From the UV observations, we also find that during optical high states the flux excess due to the intrinsic stellar variability and/or electron scattering in the disc is at most 15-20 per cent of the photospheric flux. From the data taken in discless and near-discless states (optical low states) we show that the stellar photosphere can be modelled with T-eff = 31000 K and log g = 4. With this model we derive E(B - V) = 0.39, and estimate the distance to X Per as 950 +/- 200 pc (assuming R-* = 9 R.). We fit the (quasi-)simultaneous optical and IR photometry with a simple disc model including free-bound and free-free radiation. We find that the density of the disc at the photosphere of the star varies along with the brightness of X Per, and that in optical high states the disc in X Per is among the densest of all Be stars: rho(0) = (1.5 +/- 0.3) x 10(-10) g cm(-3). The disc density at the photosphere varies by a factor of at least 20 from optical high to low states. During disc build-up and break-down phases, and also in phases when the disc is relatively stable, we find a very steep radial density gradient of the disc of X Per. This may reflect the limitations of some of the assumptions in our model. We find that in a disc-loss event the disc loses mass at a rate of about 5 x 10(-9) M. yr(-1). For a disc build-up phase we find a disc-mass growth rate of about 4 x 10(-9) M. yr(-1). This growth rate is consistent with a model that feeds the disc from the 'ordinary' mass-loss of the star, but we cannot exclude the possibility that other phenomena contribute to the disc growth as well.