Pasi Hakala | University of Turku (original) (raw)

Papers by Pasi Hakala

Research paper thumbnail of R 0751+14, the Polar-Like Intermediate Polar

Research paper thumbnail of A 106-d period in the nuclear source X-8 in M33

With an X-ray luminosity of about 10^39 erg/s, the source X-8 coincident with the optical center ... more With an X-ray luminosity of about 10^39 erg/s, the source X-8 coincident with the optical center of M33 is the most luminous X-ray source in the Local Group. However, its nature remains a mystery. We present here new and archival ROSAT observations of X-8 spread over 6 years which show variability and a ~ 106-d periodicity. This implies that (most of) the emission from M33 X-8 arises from a single object, perhaps a binary system with a ~ 10 Msol black hole primary. We suggest that the companion is a giant orbiting with a ~ 10 day period, and that the observed modulation is ``super-orbital'', analogous to that seen in Cyg X-2 and X1820-30.

Research paper thumbnail of Continuous ‘stunted’ outbursts detected from the cataclysmic variable KIC 9202990 using Kepler data

Monthly Notices of the Royal Astronomical Society, 2015

Research paper thumbnail of Using INTEGRAL and RXTE to explore the Spreading Layer of GX 9+9

GX 9+9 is a persistently bright atoll-type Low-Mass X-ray Binary. We fitted its INTEGRAL and RXTE... more GX 9+9 is a persistently bright atoll-type Low-Mass X-ray Binary. We fitted its INTEGRAL and RXTE spectra from 2002-2007 with a two-component model: a multi-temperature accretion disc blackbody, plus another blackbody representing the spreading layer (SL), an extended accretion zone on the neutron star surface. The results were mostly consistent with SL theory, although the SL temperature seemed to increase at low SL luminosities, while the approximate angular extent had a nearly linear luminosity dependency. Partial Comptonization of the SL blackbody was not required to fit the spectra. Together with the upper bound of inclination imposed by the lack of eclipses, the best-fit normalization of the accretion disc component implies a distance of ∼ 10 kpc, instead of the usually quoted 5 kpc.

Research paper thumbnail of Accretion stream mapping in eclipsing polars. an application of MEM and genetic optimisation

Astronomy and Astrophysics, Mar 30, 1995

The discovery of RE2107-05 (Hakala et al. 1993; Schwope et al. 1993), the brightest of the four k... more The discovery of RE2107-05 (Hakala et al. 1993; Schwope et al. 1993), the brightest of the four known eclipsing AM Her systems revealed that a significant proportion of the optical emission from AM Her systems originates within the accretion stream. This is clearly demonstrated by the asymmetric eclipse profiles obtained by Hakala et al. (1993). I present a new method for deriving the accretion stream brightness distribution from the eclipse profile photometry of AM Her systems using a maximum entropy method (MEM) and a Genetic Algorithm for optimisation. This technique is applied to the U band eclipse profile of RE2107-05. The qualitative results indicate, that the stream emission rises quadratically towards the white dwarf and there is a separate "hot spot" within the stream possibly due to the threading to the WD magnetosphere.

Research paper thumbnail of GRB 101225A: optical break from NOT observation

Research paper thumbnail of The GOSSIP on the MCV V347 Pav

Modelling of the polarized cyclotron emission from magnetic cataclysmic variables (MCVs) has been... more 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).

Research paper thumbnail of Re J0751+14

IAUC 5707 available at Central Bureau for Astronomical Telegrams. IAUC 5707 available at Central ... more IAUC 5707 available at Central Bureau for Astronomical Telegrams. IAUC 5707 available at Central Bureau for Astronomical Telegrams.

Research paper thumbnail of NOT observations of X-ray binaries

Research paper thumbnail of New short-period stellar pulsators at large Galactocentric distances

Monthly Notices of the Royal Astronomical Society, Jun 16, 2011

We report the discovery of 31 blue, short period, pulsators made using data taken as part of the ... more We report the discovery of 31 blue, short period, pulsators made using data taken as part of the Rapid Temporal Survey (RATS). We find they have periods between 51-83 mins and full-amplitudes between 0.05-0.65 mag. Using the period-luminosity relationship for short period pulsating stars we determine their distance. Assuming they are pulsating in either the fundamental or first over-tone radial mode the majority are located at a distance greater than 3kpc, with several being more than 20 kpc distant. Most stars are at least 1 kpc from the Galactic plane, with three being more than 10 kpc. One is located in the direction of the Galactic anti-center and has Galactocentric distance of ~30 kpc and is ~20 kpc below the plane: they are therefore potential tracers of Galactic structure. We have obtained low-resolution spectra for a small number our targets and find they have temperatures between 7200--7900K and a metal content less than Solar. The colours of the pulsators and the spectral fits to those stars for which we have spectra indicate that they are either SX Phe or delta Scuti stars. We estimate the number of SX Phe stars in our Galaxy and find significantly fewer per unit mass than reported in massive globular clusters or dwarf spheroidal galaxies.

