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We study the effects of the time-variable properties of thermonuclear X-ray bursts on modeling their millisecond-period burst oscillations. We apply the pulse profile modeling technique that is being used in the analysis of rotation-powered millisecond pulsars by the Neutron Star Interior Composition Explorer (NICER) to infer masses, radii, and geometric parameters of neutron stars. By simulating and analyzing a large set of models, we show that overlooking burst time-scale variability in temperatures and sizes of the hot emitting regions can result in substantial bias in the inferred mass and radius. To adequately infer neutron star properties, it is essential to develop a model for the time variable properties or invest a substantial amount of computational time in segmenting the data into non-varying pieces. We discuss prospects for constraints from proposed future X-ray telescopes., Comment: Accepted for publication in MNRAS

Introduction Taurus Hill Observatory (THO) [1], observatory code A95, is an amateur observatory located in Varkaus, Finland. The observatory is maintained by the local astronomical association Warkauden Kassiopeia. THO research team has observed and measured various stellar objects and phenomena. Observatory has mainly focused on exoplanet light curve measurements, observing the gamma rays burst, supernova discoveries and monitoring [2]. We also do long term monitoring projects [3]. The results and publications that pro-am based observatories, like THO, have contributed, clearly demonstrates that pro-amateurs are a significant resource for the professional astronomers now and even more in the future. High Quality Measurements The quality of the telescopes and CCD-cameras has significantly developed in 20 years. Today it is possible for pro-am's to make high quality measurements [4] with the precision that is scientifically valid. In THO we can measure exoplanet transits < 10 millimagnitude precision when the limiting magnitude of the observed object is 15 magnitudes. At very good conditions it is possible to detect as low as 1 to 2 millimagnitude variations in the light curve. Season 2021 – 2022 Exoplanet Observations Review A total of about 30 exoplanet observations and transit measurements were made during the observation season 2021/2022. All the measurements have been uploaded to the TRESCA database [5]. In total, about 250 light curve observations have now been sent directly to TRESCA from the Taurus Hill Observatory. The season highlights that we consider to be most important could be the clear time deviations from the forecasts for a few TESS candidates, and in particular the Qatar-8b transit time deviations. The TOI1582.01b transit was not detected during the predicted period at all, so it differed quite a bit from the predicted one. These observations are presented in the following figures. Figure 1: TOI1168.01b. The transit occurred about 1.7 hours earlier than predicted. Image: TRESCA. Figure 2: TOI1455.01b. The transit occurred 1.6 hours earlier than predicted. Image: TRESCA. Figure 3: TOI1582.01b. Not any clear transit was detected. Image: TRESCA. Figure 4: TOI2152.01b. The transit occurred about 20 minutes earlier than predicted. Image: TRESCA. Figure 5: Qatar-8b. The transit happened about three hours later than predicted. Image: TRESCA. Adapting a New Camera for Measurements The main equipment throughout the winter were Celestron C-14 SC telescope with a Paramount MEII tripod and an SBIG ST-8XME CCD camera with Baader Bessell BVRI photometric filters. During the spring 2022, the ASI2600MM Pro CMOS camera was tested for the first time in Taurus Hill Observatory with a Chroma I filter connected to a Meade 16” ACX -telescope (with a Paramount ME tripod) for light curve measurements in the WASP-12b observations on March 31, 2022. At the same time, the object was also detected with an SBIG ST-8XME CCD camera connected to the Celestron C-14 SC -telescope. The results were very similar, so the CMOS camera is well suited for light curve measurements. An interesting feature of the transit of the WASP-12b was that immediately after the actual transit there is a very small dimming of 3 to 5 mmag, which lasts for about 30 minutes. MaxIm DL v6.08 software was used for imaging and image calibration, AIP4Win v2.4.10 software was used for photometric measurements. The weather was even throughout the dark winter season from August to the end of April. The clearest nights were in March-April. The winter was very rainy overall, there was an exceptional amount of snow. In addition to exoplanet observations, Taurus Hill Observatory focused on comet imaging, DS imaging and the detection of GRB 220101A after-gamma glow, for which circular GCN 31356 [6] was published. Acknowledgements The Taurus Hill Observatory will acknowledge all the cooperation partners, Finnish Meteorological Institute and all financial supporters of the observatory. References [1] Taurus Hill Observatory website, http://www.taurushill.net [2] A low-energy core-collapse supernova without a hydrogen envelope; S. Valenti, A. Pastorello, E. Cappellaro, S. Benetti, P. A. Mazzali, J. Manteca, S. Taubenberger, N. Elias-Rosa, R. Ferrando, A. Harutyunyan, V.-P. Hentunen, M. Nissinen, E. Pian, M. Turatto, L. Zampieri and S. J. Smartt; Nature 459, 674-677 (4 June 2009); Nature Publishing Group; 2009. [3] A massive binary black-hole system in OJ 287 and a test of general relativity; M. J. Valtonen, H. J. Lehto, K. Nilsson, J. Heidt, L. O. Takalo, A. Sillanpää, C. Villforth, M. Kidger, G. Poyner, T. Pursimo, S. Zola, J.-H. Wu, X. Zhou, K. Sadakane, M. Drozdz, D. Koziel, D. Marchev, W. Ogloza, C. Porowski, M. Siwak, G. Stachowski, M. Winiarski, V.-P. Hentunen, M. Nissinen, A. Liakos & S. Dogru; Nature - Volume 452 Number 7189 pp781-912; Nature Publishing Group; 2008 [4] Transit timing analysis of the exoplanet TrES-5 b. Possible existence of the exoplanet TrES-5 c; Eugene N Sokov, Iraida A Sokova, Vladimir V Dyachenko, Denis A Rastegaev, Artem Burdanov, Sergey A Rusov, Paul Benni, Stan Shadick, Veli-Pekka Hentunen, Mark Salisbury, Nicolas Esseiva, Joe Garlitz, Marc Bretton, Yenal Ogmen, Yuri Karavaev,Anthony Ayiomamitis, Oleg Mazurenko, David Alonso, Sergey F Velichko; Monthly Notices of the Royal Astronomical Society, Volume 480, Issue 1, October 2018, Pages 291–301, https://doi.org/10.1093/mnras/ sty1615 [5] TRESCA; var2.astro.cz/tresca/transits.php?pozor=Veli-Pekka Hentunen&object=&page=1&lang=cz [6] Hentunen V-P, Nissinen M, Heikkinen E; GCN 31356; https://gcn.gsfc.nasa.gov/gcn/gcn3/31356.gcn3

