AM Herculis Research Articles

TESS Photometry of AM Her and AR UMa: Binary Parameters, Cyclotron Emission Modeling, and Mass Transfer Duty Cycles

Transiting Exoplanet Survey Satellite (TESS) photometry of the polars AM Herculis (AM Her) and AR Ursae Majoris (AR UMa) is presented, along with high-speed photometry. AM Her shows a variety of high states with frequent transitions between them. TESS photometry of AR UMa in the low state reveals no evidence of accretion, while the McDonald 2.1 m telescope caught AR UMa in its high accretion state. Roche-lobe overflow is shut off during low states of AR UMa, while accretion often still takes place during low states of AM Her. We derive inclinations of 50° and 70° for AM Her and AR UMa respectively. To model the high-state light curves of AM Her, we employ a self-organized map light-curve classification scheme to establish common accretion configurations. The cyclotron radiation properties then allow the production of emission region maps on the surface of the white dwarf. The accretion geometry of AM Her is most consistent with a multipolar field structure. The high-state photometry of AR UMa has stochastic accretion flaring, which we attribute to magnetically buffeted mass transfer through the inner Lagrangian point L1. To consider this possibility, we examine the magnetism of both stars and argue that the local magnetic field near L1 can initiate short-lived accretion events and affect transitions between high and low accretion states in both AM Her and AR UMa. In particular, AR UMa has the low state as its default, while AM Her and most other active polars are in the high state by default.

CSS1603+19: a low-mass polar near the cataclysmic variable period minimum

ABSTRACT CSS1603+19 is a cataclysmic variable (CV) with an orbital period of 81.96 min, near the minimal period of CVs. It is unusual in having a strong mid-infrared excess inconsistent with thermal emission from a brown dwarf companion. Here, we present time-resolved multiwavelength observations of this system. WISE photometry indicates that the mid-infrared excess displays a one-magnitude eclipsing-like variability during the orbit. We obtained near-infrared and optical spectroscopy using Gemini, MDM, and APO telescopes. Near-infrared spectra show possible cyclotron features indicating that the white dwarf has a magnetic field of about 5 MG. Optical and near-infrared spectra display double-peaked emission lines, with both components showing strong radial velocity variations during the orbital period and with the broad component leading the narrow component stably by about 0.2 of the orbital phase. We construct a physical model informed by existing observations of the system and determine that one component likely originates from the accretion column on to the magnetized white dwarf in synchronous rotation with the orbital motion and the other from the Roche overflow point. This allows us to constrain the masses of the binary components to be M1 > 0.24 M⊙ for the white dwarf accretor and M2 = 0.0644 ± 0.0074 M⊙ for the donor. We classify the system as an AM Herculis star, or a polar. It has likely completed its stint on the period gap, but has not yet gone through the period bounce.

The various accretion modes of AM Herculis: Clues from multi-wavelength observations in high accretion states

We report onXMM-NewtonandNuSTARX-ray observations of the prototypical polar, AM Herculis, supported by ground-based photometry and spectroscopy, all obtained in high accretion states. In 2005, AM Herculis was in its regular mode of accretion, showing a self-eclipse of the main accreting pole. X-ray emission during the self-eclipse was assigned to a second pole through its soft X-ray emission and not to scattering. In 2015, AM Herculis was in its reversed mode with strong soft blobby accretion at the far accretion region. The blobby acretion region was more luminous than the other, persistently accreting, therefore called main region. Hard X-rays from the main region did not show a self-eclipse indicating a pronounced migration of the accretion footpoint. Extended phases of soft X-ray extinction through absorption in interbinary matter were observed for the first time in AM Herculis. The spectral parameters of a large number of individual soft flares could be derived. SimultaneousNuSTARobservations in the reversed mode of accretion revealed clear evidence for Compton reflection of radiation from the main pole at the white dwarf surface. This picture is supported by the trace of the Fe resonance line at 6.4 keV through the whole orbit. Highly ionized oxygen lines observed with the Reflection Grating Spectrometer (RGS) were tentatively located at the bottom of the accretion column, although the implied densities are quite different from expectations. In the regular mode of accretion, the phase-dependent modulations in the ultraviolet (UV) are explained with projection effects of an accretion-heated spot at the prime pole. In the reversed mode projection effects cannot be recognized. The light curves reveal an extra source of UV radiation and extended UV absorbing dips. An HαDoppler map obtained contemporaneously with theNuSTARandXMM-Newtonobservations in 2015 lacks the typical narrow emission line from the donor star but reveals emission from an accretion curtain in all velocity quadrants, indicating widely dispersed matter in the magnetosphere.

