Pasi Hakala

Evidence for diverse optical emission from gamma-ray burst sources

Optical Transients from gamma-ray burst sources, in addition to offering a distance determination, convey important information about the physics of the emission mechanism, and perhaps also about the underlying energy source. As the gamma-ray phenomenon is extremely diverse, with timescales spanning several orders of magnitude, some diversity in optical counterpart signatures appears plausible. We have studied the optical transient that accompanied the gamma-ray burst of 1997 May 8, GRB 970508. Observations conducted at the 2.5 m Nordic Optical Telescope (NOT) and the 2.2 m telescope at the German-Spanish Calar Alto observatory (CAHA) cover the time interval starting 3 hr 5 minutes to 96 days after the high-energy event. This brackets all other published observations, including radio. When analyzed in conjunction with optical data from other observatories, evidence emerges for a composite light curve. The first interval, from 3 to 8 hr after the event, was characterized by a constant or slowly declining brightness. At a later moment, the brightness started increasing rapidly, and reached a maximum approximately 40 hr after the GRB. From that moment, the GRB brightness decayed approximately as a power law of index -1.21. The last observation, after 96 days, mR = 24.28 ± 0.10, is brighter than the extrapolated power law, and hints that a constant component, mR = 25.50 ± 0.40, is present. The optical transient is unresolved (FWHM 083) at the faintest magnitude level. The brightness of the optical transient, its duration, and the general shape of the light curve set this source apart from the single other optical transient known, that of the 1997 February 28 event.

The WEBT campaign to observe AO 0235+16 in the 2003-2004 observing season. Results from radio-to-optical monitoring and XMM-Newton observations

A multiwavelength campaign to observe the BL Lac object AO 0235+16 (z = 0.94) was set up by the Whole Earth Blazar Telescope (WEBT) collaboration during the observing seasons 2003-2004 and 2004-2005, involving radio, near-IR and optical photometric monitoring, VLBA monitoring, optical spectral monitoring, and three pointings by the XMM-Newton satellite. Here we report on the results of the first season, which involved the participation of 24 optical and near-IR telescopes and 4 radio telescopes, as well as the first XMM-Newton pointing, which occurred on January 18-19, 2004. Unpublished data from previous epochs were also collected (from 5 optical-NIR and 3 radio telescopes), in order to fill the gap between the end of the period presented in Raiteri et al. (2001) and the start of the WEBT campaign. The contribution of the southern AGN, 2 arcsec distant from the source, is taken into account. It is found to especially affect the blue part of the optical spectrum when the source is faint. In the optical and near-IR the source has been very active in the last 3 years, although it has been rather faint most of the time, with noticeable variations of more than a magnitude over a few days. In contrast, in the radio bands it appears to have been quiescent since early 2000. The major radio (and optical) outburst predicted to peak around February-March 2004 (with a six month uncertainty) has not occurred yet. When comparing our results with the historical light curves, two different behaviours seem to characterize the optical outbursts: only the major events present a radio counterpart. The X-ray spectra obtained by the three EPIC detectors are well fitted by a power law with extra-absorption at z = 0.524; the energy index in the 0.2-10 keV range is well constrained: a = 0.645 ± 0.028 and the 1 keV flux density is 0.311 ± 0.008 μJy. The analysis of the X-ray light curves reveals that no significant variations occurred during the pointing. In contrast, simultaneous dense radio monitoring with the 100 m telescope at Effelsberg shows a ∼2-3% flux decrease in 6-7 h, which, if intrinsic, would imply a brightness temperature well above the Compton limit and hence a lower limit to the Doppler factor 6 > 46.

Evidence of Magnetic Accretion in an SW Sextantis Star: Discovery of Variable Circular Polarization in LS Pegasi*

We report on the discovery of variable circular polarization in the SW Sextantis star LS Pegasi. The observed modulation has an amplitude of ≈0.3% and a period of 29.6 minutes, which we assume as the spin period of the magnetic white dwarf. We also detected periodic flaring in the blue wing of Hβ, with a period of 33.5 minutes. The difference between both frequencies is just the orbital frequency, so we relate the 33.5 minute modulation to the beat between the orbital and spin period. We propose a new accretion scenario in SW Sex stars, based on the shock of the disk-overflown gas stream against the white dwarfs magnetosphere, which extends to the corotation radius. From this geometry, we estimate a magnetic field strength of B1 ~ 5-15 MG. Our results indicate that magnetic accretion plays an important role in SW Sex stars, and we suggest that these systems are probably intermediate polars with the highest mass accretion rates.