We obtained well-sampled optical photometry of GRB 110213A, including Swift/UVOT and XRT. Combining our data from those of other ground-based telescopes, we present 15 optical multicolor light curves showing similar shapes with two peaks. In contrast, in the X-ray band, only a single peak is observed between the two optical peaks. Temporal and spectral analysis of GRB 110213A shows that the X-rays differ from the optical for Phases I–III (before the second peak of the optical band at ∼5.6 ks). Moreover, they have the same spectral behavior at late times (Phases IV–VI). These data indicate that the optical and X-ray emission are dominated by different components. The synchrotron-supported pair cascade emission is included in the standard external forward-shock model, which is dominated by synchrotron radiation and synchrotron self-Compton (SSC). We find that the optical bands of GRB 110213A are dominated by the cascade emission from synchrotron radiation + SSC at the early stage, while the primary synchrotron + SSC radiation dominates the X-ray band. At late stages, both the X-ray and optical bands are dominated by emission from primary synchrotron + SSC radiation. The cascade component can reasonably explain the first optical peak. In contrast, the primary synchrotron + SSC emission mainly contributes to the second peak.

Taurus Hill Observatory (THO) [1], observatory code A95, is an amateur observatory located in Varkaus, Finland. The observatory is maintained by the local astronomical association Warkauden Kassiopeia. THO research team has observed and measured various stellar objects and phenomena. Observatory has mainly focused on exoplanet light curve measurements (over 170 measurements so far) [4], observing the gamma rays burst, supernova discoveries and monitoring [2]. We also do long term monitoring projects [3]. The results and publications that pro-am based observatories, like THO, have contributed, clearly demonstrates that pro-amateurs are a significant resource for the professional astronomers now and even more in the future. HAT-P-38 and HAT-P-38b (Horna and Hiisi) The object is located in RA 2h 21min 32s and DE + 32 ° 14 ’46”. From Finland, the object is high in the southern sky only in autumn. In addition, the transit time of the object is such that transit occur quite rarely at night. Considering the uncertain autumn weather in Finland, the probability of detecting a complete transit is quite uncertain in Finland. Taurus Hill Observatory detected the HAT-P-38b first time on 18 September 2020 and for the second time on 8 November 2020. Based on our observations, the timing of the transit deviated from the forecast by almost an hour. The transit took place clearly ahead of schedule. It is an indication that the rotation time of the exoplanet is possibly slightly shorter than recorded in the original catalog values. In this case, the transit catalog times are no longer valid. Observations made by other observers also confirm this. It is therefore worth monitoring the object to see if such an observed change is indeed regular. In the first observation the dimming was 13.6 mmag and in the second it was clearly less, only 6.8 mmag. The length of the transit also varied slightly, from 178 minutes on the first occasion to 185 minutes on the second occasion. Figure 1: HAT-P-38b transit observed at THO. Secondary eclipse caused by the HAT-P-32b exoplanet Last winter, for the first time in Taurus Hill Observatory, a rather challenging exoplanet was observed to transit behind its own parent star. Such an observation was made in Taurus Hill Observatory on February 