arXiv subjects

Pasi Hakala

Publications and source records attributed to Pasi Hakala.

At least 55 records · Page 3Linked to original sources

arXiv2008

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 ~0.1-10MG. 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 auto-regressive analysis which models the effects of red-noise. We confirm the detection of a ~760 sec 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 sec which are just above the 3sigma 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.

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arXiv2007

Modeling of orbital modulation of Cygnus X-3 by particle simulations

The formation of the circumbinary envelope of Cygnus X-3 was studied by particle simulations of the WR (Wolf Rayet) companion wind. Light curves resulting from electron scattering absorption in this envelope were computed and compared with observed IBIS/ISGRI and BATSE light curves. The matching was relatively good. For reasonable values of binary parameters (masses, inclination) and wind velocities, a stable envelope was formed during a few binary orbits. Assuming approximately 10^-6 solar mass/year for the rate of the WR-wind, the observed light curves and accretion luminosity can be re-produced (assuming Thomson scattering opacity in the ionized He-rich envelope). The illuminated envelope can also model the CHANDRA-spectrum using the photoionizing XSTAR-code. Furthermore, we discuss observed radial velocity curves of IR emission lines in the context of simulated velocity fields and find good agreement.

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arXiv2006

XMM-Newton observations of AM CVn binaries: V396 Hya & SDSS J1240-01

We present the results of XMM-Newton observations of two AM CVn systems - V396 Hya and SDSS J1240-01. Both systems are detected in X-rays and in the UV: neither shows coherent variability in their light curves. We compare the rms variability of the X-ray and UV power spectra of these sources with other AM CVn systems. Apart from ES Cet, AM CVn sources are not strongly variable in X-rays, while in the UV the degree of variability is related to the systems apparent brightness. The X-ray spectra of V396 Hya and SDSS J1240-01 show highly non-solar abundances, requiring enhanced nitrogen to obtain good fits. We compare the UV and X-ray luminosities for 7 AM CVn systems using recent distances. We find that the X-ray luminosity is not strongly dependent upon orbital period. However, the UV luminosity is highly correlated with orbital period with the UV luminosity decreasing with increasing orbital period. We expect that this is due to the accretion disk making an increasingly strong contribution to the UV emission at shorter periods. The implied luminosities are in remarkably good agreement with predictions.

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arXiv2006

RApid Temporal Survey - RATS II: Followup observations of 4 newly discovered short period variables

The RApid Temporal Survey (RATS) is a survey to detect objects whose optical intensity varies on timescales of less than ~70 min. In our pilot dataset taken with the INT and the Wide Field Camera in Nov 2003 we discovered nearly 50 new variable objects. Many of these varied on timescales much longer than 1 hr. However, only 4 objects showed a modulation on a timescale of 1 hour or less. This paper presents followup optical photometry and spectroscopy of these 4 objects. We find that RAT J0455+1305 is a pulsating (on a period of 374 sec) subdwarf B (sdB) star of the EC 14026 type. We have modelled its spectrum and determine Teff = 29,200+/- 1900K and log g = 5.2+/-0.3 which locates it on the cool edge of the EC 14026 instability strip. It has a modulation amplitude which is one of the highest of any known EC 14026 star. Based on their spectra, photometric variability and their infra-red colours, we find that RAT J0449+1756, RAT J0455+1254 and RAT J0807+1510 are likely to be SX Phe stars - dwarf Delta Sct stars. Our results show that our observing strategy is a good method for finding rare pulsating stars.

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arXiv2006

XMM-Newton and Chandra observations of the ultra-compact binary RX J1914+24

The nature of the X-ray source RX J1914+24 has been the subject of much debate. It shows a prominent period of 569 sec in X-rays and the optical/infra-red: in most models this has been interpreted as the binary orbital period. We present our analysis of new XMM-Newton and Chandra data. We find a longer term trend in the XMM-Newton data and power at 556 and 585 sec in 5 sets of data. It is not clear if they are produced as a result of a beat between a longer intrinsic period and the 569 sec modulation or if they are due to secular variations. We obtain a good fit to the XMM-Newton spectrum with a low temperature thermal plasma model with an edge at 0.83keV. This model implies an unabsorbed bolometric X-ray luminosity of 1x10^{33} ergs/s (for a distance of 1kpc) - this is 2 orders of magnitude lower than our previous estimate (derived using a different model). If the distance is much less, as the absorption derived from the X-ray fits suggest, then it is even lower at \~3x10^{31} ergs/s.

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arXiv2005

XMM-Newton observations of AM CVn binaries

We present the results of XMM-Newton observations of four AM CVn systems -- AM CVn, CR Boo, HP Lib and GP Com. Their light curves show very different characteristics. The X-ray light curves show no coherent pulsations, suggesting the accreting white dwarfs have relatively low magnetic field strengths. Their spectra were best modelled using a multi-temperature emission model and a strong UV component. We find that CR Boo and HP Lib have X-ray spectra with abundances consistent with relatively low temperature CNO processed material, while AM CVn and GP Com show an enhancement of nitrogen. A large fraction of the accretion luminosity is emitted in the UV. We determine accretion luminosities of ~1.6x10^{33} ergs/s and 1.7x10^{31} ergs/s for AM CVn and GP Com respectively. Comparing the implied mass transfer rates with that derived using model fits to optical and UV spectra, we find evidence that in the case of AM CVn, we do not detect a significant proportion of the accretion energy. This missing component could be lost in the form of a wind.