PTF1J2224+17: A short‐period, high‐field polar

AbstractWe present time‐resolved photometry of the cataclysmic variable PTF1J2224+17 obtained during four nights in October 2018 and January 2019 from Inastars observatory. The object is variable on a period of 103.82 min. Archival Catalina Real‐Time Transient Survey (CRTS), Palomar Transient Factory, and Zwicky Transient Facility‐data show frequent changes between high and low states. Based on its photometric properties and the cyclotron humps in the identification spectrum the object is certainly classified as an AM Herculis star (or polar) with a likely magnetic field strength of B ∼ 65 MG. Its accretion duty cycle was estimated from 9 years of photometric monitoring to be about 35%.

CRTS J035010.7 + 323230, a new eclipsing polar in the cataclysmic variable period gap

ABSTRACT We report the discovery of a new eclipsing polar, CRTS J035010.7+323230 (hereafter CRTS J0350+3232). We identified this cataclysmic variable (CV) candidate as a possible polar from its multiyear Catalina Real-Time Transient Survey (CRTS) optical light curve. Photometric monitoring of 22 eclipses in 2015 and 2017 was performed with the 2.1-m Otto Struve Telescope at McDonald Observatory. We derive an unambiguous high-precision ephemeris. Strong evidence that CRTS J0350 + 3232 is a polar comes from optical spectroscopy obtained over a complete orbital cycle using the Apache Point Observatory 3.5-m telescope. High velocity Balmer and He ii λ4686Å emission-line equivalent width ratios, structures, and variations are typical of polars and are modulated at the same period, 2.37 h (142.3 min), as the eclipse to within uncertainties. The spectral energy distribution and luminosity is found to be comparable to that of AM Herculis. Pre-eclipse dips in the light curve show evidence for stream accretion. We derive the following tentative binary and stellar parameters assuming a helium composition white dwarf and a companion mass of 0.2 M⊙: inclination i = 74.68° ± 0.03°, semimajor axis a = 0.942 ± 0.024 R⊙, and masses and radii of the white dwarf and companion, respectively: M1 = 0.948 $^{+0.006}_{-0.012}$ M⊙, R1 = 0.00830 $^{+0.00012}_{-0.00006}$ R⊙, and R2 = 0.249 ± 0.002 R⊙. As a relatively bright (V ∼ 17–19 mag), eclipsing, period-gap polar, CRTS J0350 + 3232 will remain an important laboratory for the study of accretion and angular momentum evolution in polars.

Comparison of Dimensionless Parameters in Astrophysical MHD Systems and in Laboratory Experiments

Estimates of typical parameters of accretion flows in the representative intermediate polar EX Hydrae, the polar AM Herculis, and the “hot Jupiter” WASP-12b are presented. Dimensionless parameters of astrophysical systems are compared with those of laboratory experiments on laser ablation in magnetic fields. It is shown that laboratory simulations of astrophysical flows is possible in principle, provided that some adjustment to the magnetic field, plasma density, and plasma velocity are made.

Another look at AM Herculis – radio-astrometric campaign with the e-EVN at 6 cm

We conducted radio-interferometric observations of the well known binary cataclysmic system AM Herculis. This particular system is formed by the magnetic white dwarf (primary) and the red dwarf (secondary), and is the prototype of so-called polars. Our observations were conducted with the European VLBI Network (EVN) in the e-EVN mode at 5 GHz. We obtained six astrometric measurements spanning one year, which make it possible to update the annual parallax for this system with the best precision to date ($\pi=11.29\pm0.08$ mas), equivalent to the distance of $88.6\pm0.6$ pc. The system was observed mostly in the quiescent phase (visual magnitude $m_v\sim15.3$), when the radio emission was at the level of about $300~\mu$Jy. Our analysis suggests that the radio flux of AM Herculis is modulated with the orbital motion. Such specific properties of the radiation can be explained using the emission mechanism similar to the scenario proposed for V471 Tauri and, in general, for RS CVn type stars. In this scenario the radio emission arises near the surface of the red dwarf, where the global magnetic field strength may reach a few kG. We argue that the quiescent radio emission distinguishes AM Herculis together with AR Ursae Majoris (the second known persistent radio polar) from other polars, as the systems with a magnetized secondary star.

Resynchronization of the Asynchronous Polar CD Ind

CD Ind is one of only four confirmed asynchronous polars (APs). APs are strongly magnetic cataclysmic variables of the AM Herculis subclass with the characteristic that their white dwarfs rotate a few percent out of synchronism with their binary orbit. Theory suggests that nova eruptions disrupt previously synchronized states. Following the eruption, the system is expected to rapidly resynchronize over a timescale of centuries. The other three asynchronous polars—V1432 Aql, BY Cam, and V1500 Cyg—have resynchronization time estimates ranging from 100 to more than 3500 years, with all but one being less than 1200 years. We report on the analysis of over 46,000 observations of CD Ind taken between 2007 and 2016, combined with previous observations from 1996, and estimate a CD Ind resynchronization time of 6400 ± 800 years. We also estimate an orbital period of 110.820(1) minutes and a current (2016.4) white dwarf spin period of 109.6564(1) minutes.