17, 2021 from the exoplanet HAT-P-32b. Normal transit of a similar object had been observed in Taurus Hill Observatory a few times before. After an observation tip from the Pulkova Observatory, an attempt was made to observe this secondary eclipse in Taurus Hill Observatory. According to forecasts, the subject would have to dim about 3-4 mmag and the duration of the eclipse would be about 120 minutes. The fading according to the forecasts was barely visible in the measurements of the Taurus Hill Observatory Observatory. Although the detection of a “behind transit” of a star would require better accuracy, at least the measurement results obtained from the light curve, the timing, and the intensity of the dimming were fully consistent with the predictions. Thus, there is strong evidence that the first observation of secondary eclipse in Taurus Hill Observatory was real. Figure 2: Secondary eclipse caused by the HAT-P-32b observed at THO. TESS candidates have joined as new targets The Taurus Hill Observatory began observing TESS candidates, or TOI objects, in the autumn of 2020. In total, these objects have now been observed 21 times in Taurus Hill Observatory. There have been seven different TESS candidates on the list. These selected targets have been fairly easy to detect, with a change in brightness caused by transits in between 7 and 20 mmag. The findings have been uploaded to the TRESCA ETD database. Although transits have been clearly observable in all observations, the timing of transits or the magnitude of dimming in most of them have been somewhat different from the catalog values, according to measurements by the Taurus Hill Observatory. This is probably mainly due to the huge number of observations of new uncertain objects and the rather modest resolution equipment of the TESS satellite itself. It is very possible that not nearly all of the observed TESS candidates will be confirmed as new exoplanets. Figure 3: TOI1516.01b transit observed at THO. Acknowledgements The Taurus Hill Observatory will acknowledge the cooperation partners, Pulkova Observatory, Finnish Meteorological Institute and all financial supporters of the observatory. References [1] Taurus Hill Observatory website (http://www.taurushill.net) [2] A low-energy core-collapse supernova without a hydrogen envelope; S. Valenti, A. Pastorello, E. Cappellaro, S. Benetti, P. A. Mazzali, J. Manteca, S. Taubenberger, N. Elias-Rosa, R. Ferrando, A. Harutyunyan, V.-P. Hentunen, M. Nissinen, E. Pian, M. Turatto, L. Zampieri and S. J. Smartt; Nature 459, 674-677 (4 June 2009); Nature Publishing Group; 2009. [3] A massive binary black-hole system in OJ 287 and a test of general relativity; M. J. Valtonen, H. J. Lehto, K. Nilsson, J. Heidt, L. O. Takalo, A. Sillanpää, C. Villforth, M. Kidger, G. Poyner, T. Pursimo, S. Zola, J.-H. Wu, X. Zhou, K. Sadakane, M. Drozdz, D. Koziel, D. Marchev, W. Ogloza, C. Porowski, M. Siwak, G. Stachowski, M. Winiarski, V.-P. Hentunen, M. Nissinen, A. Liakos & S. Dogru; Nature - Volume 452 Number 7189 pp781-912; Nature Publishing Group; 2008 [4] Transit timing analysis of the exoplanet TrES-5 b. Possible existence of the exoplanet TrES-5 c; Eugene N Sokov, Iraida A Sokova, Vladimir V Dyachenko, Denis A Rastegaev, Artem Burdanov, Sergey A Rusov, Paul Benni, Stan Shadick, Veli-Pekka Hentunen, Mark Salisbury, Nicolas Esseiva, Joe Garlitz, Marc Bretton, Yenal Ogmen, Yuri Karavaev,Anthony Ayiomamitis, Oleg Mazurenko, David Alonso, Sergey F Velichko; Monthly Notices of the Royal Astronomical Society, Volume 480, Issue 1, October 2018, Pages 291–301, https://doi.org/10.1093/mnras/ sty1615