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arXiv2005

RApid Temporal Survey - RATS I: Overview and first results

We present the aim and first results of the RApid Temporal Survey (RATS) made using the Wide Field Camera on the Isaac Newton Telescope. Our initial survey covers 3 square degrees, reaches a depth of V~22.5 and is sensitive to variations on timescales as short as 2 minutes: this is a new parameter space. Each field was observed for over 2 hours in white light, with 12 fields being observed in total. Our initial analysis finds 46 targets which show significant variations. Around half of these systems show quasi-sinusoidal variations: we believe they are contact or short period binaries. We find 4 systems which show variations on a timescale less than 1 hour. The shortest period system has a period of 374 sec. We find two systems which show a total eclipse. Further photometric and spectroscopic observations are required to fully identify the nature of these systems. We outline our future plans and objectives.

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arXiv2004

XMM-Newton observations of the ultra-compact binary RX J1914+24

We present XMM-Newton observations of the 569 sec period system RX J1914+24 (V407 Vul). This period is believed to represent the binary orbital period making it an ultra-compact binary system. By comparing the phase of the rise to maximum X-ray flux at various epochs (this includes observations made using ROSAT, ASCA and Chandra) we find that the system is spinning up at a rate of 3.17+/-0.07x10^{-12} s/s. We find that the spectra softens as the X-ray flux declines towards the off-phase of the 569 sec period. Further, the spectra are best fitted by an absorbed blackbody component together with a broad emission feature around 0.59keV. This emission feature is most prominent at the peak of the on-phase. We speculate on its origin.

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arXiv2004

Accretion stream mapping with genetically modified "fire-flies"

We apply an eclipse mapping technique using `genetically modified fire-flies' to the eclipse light curves of HU Aqr and EP Dra. The technique makes as few assumptions as possible about the location of accretion stream material, allowing the emission to be located anywhere within the Roche lobe of the white dwarf. We model two consecutive eclipses in the UBVR_c band for HU Aqr, and four consecutive `white'-light eclipses for EP Dra, to determine the changing brightness distribution of stream material. We find fire-fly distributions which are consistent with accretion through a curtain of material in both HU Aqr and EP Dra, and show that the previously assumed two part ballistic and magnetic trajectory is a good approximation for polars. Model fits to the colour band data of HU Aqr indicate that the material confined to the magnetic field lines is brightest, and most of the emission originates from close to the white dwarf. There is evidence for emission from close to a calculated ballistic stream in both HU Aqr and EP Dra.We propose that a change in the stream density causes a change in the location of the bright material in the accretion stream in EP Dra.

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arXiv2004

XMM-Newton observations of the dwarf nova YZ Cnc in quiescence

We present results from the XMM observations of the dwarf nova YZ Cnc in a quiescent state. We have performed a detailed time series analysis of the resulting light curves. Unusually, we do not detect any orbital modulation in the UV, with only marginal evidence for X-ray modulation on this period. Although there are peaks in the X-ray periodograms at periods less than 5000 s, we attribute them to red noise effects and assign significance to them using a novel approach. The variability in the UV and optical bands can also be modelled as a result of aperiodic variability (red noise) in the system. There is evidence that the UV and X-ray fluxes are anti-correlated with a time delay of about 100 s, with the UV lagging behind the X-ray emission. This anti-correlation is intriguing, but is only present on two occasions lasting several 1000 s each. The X-ray spectrum shows similar emission features to other dwarf novae and is well fitted using a multi-temperature emission model. We measure a relatively high X-ray luminosity of ~1.4x10^{32} ergs/s, although this is consistent with a low binary inclination. Finally, we find evidence for a possible -1200km/s blue shift in the fitted Fe K line energies, possibly indicating the presence of an outflow in this low inclination system.

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arXiv2003

XMM-Newton observations of OY Car III: OM light curve modelling, X-ray timing and spectral studies

We revisit the XMM-Newton observations of the dwarf nova OY Car taken in July 2000 which occured shortly after an outburst. Ramsay et al (2001a) found a prominent energy dependent modulation at a period of 2240 sec: this modulation was only seen for app. 1/3 of the observation duration. In our new analysis, we examine this time interval in greater detail. In addition to the 2240 sec period we find evidence for other periods, the most prominent being near 3500 sec. Both these modulations are most likely due to changes in photoelectric absorption over this period: this is supported by phase-resolved spectroscopy. This may indicate the presence of matter above the accretion disc or a presence of a magnetic accretion curtain. In this case the 2240 sec period could represent a spin period of the white dwarf and the 3500 sec period a beat period between the spin and orbital periods. We also model the B band and UV eclipse profiles and light curves using a new technique to map the spatial extent of the accretion disc. As a result we find that whilst the optical emission is dominated by both the emission close to the accretion disc boundary layer and the hot spot where the accretion stream hits the disc, the UV emission is mainly dominated by the inner disc/boundary layer only.

