Final Report Summary - PROMOPS (Probing models of pulsars through multi wavelength observations)
A summary description of the project objectives:
The main objective of the research reintegration grant was to investigate isolated and binary compact objects like neutron stars and white dwarfs. Observations of that objects were performed as photometric and polarimetric observations with different time resolution going down to nanosecond time scales. Many of them were performed with the specially designed and constructed very fast 'optical pulsar timing analyser' (OPTIMA) photo-polarimeter. The researcher is a member of the OPTIMA project group. With the financial support from the reintegration grant she was able to join three observing campaigns of OPTIMA at the Skinakas astronomical observatory in Crete, Greece.
A description of the work performed since the beginning of the project and a description of the main results achieved so far:
SAX J2103.5+4545 is the Be/X-ray binary (BeX) with the shortest orbital period. It shows extended bright and faint X-ray states that last for a few hundred days. The main objective of this work was to investigate the relationship between the X-ray and optical variability and to characterise the spectral and timing properties of the bright and faint states. We have found a correlation between the spectral and temporal parameters that fit the energy and power spectra. Softer energy spectra correspond to softer power spectra. Ha emission, indicative of an equatorial disc around the B-type star, is detected whenever the source is bright in X-rays. When the disc is absent, the X-ray emission decreases significantly. The long-term variability of SAX J2103.5+4545 is characterised by fast episodes of disc loss and subsequent reformation. The time-scales for the loss and reformation of the disc (about two years) are the fastest among BeXs.
We presented fast timing photometric observations of the intermediate polar V2069 Cygni (RX J2123.7+4217). The optical light curve of V2069 Cygni revealed a double-peaked pulsation with a period of 743.38 ± 0.25 s. We suggest that this period represents the spin of the white dwarf accretor. Moreover, we presented results from a detailed analysis of the X-ray multi-mirror mission (XMM)-Newton observations, which also show a double-peaked modulation, however shifted in phase, with a period of 742.35 ± 0.23 s. The X-ray spectra obtained from the XMM-Newton European photon imaging camera (EPIC) instruments were modelled by a plasma emission and a soft blackbody component with a partial covering photoelectric absorption model with a covering fraction of 0.65. An additional Gaussian emission line at 6.385 keV with an equivalent width of 243 eV is required to account for fluorescent emission from neutral iron. The iron fluorescence (approximately 6.4 keV) and Fe XXVI lines (approximately 6.95 keV) are clearly resolved in the EPIC spectra. In the Porb-Pspin diagram of intermediate polars, V2069 Cyg shows a low spin-to-orbit ratio of approximately 0.0276 in comparison with approximately 0.1 for other intermediate polars.
The 1 700 year old PSR B0540-69 in the large Magellanic clouds (LMC) is considered to be the twin of the Crab pulsar because of its similar spin parameters, magnetic field and energetics. PSR B0540-69 (V 22.5) is also one of the very few pulsars for which both optical pulsations and polarised emission have been measured. We could clearly identify PSR B0540-69 in our J, H and KS-band images and measure its flux (J = 20.14 H = 19.33 KS = 18.55 with an overall error of ± 0.1 mag in each band). The joint fit to the available optical and near infrared (nIR) photometry with a power-law spectrum gives a spectral index of 0.70 ± 0.04 slightly more precise than measured in the optical only. This clearly implies that there is no spectral break between the optical and the nIR. We also detected, for the first time, the PSR B0540-69 pulsar wind nebula (PWN) in the nIR. The comparison between our Nasmyth adaptive optics system nIR imager and spectrograph (NACO) images and Hubble space telescope (HST) optical ones does not reveal any apparent difference in the PWN morphology as a function of wavelength. The PWN optical-to-nIR spectrum can also be fitted by a single power-law, with spectral index of 0.56 ± 0.03 slightly flatter than the pulsar's. Using NACO at the very large telescope (VLT), we obtained the first detection of PSR B0540-69 and its PWN in the nIR. Due to the small angular scale of the PWN (approximately four parcsec) only the spatial resolution of the James Webb space telescope (JWST) will make it possible to extend the study of the pulsar and PWN spectrum towards the mid-IR.