IXPE is a NASA/ASI Small Explorer Mission. It will probe the X-ray polarization properties of celestial sources. In particular, for Accretion-powered Millisecond Pulsars (AMPs), IXPE can provide us with unique information on their geometry. These information together with pulse shape modelling will strongly boost the achievable sensitivity on measuring the AMPs mass and the radius. As a case-study, we simulated an observation of SAXJ1808.4-3658 and studied the accuracy that can be achieved in the measured time-dependent Stokes profiles. From these data we estimated how well IXPE will be able to constraint NS geometrical parameters, such as the inclination and hot-spot co-latitude angles.

We have simulated the X-ray polarization data that can be obtained with the Imaging X-ray Polarimetry Explorer, when observing accretion-powered millisecond pulsars. We estimated the necessary exposure times for SAX J1808.4$-$3658 in order to obtain different accuracies in the measured time-dependent Stokes profiles integrated over all energy channels. We found that the measured relative errors depend strongly on the relative configuration of the observer and the emitting hotspot. The improvement in the minimum relative error in Stokes $Q$ and $U$ parameters as a function of observing time $t$ scales as $1/\sqrt{t}$, and spans the range from 30-90% with 200 ks exposure time to 20-60% with 500 ks exposure time (in case of data binned in 19 phase bins). The simulated data were also used to predict how accurate measurements of the geometrical parameters of the neutron star can be made when modelling only $Q$ and $U$ parameters, but not the flux. We found that the observer inclination and the hotspot co-latitude could be determined with better than 10 deg accuracy for most of the cases we considered. These measurements can be used to further constrain neutron star mass and radius when combined with modelling of the X-ray pulse profile., Comment: 12 pages, 11 figures, published in A&A