Detection of the optical counterpart of the proposed double degenerate polar RX J1914+24

We have detected the optical counterpart of the proposed double degenerate polar RX J1914+24. The I-band light curve is modulated on the 9.5-min period seen in X-rays. There is no evidence for any other periods. No significant modulation is seen in J. The infrared colours of RX J1914+24 are not consistent with a main-sequence dwarf secondary star. Our ASCA spectrum of RX J1914+24 is typical of a heavily absorbed polar and our ASCA light curve also shows only the 9.5-min period. We find that the folded I band and X-ray light curves are out of phase. We attribute the I-band flux to the irradiated face of the donor star. The long-term X-ray light curve shows a variation in the observed flux of up to an order of magnitude. These observations strengthen the view that RX J1914+24 is indeed the first double degenerate polar to be detected. In this light, we discuss the synchronizing mechanisms in such a close binary and other system parameters.

Characterizing a new class of variability in GRS 1915+105 with simultaneous INTEGRAL/RXTE observations

We report on the analysis of 100 ks INTEGRAL observations of the Galactic microquasar GRS 1915+105. We focus on INTEGRAL Revolution number 48 when the source was found to exhibit a new type of variability as preliminarily reported in Hannikainen (2003, A&A, 411, L415). The variability pattern, which we name ξ, is characterized by a pulsing behaviour, consisting of a main pulse and a shorter, softer, and smaller amplitude precursor pulse, on a timescale of 5 min in the JEM-X 3-35 keV lightcurve. We also present simultaneous RXTE data. From a study of the individual RXTE/PCA pulse profiles we find that the rising phase is shorter and harder than the declining phase, which is opposite to what has been observed in other otherwise similar variability classes in this source. The position in the colour-colour diagram throughout the revolution corresponds to Stale A (Belloni et al. 2000, A&A, 355, 271) but not to any previously known variability class. We separated the INTEGRAL data into two subsets covering the maxima and minima of the pulses and fitted the resulting two broadband spectra with a hybrid thermal-non-thermal Comptonization model. The fits show the source to be in a soft state characterized by a strong disc component below ∼6 keV and Comptonization by both thermal and non-thermal electrons at higher energies.

VLT/FORS spectroscopy of faint cataclysmic variables discovered by the Sloan Digital Sky Survey

We present medium-resolution VLT/FORS2 spectroscopy of six cataclysmic variables (CVs) discovered by the Sloan Digital Sky Survey (SDSS). We determine orbital periods for SDSS J023322.61+005059.5 (96.08 +/- 0.09 min), SDSS J091127.36+084140.7 (295.74 +/- 0.22 min), SDSS J103533.02+055158.3 (82.10 +/- 0.09 min) and SDSS J121607.03+052013.9 (most likely 98.82 +/- 0.16 min, but the one-day aliases at 92 and 107 min are also possible) using radial velocities measured from their H alpha and H beta emission lines. Three of the four orbital periods measured here are close to the observed 75-80 min minimum period for CVs, indicating that the properties of the population of these objects discovered by the SDSS are substantially different to those of the CVs found by other means. Additional photometry of SDSS J023322.61+005059.5 reveals a periodicity of approximately 60 min which we interpret as the spin period of the white dwarf, suggesting that this system is an intermediate polar with a low accretion rate. SDSS J103533.02+055158.3 has a period right at the observed minimum value, a spectrum dominated by the cool white dwarf primary star and exhibits deep eclipses, so is an excellent candidate for an accurate determination of the parameters of the system. The spectroscopic orbit of SDSS J121607.03+052013.9 has a velocity amplitude of only 13.8 +/- 1.6 km s(-1), implying that this system has an extreme mass ratio. From several physical constraints we find that this object must contain either a high-mass white dwarf or a brown-dwarf-mass secondary component or both.