We investigated the eclipse timing of the polar binary HU Aquarii that has been observed for almost two decades. Recently, Qian et al. attributed large (O-C) deviations between the eclipse ephemeris and observations to a compact system of two massive Jovian companions. We improved the Keplerian, kinematic model of the light travel time effect and re-analysed the whole currently available data set. We added almost 60 new, yet unpublished, mostly precision light curves obtained using the time high-resolution photopolarimeter OPTIMA, as well as photometric observations performed at the monitoring network of telescopes/north, physics innovations robotic astronomical telescope explorer and Carlos Sánchez telescope. We determined new mid-egress times with a mean uncertainty at the level of 1 s or better. We claimed that because the observations that currently exist in the literature are non-homogeneous with respect to spectral windows (ultraviolet, X-ray, visual and polarimetric mode) and the reported mid-egress measurements errors, they may introduce systematics that affect orbital fits. Indeed, we found that the published data, when taken literally, cannot be explained by any unique solution. Many qualitatively different and best-fit two-planet configurations, including self-consistent, Newtonian N-body solutions may be able to explain the data. However, using high-resolution, precision OPTIMA light curves, we find that the observed minus calculated (O-C) deviations are best explained by the presence of a single circumbinary companion orbiting at a distance of approximately 4.5 au with a small eccentricity and having approximately seven Jupiter masses. This object could be the next circumbinary planet detected from the ground, similar to the announced companions around close binaries HW Vir, NN Ser, UZ For, DP Leo, FS Aur or SZ Her and planets of this type around Kepler-16, Kepler-34 and Kepler-35.
We focussed on short-period eclipsing binaries that belong to a class of cataclysmic variables (CVs). They are known as polars and intermediate polars, closely resembling their prototype AM Herculis. These binaries consist of a red dwarf and a strongly magnetic white dwarf, having orbital periods of only a few hours. Monitoring the eclipses of these typically faint sources demands high-time resolution photometry. We described the very recent results obtained for two CVs, HU Aqr and DQ Her, which were observed with OPTIMA. The new observations of HU Aqr confirm that the O-C diagrams exhibit variations known for this binary which can be explained by a single, massive Jupiter-like planet, possibly accompanied by a very distant companion.
The expected final results and their potential impact and use (including the socio-economic impact and the wider societal implications of the project so far):
There were three observing campaigns with the OPTIMA photo-polarimeter. There is a lot of data collected during these campaigns that are not published yet. For some sources, as for example for polars that might host exoplanets, monitoring of their eclipses for many years is needed to cover wide enough time span and to allow to fit Keplerian kinematic model. There are also some polarimetric observations of propeller AE Aqr.
Additionally, we also have the radio polarimetric data of the Crab pulsar at 610 MHz and 1.4 GHz with the highest possible time resolution available now. Data analysis is in progress and the results might reveal competently new picture of the coherent and non-coherent radiation of pulsars. Such comparison between radio and optical polarimetric features of pulsars is performed for the first time and can give unexpected results.
The main objective of the research reintegration grant was to investigate isolated and binary compact objects like neutron stars and white dwarfs. Observations of that objects were performed as photometric and polarimetric observations with different time resolution going down to nanosecond time scales. Many of them were performed with the specially designed and constructed very fast 'optical pulsar timing analyser' (OPTIMA) photo-polarimeter. The researcher is a member of the OPTIMA project group. With the financial support from the reintegration grant she was able to join three observing campaigns of OPTIMA at the Skinakas astronomical observatory in Crete, Greece.
A description of the work performed since the beginning of the project and a description of the main results achieved so far:
SAX J2103.5+4545 is the Be/X-ray binary (BeX) with the shortest orbital period. It shows extended bright and faint X-ray states that last for a few hundred days. The main objective of this work was to investigate the relationship between the X-ray and optical variability and to characterise the spectral and timing properties of the bright and faint states. We have found a correlation between the spectral and temporal parameters that fit the energy and power spectra. Softer energy spectra correspond to softer power spectra. Ha emission, indicative of an equatorial disc around the B-type star, is detected whenever the source is bright in X-rays. When the disc is absent, the X-ray emission decreases significantly. The long-term variability of SAX J2103.5+4545 is characterised by fast episodes of disc loss and subsequent reformation. The time-scales for the loss and reformation of the disc (about two years) are the fastest among BeXs.
We presented fast timing photometric observations of the intermediate polar V2069 Cygni (RX J2123.7+4217). The optical light curve of V2069 Cygni revealed a double-peaked pulsation with a period of 743.38 ± 0.25 s. We suggest that this period represents the spin of the white dwarf accretor. Moreover, we presented results from a detailed analysis of the X-ray multi-mirror mission (XMM)-Newton observations, which also show a double-peaked modulation, however shifted in phase, with a period of 742.35 ± 0.23 s. The X-ray spectra obtained from the XMM-Newton European photon imaging camera (EPIC) instruments were modelled by a plasma emission and a soft blackbody component with a partial covering photoelectric absorption model with a covering fraction of 0.65. An additional Gaussian emission line at 6.385 keV with an equivalent width of 243 eV is required to account for fluorescent emission from neutral iron. The iron fluorescence (approximately 6.4 keV) and Fe XXVI lines (approximately 6.95 keV) are clearly resolved in the EPIC spectra. In the Porb-Pspin diagram of intermediate polars, V2069 Cyg shows a low spin-to-orbit ratio of approximately 0.0276 in comparison with approximately 0.1 for other intermediate polars.