Taurus Hill Observatory (THO) [1], observatory code A95, is an amateur observatory located in Varkaus, Finland. The observatory is maintained by the local astronomical association Warkauden Kassiopeia. THO research team has observed and measured various stellar objects and phenomena. Observatory has mainly focused on exoplanet light curve measurements, observing the gamma rays burst, supernova discoveries and monitoring [2]. We also do long term monitoring projects [3] [4]. The results and publications that pro-am based observatories, like THO, have contributed, clearly demonstrates that pro-amateurs are a significant resource for the professional astronomers now and even more in the future. High Quality Measurements The quality of the telescopes and CCD-cameras has significantly developed in 20 years. Today it is possible for pro-am's to make high quality measurements with the precision that is scientifically valid. In THO we can measure exoplanet transits < 10 millimagnitude precision when the limiting magnitude of the observed object is 15 magnitudes. At very good conditions it is possible to detect as low as 1 to 2 millimagnitude variations in the light curve. Exoplanet Transit Observations in THO To this date the team has measured over 70 different exoplanet light curves, some of them several times. Most of the transit measurements have been stored in the EDT (Exoplanet Transit Database) maintained by Variable Star and Exoplanet of Czech Astronomical Society. Here are some recent examples of the exoplanet measurements from THO. In Figure 1 below is the exoplanet measurement of the exoplanet HAT-P-12b. The exoplanet is located 468 light-years away. It’s probably a hot gas giant about Jupiter in diameter, orbiting its parent star in 3.2 days. In this measurement, the brightness of the parent star dimmed by 31 mmag due to transit. The transit took about 147 minutes. Figure 1: HAT-P-12b light curve 22./23.4.2020; C-14, Paramount MEII, SBIG ST-8XME. Second example concerns two measurements from the same night. HAT-P-3b and HAT-P-36b exoplanet transits were observed on 30./31.3.20202 (Figure 2). Figure 2: HAT-P-3b and HAT-P-36b 30./31.3.20202; C-14, Paramount MEII, SBIG ST-8XME. Third example (Figure 3) concerns about KPS-1b exoplanet transit observation. Despite the rather hazy sky, however, the transit of the planet was visible in the light curve reasonably well. Figure 3: KPS-1b 22./23.3.2020; C-14, Paramount MEII, SBIG ST-8XME. Summary and Conclusions Taurus Hill Observatory and other similar pro-amateur based observatories have a good record in field of astronomy and especially in the light curve measurements and photometric monitoring. The research teams have the knowledge for making a good and high quality photometric light curve measurements. The publication records are one of the good examples from this knowledge. In the future the THO research team aims for more challenging astronomical research projects with professional astronomers and observatories. As a conclusion it can be stated that it is possible to do high quality astronomical research with pro-amateur astronomy equipment if you just have the enthusiasm and knowledge to use your equipment in the right way. Acknowledgements The Taurus Hill Observatory will acknowledge the cooperation partners, Pulkova Observatory, Finnish Meteorological Institute and all financial supporters of the observatory. References [1] Taurus Hill Observatory website, http://www.taurushill.net [2] A low-energy core-collapse supernova without a hydrogen envelope; S. Valenti, A. Pastorello, E. Cappellaro, S. Benetti, P. A. Mazzali, J. Manteca, S. Taubenberger, N. Elias-Rosa, R. Ferrando, A. Harutyunyan, V.-P. Hentunen, M. Nissinen, E. Pian, M. Turatto, L. Zampieri and S. J. Smartt; Nature 459, 674-677 (4 June 2009); Nature Publishing Group; 2009. [3] A massive binary black-hole system in OJ 287 and a test of general relativity; M. J. Valtonen, H. J. Lehto, K. Nilsson, J. Heidt, L. O. Takalo, A. Sillanpää, C. Villforth, M. Kidger, G. Poyner, T. Pursimo, S. Zola, J.-H. Wu, X. Zhou, K. Sadakane, M. Drozdz, D. Koziel, D. Marchev, W. Ogloza, C. Porowski, M. Siwak, G. Stachowski, M. Winiarski, V.-P. Hentunen, M. Nissinen, A. Liakos & S. Dogru; Nature - Volume 452 Number 7189 pp781-912; Nature Publishing Group; 2008 [4] Transit timing analysis of the exoplanet TrES-5 b. Possible existence of the exoplanet TrES-5 c; Eugene N Sokov, Iraida A Sokova, Vladimir V Dyachenko, Denis A Rastegaev, Artem Burdanov, Sergey A Rusov, Paul Benni, Stan Shadick, Veli-Pekka Hentunen, Mark Salisbury, Nicolas Esseiva, Joe Garlitz, Marc Bretton, Yenal Ogmen, Yuri Karavaev,Anthony Ayiomamitis, Oleg Mazurenko, David Alonso, Sergey F Velichko; Monthly Notices of the Royal Astronomical Society, Volume 480, Issue 1, October 2018, Pages 291–301, https://doi.org/10.1093/mnras/ sty1615

We have developed a complete theory for the calculation of the observed Stokes parameters for radiation emitted from the surface of a rapidly rotating neutron star (NS) using the oblate Schwarzschild approximation. We accounted for the rotation of the polarization plane due to relativistic effects along the path from the stellar surface to the observer. The results were shown to agree with those obtained by performing full numerical general relativistic ray-tracing with the \textsc{arcmancer} code. We showed that the obtained polarization angle (PA) profiles may differ substantially from those derived for a spherical star. We demonstrated that assuming incorrect shape for the star can lead to biased constraints for NS parameters when fitting the polarization data. Using a simplified model, we also made a rough estimate of how accurately the geometrical parameters of an accreting NS can be determined using the X-ray polarization measurements of upcoming polarimeters like the Imaging X-ray Polarimeter Explorer (IXPE) or the enhanced X-ray Timing and Polarimetry (eXTP) mission., 11 pages, 10 figures, accepted in A&A