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arXiv2003

Spin up in RX J0806+15 - the shortest period binary

RX J0806+15 has recently been identified as the binary system with the shortest known orbital period. We present a series of observations of RX J0806+15 including new optical observations taken one month apart. Using these observations and archival data we find that the period of this system is decreasing over time. Our measurements imply f_dot = 6.11x10^-16 Hz/s, which is in agreement with a rate expected from the gravitational radiation for two white dwarfs orbiting at a given period. However, a smaller value of f_dot = 3.14x10^-16 Hz/s cannot be ruled out. Our result supports the idea that the 321.5 s period is the orbital period and that the system is the shortest period binary known so far and that it is one of the strongest sources of constant gravitational radiation in the sky. Furthermore, the decrease of the period strongly favours the unipolar inductor (or electric star) model rather than the accretion models.

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arXiv2003

Stream eclipse mapping with 'fire-flies'

We apply a new method of eclipse mapping to the light curves of eclipsing polars. The technique aims to locate the bright emission associated with the accretion stream, using a technique that makes the fewest prior assumptions about the location of the accretion stream material. We have obtained data of EP Dra and HU Aqr with the S-Cam 2 superconducting tunnel junction camera using the William Herschel Telescope. The location of emission regions in both systems show that previously assumed trajectories are consistent with those found using our technique. Most of the emission is located in a region where we expect material to be confined to magnetic field lines, particularly for HU Aqr, while there appears to be less emission from where we conventionally expect material to follow a ballistic trajectory from the L1-point.

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arXiv2003

Ultra-Short Period Double-Degenerate Binaries

We review the current observational status of the ROSAT sources RX J1914.4+2456 and RX J0806.3+1527, and the evidence that these are ultra-short period (<10min) binary systems. We argue that an Intermediate Polar interpretation can be ruled out, that they are indeed compact binaries with a degenerate secondary, and that the period seen in the X-ray and optical is the orbital period. A white dwarf primary is preferred, but a neutron star cannot be excluded. We examine the capability of the three current double-degenerate models (Polar, Direct Accretor and Electric Star) to account for the observational characteristics of these systems. All models have difficulties with some aspects of the observations, but none can be excluded with confidence at present. The Electric Star model provides the best description, but the lifetime of this phase requires further investigation. These ultra-short period binaries will be strong gravitational wave emitters in the LISA bandpass, and because of their known source properties will be important early targets for gravitational wave studies.

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arXiv1999

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 synchronising mechanisms in such a close binary and other system parameters.

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arXiv1998

Evidence for evolving accretion disk structure in 4U 1957+115

We present results of the first multicolour photometric study of 4U1957+115, one of the optically less studied low mass X-ray binaries (LMXB). Our quasi-simultaneous UBVRI observations reveal that the light curve pulse shape over the 9.33 hr period discovered by Thorstensen (1987) has changed substantially since his earlier V band studies. The light curve now shows clear asymmetry and an amplitude more than double of that seen by Thorstensen (1987). The light curve also shows colour dependence that seems to rule out the X-ray heating model as an origin for photometric variability. We believe that the changes observed in the light curve shape are indicative of evolving accretion disk structure. We discuss implications of this on the origin of optical emission in 4U1957+115 and disk accreting binaries in general.

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arXiv1998

The optical counterpart of the double degenerate polar RX J1914+24

Cropper et al 1998 presented X-ray observations of RX J1914+24 and claimed that the most likely interpretation of their data was that of a double degenerate Polar. Here we show the preliminary results of optical and further X-ray observations. We identify the optical counterpart and show that no other period apart from the 569 sec period seen in X-rays is identified. Although the optical counterpart has no significant polarised flux in the I band, our ASCA spectrum (which is typical of a Polar) together with our photometric result adds weight to the conclusion that RX J1914+24 is the first double degenerate Polar to be discovered.

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arXiv1998

The GOSSIP on the MCV V347 Pav

Modelling of the polarized cyclotron emission from magnetic cataclysmic variables (MCVs) has been a powerful technique for determining the structure of the accretion zones on the white dwarf. Until now, this has been achieved by constructing emission regions (for example arcs and spots) put in by hand, in order to recover the polarized emission. These models were all inferred indirectly from arguments based on polarization and X-ray light curves. Potter, Hakala & Cropper (1998) presented a technique (Stokes imaging) which objectively and analytically models the polarized emission to recover the structure of the cyclotron emission region(s) in MCVs. We demonstrate this technique with the aid of a test case, then we apply the technique to polarimetric observations of the AM Her system V347 Pav. As the system parameters of V347 Pav (for example its inclination) have not been well determined, we describe an extension to the Stokes imaging technique which also searches the system parameter space (GOSSIP).

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