The 1 700 year old PSR B0540-69 in the large Magellanic clouds (LMC) is considered to be the twin of the Crab pulsar because of its similar spin parameters, magnetic field and energetics. PSR B0540-69 (V 22.5) is also one of the very few pulsars for which both optical pulsations and polarised emission have been measured. We could clearly identify PSR B0540-69 in our J, H and KS-band images and measure its flux (J = 20.14 H = 19.33 KS = 18.55 with an overall error of ± 0.1 mag in each band). The joint fit to the available optical and near infrared (nIR) photometry with a power-law spectrum gives a spectral index of 0.70 ± 0.04 slightly more precise than measured in the optical only. This clearly implies that there is no spectral break between the optical and the nIR. We also detected, for the first time, the PSR B0540-69 pulsar wind nebula (PWN) in the nIR. The comparison between our Nasmyth adaptive optics system nIR imager and spectrograph (NACO) images and Hubble space telescope (HST) optical ones does not reveal any apparent difference in the PWN morphology as a function of wavelength. The PWN optical-to-nIR spectrum can also be fitted by a single power-law, with spectral index of 0.56 ± 0.03 slightly flatter than the pulsar's. Using NACO at the very large telescope (VLT), we obtained the first detection of PSR B0540-69 and its PWN in the nIR. Due to the small angular scale of the PWN (approximately four parcsec) only the spatial resolution of the James Webb space telescope (JWST) will make it possible to extend the study of the pulsar and PWN spectrum towards the mid-IR.
We investigated the eclipse timing of the polar binary HU Aquarii that has been observed for almost two decades. Recently, Qian et al. attributed large (O-C) deviations between the eclipse ephemeris and observations to a compact system of two massive Jovian companions. We improved the Keplerian, kinematic model of the light travel time effect and re-analysed the whole currently available data set. We added almost 60 new, yet unpublished, mostly precision light curves obtained using the time high-resolution photopolarimeter OPTIMA, as well as photometric observations performed at the monitoring network of telescopes/north, physics innovations robotic astronomical telescope explorer and Carlos Sánchez telescope. We determined new mid-egress times with a mean uncertainty at the level of 1 s or better. We claimed that because the observations that currently exist in the literature are non-homogeneous with respect to spectral windows (ultraviolet, X-ray, visual and polarimetric mode) and the reported mid-egress measurements errors, they may introduce systematics that affect orbital fits. Indeed, we found that the published data, when taken literally, cannot be explained by any unique solution. Many qualitatively different and best-fit two-planet configurations, including self-consistent, Newtonian N-body solutions may be able to explain the data. However, using high-resolution, precision OPTIMA light curves, we find that the observed minus calculated (O-C) deviations are best explained by the presence of a single circumbinary companion orbiting at a distance of approximately 4.5 au with a small eccentricity and having approximately seven Jupiter masses. This object could be the next circumbinary planet detected from the ground, similar to the announced companions around close binaries HW Vir, NN Ser, UZ For, DP Leo, FS Aur or SZ Her and planets of this type around Kepler-16, Kepler-34 and Kepler-35.
We focussed on short-period eclipsing binaries that belong to a class of cataclysmic variables (CVs). They are known as polars and intermediate polars, closely resembling their prototype AM Herculis. These binaries consist of a red dwarf and a strongly magnetic white dwarf, having orbital periods of only a few hours. Monitoring the eclipses of these typically faint sources demands high-time resolution photometry. We described the very recent results obtained for two CVs, HU Aqr and DQ Her, which were observed with OPTIMA. The new observations of HU Aqr confirm that the O-C diagrams exhibit variations known for this binary which can be explained by a single, massive Jupiter-like planet, possibly accompanied by a very distant companion.
The expected final results and their potential impact and use (including the socio-economic impact and the wider societal implications of the project so far):
There were three observing campaigns with the OPTIMA photo-polarimeter. There is a lot of data collected during these campaigns that are not published yet. For some sources, as for example for polars that might host exoplanets, monitoring of their eclipses for many years is needed to cover wide enough time span and to allow to fit Keplerian kinematic model. There are also some polarimetric observations of propeller AE Aqr.
Additionally, we also have the radio polarimetric data of the Crab pulsar at 610 MHz and 1.4 GHz with the highest possible time resolution available now. Data analysis is in progress and the results might reveal competently new picture of the coherent and non-coherent radiation of pulsars. Such comparison between radio and optical polarimetric features of pulsars is performed for the first time and can give unexpected results.