arXiv subjects

P. Hakala

Publications and source records attributed to P. Hakala.

41 records · Page 3Linked to original sources

arXiv2000

Evidence of magnetic accretion in an SW Sex star: discovery of variable circular polarization in LS Pegasi

We report on the discovery of variable circular polarization in the SW Sex 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 Hbeta, with a period of 33.5 minutes. The difference between both frequencies is just the orbital frequency, so we relate the 33.5-min 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 dwarf's magnetosphere, which extends to the corotation radius. From this geometry, we estimate a magnetic field strength of B(1) ~ 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.

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arXiv2000

On the multi-periodicities in the X-ray dipper XB 1916-053

Using the Rossi X-ray Timing Explorer and the Nordic Optical Telescope we have obtained the highest ever quality X-ray/white-light high-speed photometry of XB 1916-053. We refine the X-ray period (P_X) to 3000.6+/-0.2s via a restricted cycle counting approach. Using our complete optical lightcurve, we have extended the optical period (P_opt) ephemeris by another 4 years, providing further evidence for its stability, although a slightly longer period of 3027.555+/-0.002s now provides a marginally better fit. Moreover, modulations at both P_X and P_opt are present in the optical data, with the former dominating the nightly lightcurves (i.e. a few cycles of data). We have also attempted to determine the ``beat'' period, as seen in the repeating evolution of the X-ray dip structure, and the variation in primary dip phase. We find that a quasi-period of 4.74+/-0.05d provides the best fit to the data, even then requiring phase shifts between cycles, with the expected 3.90d ``beat'' of P_X and P_opt appearing to be less likely. Finally, considering the nature of each of these temporal phenomena, we outline possible models, which could explain all of the observed behaviour of this enigmatic source, focusing on which of P_X or P_opt is the binary period.

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arXiv1998

An ``outside-in'' outburst of Aql X--1

We present optical spectroscopy and optical and infrared photometry of the neutron star soft X-ray transient Aql X--1 during its X-ray outburst of August 1997. By modelling the X-ray, optical, and IR light curves, we find a 3 day delay between the IR and X-ray rise times, analogous to the UV-optical delay seen in dwarf novae outbursts and black hole X-ray transients. We interpret this delay as the signature of an ``outside-in'' outburst, in which a thermal instability in the outer disc propagates inward. This outburst is the first of this type definitively identified in a neutron star X-ray transient.

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arXiv1997

UV Spectroscopy of AB Doradus with the Hubble Space Telescope. Impulsive flares and bimodal profiles of the CIV 1549 line in a young star

We observed AB Doradus, a young and active late type star (K0 - K2 IV-V, P= 0.514 d) with the Goddard High Resolution Spectrograph of the post-COSTAR Hubble Space Telescope with the time and spectral resolutions of 27 s and 15 km, respectively. The wavelength band (1531 - 1565 A) included the strong CIV doublet (1548.202 and 1550.774, formed in the transition region at 100 000 K). The mean quiescent CIV flux state was close to the saturated value and 100 times the solar one. The line profile (after removing the rotational and instrumental profiles) is bimodal consisting of two Gaussians, narrow (FWHM = 70 km/s) and broad (FWHM =330km/s). This bimodality is probably due to two separate broadening mechanisms and velocity fields at the coronal base. It is possible that TR transient events (random multiple velocities), with a large surface coverage, give rise to the broadening of the narrow component,while true microflaring is responsible for the broad one. The transition region was observed to flare frequently on different time scales and magnitudes. The largest impulsive flare seen in the CIV 1549 emission reached in less than one minute the peak differential emission measure (10**51.2 cm-3) and returned exponentially in 5 minutes to the 7 times lower quiescent level.The 3 min average line profile of the flare was blue-shifted (-190 km/s) and broadened (FWHM = 800 km/s). This impulsive flare could have been due to a chromospheric heating and subsequent evaporation by an electron beam, accelerated (by reconnection) at the apex of a coronal loop.

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arXiv1997

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 on the physics of the emission mechanism, and perhaps also about the underlying energy source. As the gamma-ray phenomenon is extremely diverse, with time scales spanning several orders of magnitude, some diversity in optical counterpart signatures appears plausible. We have studied the Optical Transient, which accompanied the gamma-ray burst of May 8, 1997 (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 hours 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 hours 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 hours 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, m_R = 24.28+-0.10, is brighter than the extrapolated power-law, and hints that a constant component, m_R = 25.50+-0.40 is present. The OT is unresolved (FWHM 0.83") at the faintest magnitude level. The brightness of the optical transient, its duration and the general shape of the light curve sets this source apart from the single other optical transient known, that of the February 28, 1997 event.

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