Research paper thumbnail of Searching for I -band variability in stars in the M/L spectral transition region

Monthly Notices of the Royal Astronomical Society, 2015

Research paper thumbnail of The defining characteristics of intermediate polars – the case of three‐candidate systems

Monthly Notices of the Royal Astronomical Society, Apr 8, 2008

Intermediate polars (IPs) are a group of cataclysmic variables (CVs) which are thought to contain... more 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 autoregressive analysis which models the effects of red noise. We confirm the detection of a ~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 3σ 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.

Research paper thumbnail of GRB 101225A: optical break from NOT observation

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

Mon Notic Roy Astron Soc, Jun 26, 2006

The RApid Temporal Survey (RATS) is a survey to detect objects whose optical intensity varies on ... more 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.

Research paper thumbnail of Extreme Circular Polarization Discovered in the Magnetic CV RE 1307+535

Research paper thumbnail of RApid Temporal Survey (RATS) - I. Overview and first results

Monthly Notices of the Royal Astronomical Society, 2005

We present the aim and first results of the RApid Temporal Survey (RATS) made using the Wide Fiel... more 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 time-scales as short as 2 min: this is a ...

Research paper thumbnail of SDSS. II. Measurements for six objects, including two eclipsing systems

Research paper thumbnail of Monitoring the spin up in RX J0806+15

Monthly Notices of the Royal Astronomical Society, 2004

RX J0806+15 shows a prominent intensity variation on a period of 321.5 sec. This has widely been ... more RX J0806+15 shows a prominent intensity variation on a period of 321.5 sec. This has widely been interpreted as the binary orbital period, although this remains controversial. We have been monitoring the precise period of RX J0806+15 for a number of years. By measuring the rate of change we can help distinguish between competing physical models. New observations obtained between Nov 2003 and Feb 2004 show that the period decrease already reported by and Strohmayer is continuing. We discuss how reliably we can determine the period of RX J0806+15 using our technique and evaluate the current models which have been proposed to account for the observational properties of this source.

Research paper thumbnail of UBVRI-photopolarimetry and the X ray dips of V471 Tau

Research paper thumbnail of Optical counterpart of HLX-1 during the 2010 outburst

Monthly Notices of the Royal Astronomical Society, 2012

We studied the optical counterpart of the intermediate-mass black hole candidate HLX-1 in ESO 243... more We studied the optical counterpart of the intermediate-mass black hole candidate HLX-1 in ESO 243-49. We used a set of Very Large Telescope imaging observations from 2010 November, integrated by Swift X-ray data from the same epoch. We measured standard Vega brightnesses U = 23.89 ± 0.18 mag, B = 25.19 ± 0.30 mag, V = 24.79 ± 0.34 mag and R = 24.71 ± 0.40 mag. Therefore, the source was ≈ 1 mag fainter in each band than in a set of Hubble Space Telescope images taken a couple of months earlier, when the X-ray flux was a factor of 2 higher. We conclude that during the 2010 September observations, the optical counterpart was dominated by emission from an irradiated disk (which responds to the varying X-ray luminosity), rather than by a star cluster around the black hole (which would not change). We modelled the Comptonized, irradiated X-ray spectrum of the disk, and found that the optical luminosity and colours in the 2010 November data are still consistent with emission from the irradiated disk, with a characteristic outer radius r out ≈ 2800r in ∼ 10 13 cm and a reprocessing fraction ≈ 2 × 10 −3 . The optical colours are also consistent with a stellar population with age 6 Myr (at solar metallicity) and mass ≈ 10 4 M ⊙ ; this is only an upper limit to the mass, if there is also a significant contribution from an irradiated disk. We strongly rule out the presence of a young super-star-cluster, which would be too bright. An old globular cluster might be associated with HLX-1, as long as its mass 2 × 10 6 M ⊙ for an age of 10 Gyr, but it cannot significantly contribute to the observed very blue and variable optical/UV emission.