In this White Paper we present the potential of the Enhanced X-ray Timing and Polarimetry (eXTP) mission for determining the nature of dense matter; neutron star cores host an extreme density regime which cannot be replicated in a terrestrial laboratory. The tightest statistical constraints on the dense matter equation of state will come from pulse profile modelling of accretion-powered pulsars, burst oscillation sources, and rotation-powered pulsars. Additional constraints will derive from spin measurements, burst spectra, and properties of the accretion flows in the vicinity of the neutron star. Under development by an international Consortium led by the Institute of High Energy Physics of the Chinese Academy of Science, the eXTP mission is expected to be launched in the mid 2020s., Comment: Accepted for publication on Sci. China Phys. Mech. Astron. (2019)

The aim of this work is to study the possible effects and biases on the radius constraints for rotation-powered millisecond pulsars when using Thomson approximation to describe electron scattering in the atmosphere models, instead of using exact formulation for Compton scattering. We compare the differences between the two models in the energy spectrum and angular distribution of the emitted radiation. We also analyse a self-generated synthetic phase-resolved energy spectrum, based on Compton atmosphere and the most X-ray luminous rotation-powered millisecond pulsars observed by the Neutron star Interior Composition ExploreR (NICER). We derive constraints for the neutron star parameters using both the Compton and Thomson models. The results show that the method works by reproducing the correct parameters with the Compton model. However, biases are found in size and the temperature of the emitting hot spot, when using the Thomson model. The constraints on the radius are still not significantly changed, and therefore the Thomson model seems to be adequate if we are interested only in the radius measurements using NICER., Comment: 6 pages, 9 figures, published in A&A

We computed accurate atmosphere models of rotation-powered millisecond pulsars in which the polar caps of a neutron star (NS) are externally heated by magnetospheric return currents. The external ram pressure, energy losses, and stopping depth of the penetrating charged particles were computed self-consistently with the atmosphere model, instead of assuming a simplified deep-heated atmosphere in radiative equilibrium. We used exact Compton scattering formalism to model the properties of the emergent X-ray radiation. The deep-heating approximation was found to be valid only if most of the heat originates from ultra-relativistic bombarding particles with Lorentz factors of $\gamma \gtrsim 100$. In the opposite regime, the atmosphere attains a distinct two-layer structure with an overheated optically thin skin on top of an optically thick cool plasma. The overheated skin strongly modifies the emergent radiation: it produces a Compton-upscattered high-energy tail in the spectrum and alters the radiation beaming pattern from limb darkening to limb brightening for emitted hard X-rays. This kind of drastic change in the emission properties can have a significant impact on the inferred NS pulse profile parameters as performed, for example, by Neutron star Interior Composition ExploreR. Finally, the connection between the energy distribution of the return current particles and the atmosphere emission properties offers a new tool to probe the exact physics of pulsar magnetospheres., Comment: 13 pages, 10 figures, published in A&A

Thermonuclear X-ray bursts on the surface of neutron stars (NSs) can enrich photosphere with metals, which may imprint the photoionization edges on the burst spectra. We report here the discovery of absorption edges in the spectra of the type I X-ray burst from the NS low-mass X-ray binary GRS 1747$-$312 in Terzan 6 during observations by the Rossi X-ray Timing Explorer. We find that the edge energy evolves from $9.45\pm0.51$ keV to $\sim6$ keV and then back to $9.44\pm0.40$ keV during the photospheric radius expansion phase and remains at $8.06\pm0.66$ keV in the cooling tail. The photoionization absorption edges of hydrogen-like Ni, Fe or Fe/Ni mixture and the bound-bound transitions of metals may be responsible for the observed spectral features. The ratio of the measured absorption edge energy in the cooling tail to the laboratory value of hydrogen-like Ni(Fe) edge energy allows us to estimate the gravitational redshift factor $1+z=1.34\pm0.11$($1+z=1.15\pm0.09$). The evolution of the spectral parameters during the cooling tail are well-described by the metal-rich atmosphere models. The combined constraints on the NS mass and radius from the direct cooling method and the tidal deformability strongly suggest very high atmospheric abundance of the iron group elements and limit the distance to the source to $11\pm1$ kpc., matches the published version