SDSS J0926+3624 : the shortest period eclipsing binary star

With orbital periods of the order of tens of minutes or less, the AM Canum Venaticorum stars are ultracompact, hydrogen-deficient binaries with the shortest periods of any binary subclass, and are expected to be among the strongest gravitational wave sources in the sky. To date, the only known eclipsing source of this type is the P= 28 min binary SDSS J0926+3624. We present multiband, high time resolution light curves of this system, collected with William Herschel Telescope (WHT)/ULTRACAM in 2006 and 2009. We supplement these data with additional observations made with Liverpool Telescope/Rapid Imager to Search for Exoplanets (LT/RISE), XMM–Newton and the Catalina Real-Time Transient Survey. From light curve models we determine the mass ratio to be q=M2/M1= 0.041 ± 0.002 and the inclination to be . We calculate the mass of the primary white dwarf to be 0.85 ± 0.04 M⊙ and the donor to be 0.035 ± 0.003 M⊙, implying a partially degenerate state for this component. We observe superhump variations that are characteristic of an elliptical, precessing accretion disc. Our determination of the superhump period excess is in agreement with the established relationship between this parameter and the mass ratio, and is the most precise calibration of this relationship at low q. We also observe a quasi-periodic oscillation in the 2006 data, and we examine the outbursting behaviour of the system over a 4.5 year period.

Optical spectroscopy and photometry of SAX J1808.4−3658 in outburst

We present phase resolved optical spectroscopy and photometry of V4580 Sagittarii, the optical counterpart to the accretion powered millisecond pulsar SAX J1808.4−3658, obtained during the 2008 September/October outburst. Doppler tomography of the N iii λ4640.64 Bowen blend emission line reveals a focused spot of emission at a location consistent with the secondary star. The velocity of this emission occurs at 324 ± 15 km s 1 ; applying a “K-correction”, we find the velocity of the secondary star projected onto the line of sight to be 370 ± 40 km s 1 . Based on existing pulse timing measurements, this constrains the mass ratio of the system to be 0.044 +0.005 0.004, and the mass function for the pulsar to be 0.44 +0.16 0.13 M⊙. Combining this mass function with various inclination estimates from other authors, we find no evidence to suggest that the neutron star in SAX J1808.4−3658 is more massive than the canonical value of 1.4 M⊙. Our optical light curves exhibit a possible superhump modulation, expected for a system with such a low mass ratio. The equivalent width of the Ca ii H and K interstellar absorption lines suggest that the distance to the source is ∼2.5 kpc. This is consistent with previous distance estimates based on type-I Xray bursts which assume cosmic abundances of hydrogen, but lower than more recent estimates which assume helium-rich bursts.

The X-ray transient XTE J1859 + 226 in outburst and quiescence

We present optical photometry and spectroscopy of the X-ray transient XTE J1859+226, obtained during outburst and its subsequent decay to quiescence. Both the X-ray and optical properties are very similar to those of well-studied black hole soft X-ray transients. We have detected three mini-outbursts, when XTE J1859+226 was approaching quiescence, as has been previously detected in the soft X-ray transients GRO J0422+32 and GRS 1009-45. By 2000 August 24 the system had reached quiescence with R = 22.48 ′ 0.07. The estimated distance to the source is ∼11 kpc. Photometry taken during quiescence shows a sinusoidal modulation with a peak to peak amplitude of about 0.4 mag. A period analysis suggests that periods from 0.28 to 0.47 d are equally possible at the 68 per cent confidence level. The amplitude of the quiescent light curve and the relatively low ratio of X-ray to optical flux indicate that the binary inclination should be high. The measured colours during the outburst allow us to obtain the basic properties of the disc, which agrees well with irradiated disc model predictions.

The defining characteristics of intermediate polars - the case of three-candidate systems

Intermediate polars (IPs) are a group of cataclysmic variables (CVs) which are thought to contain white dwarfs which have a magnetic field strength in the range similar to 0.1-10 MG. A significant fraction of the X-ray sources detected in recent deep surveys has been postulated to consist of IPs. Until now two of the defining characteristics of IPs have been the presence of high (and complex) absorption in their X-ray spectra and the presence of a stable modulation in the X-ray light curve which is a signature of the spin period, or the beat period, of the accreting white dwarf. Three CVs, V426 Oph, EI UMa and LS Peg, have characteristics which are similar to IPs. However, there has been only tentative evidence for a coherent period in their X-ray light curve. We present the results of a search for coherent periods in XMM-Newton data of these sources using an autoregressive analysis which models the effects of red noise. We confirm the detection of a similar to 760 s period in the soft X-ray light curve of EI UMa reported by Reimer et al. and agree that this represents the spin period. We also find evidence for peaks in the power spectrum of each source in the range 100-200 s which are just above the 3 sigma confidence level. We do not believe that they represent genuine coherent modulations. However, their X-ray spectra are very similar to those of known IPs. We believe that all three CVs are bona fide IPs. We speculate that V426 Oph and LS Peg do not show evidence for a spin period since they have closely aligned magnetic and spin axes. We discuss the implications that this has for the defining characteristics of IPs.