Research paper thumbnail of R 0751+14, the Polar-Like Intermediate Polar

Research paper thumbnail of A 106-d period in the nuclear source X-8 in M33

With an X-ray luminosity of about 10^39 erg/s, the source X-8 coincident with the optical center ... more With an X-ray luminosity of about 10^39 erg/s, the source X-8 coincident with the optical center of M33 is the most luminous X-ray source in the Local Group. However, its nature remains a mystery. We present here new and archival ROSAT observations of X-8 spread over 6 years which show variability and a ~ 106-d periodicity. This implies that (most of) the emission from M33 X-8 arises from a single object, perhaps a binary system with a ~ 10 Msol black hole primary. We suggest that the companion is a giant orbiting with a ~ 10 day period, and that the observed modulation is ``super-orbital'', analogous to that seen in Cyg X-2 and X1820-30.

Research paper thumbnail of Continuous ‘stunted’ outbursts detected from the cataclysmic variable KIC 9202990 using Kepler data

Monthly Notices of the Royal Astronomical Society, 2015

Research paper thumbnail of Using INTEGRAL and RXTE to explore the Spreading Layer of GX 9+9

GX 9+9 is a persistently bright atoll-type Low-Mass X-ray Binary. We fitted its INTEGRAL and RXTE... more GX 9+9 is a persistently bright atoll-type Low-Mass X-ray Binary. We fitted its INTEGRAL and RXTE spectra from 2002-2007 with a two-component model: a multi-temperature accretion disc blackbody, plus another blackbody representing the spreading layer (SL), an extended accretion zone on the neutron star surface. The results were mostly consistent with SL theory, although the SL temperature seemed to increase at low SL luminosities, while the approximate angular extent had a nearly linear luminosity dependency. Partial Comptonization of the SL blackbody was not required to fit the spectra. Together with the upper bound of inclination imposed by the lack of eclipses, the best-fit normalization of the accretion disc component implies a distance of ∼ 10 kpc, instead of the usually quoted 5 kpc.

Research paper thumbnail of Accretion stream mapping in eclipsing polars. an application of MEM and genetic optimisation

Astronomy and Astrophysics, Mar 30, 1995

The discovery of RE2107-05 (Hakala et al. 1993; Schwope et al. 1993), the brightest of the four k... more The discovery of RE2107-05 (Hakala et al. 1993; Schwope et al. 1993), the brightest of the four known eclipsing AM Her systems revealed that a significant proportion of the optical emission from AM Her systems originates within the accretion stream. This is clearly demonstrated by the asymmetric eclipse profiles obtained by Hakala et al. (1993). I present a new method for deriving the accretion stream brightness distribution from the eclipse profile photometry of AM Her systems using a maximum entropy method (MEM) and a Genetic Algorithm for optimisation. This technique is applied to the U band eclipse profile of RE2107-05. The qualitative results indicate, that the stream emission rises quadratically towards the white dwarf and there is a separate "hot spot" within the stream possibly due to the threading to the WD magnetosphere.

Research paper thumbnail of GRB 101225A: optical break from NOT observation

Research paper thumbnail of The GOSSIP on the MCV V347 Pav

Modelling of the polarized cyclotron emission from magnetic cataclysmic variables (MCVs) has been... more 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).

Research paper thumbnail of Re J0751+14

IAUC 5707 available at Central Bureau for Astronomical Telegrams. IAUC 5707 available at Central ... more IAUC 5707 available at Central Bureau for Astronomical Telegrams. IAUC 5707 available at Central Bureau for Astronomical Telegrams.

Research paper thumbnail of NOT observations of X-ray binaries

Research paper thumbnail of New short-period stellar pulsators at large Galactocentric distances

Monthly Notices of the Royal Astronomical Society, Jun 16, 2011

We report the discovery of 31 blue, short period, pulsators made using data taken as part of the ... more We report the discovery of 31 blue, short period, pulsators made using data taken as part of the Rapid Temporal Survey (RATS). We find they have periods between 51-83 mins and full-amplitudes between 0.05-0.65 mag. Using the period-luminosity relationship for short period pulsating stars we determine their distance. Assuming they are pulsating in either the fundamental or first over-tone radial mode the majority are located at a distance greater than 3kpc, with several being more than 20 kpc distant. Most stars are at least 1 kpc from the Galactic plane, with three being more than 10 kpc. One is located in the direction of the Galactic anti-center and has Galactocentric distance of ~30 kpc and is ~20 kpc below the plane: they are therefore potential tracers of Galactic structure. We have obtained low-resolution spectra for a small number our targets and find they have temperatures between 7200--7900K and a metal content less than Solar. The colours of the pulsators and the spectral fits to those stars for which we have spectra indicate that they are either SX Phe or delta Scuti stars. We estimate the number of SX Phe stars in our Galaxy and find significantly fewer per unit mass than reported in massive globular clusters or dwarf spheroidal galaxies.

Research paper thumbnail of Searching for I -band variability in stars in the M/L spectral transition region

Monthly Notices of the Royal Astronomical Society, 2015

Research paper thumbnail of The defining characteristics of intermediate polars – the case of three‐candidate systems

Monthly Notices of the Royal Astronomical Society, Apr 8, 2008

Intermediate polars (IPs) are a group of cataclysmic variables (CVs) which are thought to contain... more 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 autoregressive analysis which models the effects of red noise. We confirm the detection of a ~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 3σ 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.

Research paper thumbnail of GRB 101225A: optical break from NOT observation

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

Mon Notic Roy Astron Soc, Jun 26, 2006

The RApid Temporal Survey (RATS) is a survey to detect objects whose optical intensity varies on ... more 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.

Research paper thumbnail of Extreme Circular Polarization Discovered in the Magnetic CV RE 1307+535

Research paper thumbnail of RApid Temporal Survey (RATS) - I. Overview and first results

Monthly Notices of the Royal Astronomical Society, 2005

We present the aim and first results of the RApid Temporal Survey (RATS) made using the Wide Fiel... more 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 time-scales as short as 2 min: this is a ...

Research paper thumbnail of SDSS. II. Measurements for six objects, including two eclipsing systems

Research paper thumbnail of Monitoring the spin up in RX J0806+15

Monthly Notices of the Royal Astronomical Society, 2004

RX J0806+15 shows a prominent intensity variation on a period of 321.5 sec. This has widely been ... more RX J0806+15 shows a prominent intensity variation on a period of 321.5 sec. This has widely been interpreted as the binary orbital period, although this remains controversial. We have been monitoring the precise period of RX J0806+15 for a number of years. By measuring the rate of change we can help distinguish between competing physical models. New observations obtained between Nov 2003 and Feb 2004 show that the period decrease already reported by and Strohmayer is continuing. We discuss how reliably we can determine the period of RX J0806+15 using our technique and evaluate the current models which have been proposed to account for the observational properties of this source.

Research paper thumbnail of UBVRI-photopolarimetry and the X ray dips of V471 Tau

Research paper thumbnail of Optical counterpart of HLX-1 during the 2010 outburst

Monthly Notices of the Royal Astronomical Society, 2012

We studied the optical counterpart of the intermediate-mass black hole candidate HLX-1 in ESO 243... more We studied the optical counterpart of the intermediate-mass black hole candidate HLX-1 in ESO 243-49. We used a set of Very Large Telescope imaging observations from 2010 November, integrated by Swift X-ray data from the same epoch. We measured standard Vega brightnesses U = 23.89 ± 0.18 mag, B = 25.19 ± 0.30 mag, V = 24.79 ± 0.34 mag and R = 24.71 ± 0.40 mag. Therefore, the source was ≈ 1 mag fainter in each band than in a set of Hubble Space Telescope images taken a couple of months earlier, when the X-ray flux was a factor of 2 higher. We conclude that during the 2010 September observations, the optical counterpart was dominated by emission from an irradiated disk (which responds to the varying X-ray luminosity), rather than by a star cluster around the black hole (which would not change). We modelled the Comptonized, irradiated X-ray spectrum of the disk, and found that the optical luminosity and colours in the 2010 November data are still consistent with emission from the irradiated disk, with a characteristic outer radius r out ≈ 2800r in ∼ 10 13 cm and a reprocessing fraction ≈ 2 × 10 −3 . The optical colours are also consistent with a stellar population with age 6 Myr (at solar metallicity) and mass ≈ 10 4 M ⊙ ; this is only an upper limit to the mass, if there is also a significant contribution from an irradiated disk. We strongly rule out the presence of a young super-star-cluster, which would be too bright. An old globular cluster might be associated with HLX-1, as long as its mass 2 × 10 6 M ⊙ for an age of 10 Gyr, but it cannot significantly contribute to the observed very blue and variable optical/UV emission.