Properties of the long-term optical activity of the prototype polar AM Herculis

Properties of the long-term optical activity of the prototype polar AM Herculis

Laboratory analogue of a supersonic accretion column in a binary star system

Astrophysical flows exhibit rich behaviour resulting from the interplay of different forms of energy—gravitational, thermal, magnetic and radiative. For magnetic cataclysmic variable stars, material from a late, main sequence star is pulled onto a highly magnetized (B>10 MG) white dwarf. The magnetic field is sufficiently large to direct the flow as an accretion column onto the poles of the white dwarf, a star subclass known as AM Herculis. A stationary radiative shock is expected to form 100–1,000 km above the surface of the white dwarf, far too small to be resolved with current telescopes. Here we report the results of a laboratory experiment showing the evolution of a reverse shock when both ionization and radiative losses are important. We find that the stand-off position of the shock agrees with radiation hydrodynamic simulations and is consistent, when scaled to AM Herculis star systems, with theoretical predictions.

OPTICAL STUDIES OF 13 HARD X-RAY SELECTED CATACLYSMIC BINARIES FROM THESWIFT-BAT SURVEY

From a set of thirteen cataclysmic binaries that were discovered in the Swift Burst Alert Telescope (BAT) survey, we conducted time-resolved optical spectroscopy and/or time-series photometry of eleven, with the goal of measuring their orbital periods and searching for spin periods. Seven of the objects in this study are new optical identifications. Orbital periods are found for seven targets, ranging from 81 minutes to 20.4 hours. PBC J0706.7+0327 is an AM Herculis star (polar) based on its emission-line variations and large amplitude photometric modulation on the same period. Swift J2341.0+7645 may be a polar, although the evidence here is less secure. Coherent pulsations are detected from two objects, Swift J0503.7-2819 (975 s) and Swift J0614.0+1709 (1412 s and 1530 s, spin and beat periods, respectively), indicating that they are probable intermediate polars (DQ Herculis stars). For two other stars, longer spin periods are tentatively suggested. We also present the discovery of a 2.00 hour X-ray modulation from RX J2015.6+3711, possibly a contributor to Swift J2015.9+3715, and likely a polar.

FOLLOW UP OBSERVATIONS OF SDSS AND CRTS CANDIDATE CATACLYSMIC VARIABLES

We present photometry of 11 and spectroscopy of 35 potential cataclysmic variables from the Sloan Digital Sky Survey, the Catalina Real-Time Transient Survey and vsnet-alerts. The photometry results include quasi-periodic oscillations during the decline of V1363 Cyg, nightly accretion changes in the likely Polar (AM Herculis binary) SDSS J1344+20, eclipses in SDSS J2141+05 with an orbital period of 76+/-2 min, and possible eclipses in SDSS J2158+09 at an orbital period near 100 min. Time-resolved spectra reveal short orbital periods near 80 min for SDSS J0206+20, 85 min for SDSS J1502+33, and near 100 min for CSS J0015+26, RXS J0150+37, SDSS J1132+62, SDSS J2154+15 and SDSS J2158+09. The prominent HeII line and velocity amplitude of SDSS J2154+15 are consistent with a Polar nature for this object, while the lack of this line and a low velocity amplitude argue against this classification for RXS J0150+37. Single spectra of 10 objects were obtained near outburst and the rest near quiescence, confirming the dwarf novae nature of these objects.

OPTICAL AND X-RAY STUDIES OF 10 X-RAY-SELECTED CATACLYSMIC BINARIES

We report on ground-based optical observations of ten cataclysmic binaries that were discovered through their X-ray emission. Time-resolved radial velocity spectroscopy yields unambiguous orbital periods for eight objects and ambiguous results for the remaining two. The orbital periods range from 87 min to 9.38 hr. We also obtained time-series optical photometry for six targets, four of which have coherent pulsations. These periods are 1218 s for 1RXS J045707.4+452751, 628 s for AX J1740.2-2903, 477 s for AX J1853.3-0128, and 935 s for IGR J19267+1325. A total of seven of the sources have coherent oscillations in X-rays or optical, indicating that they are intermediate polars (DQ Herculis stars). Time-resolved spectroscopy of one object, Swift J2218.4+1925, shows that it is an AM Herculis star, or polar, and IGR J19552+0044 may also be in that class. For another object, Swift J0476.2-1611, we find an orbital period of 9.384 hr and detect the spectrum of the secondary star. The secondary's spectral contribution implies a distance of 900 (+190, -150) pc, where the error bars are estimated using a Monte Carlo technique to account for correlated uncertainties.

UPDATED PHOTOMETRY AND ORBITAL PERIOD ANALYSIS FOR THE POLAR AM HERCULIS ON THE UPPER EDGE OF THE PERIOD GAP

Twenty-one new optical light curves, including five curves obtained in 2009 and sixteen curves detected from the AAVSO International Database spanning from 1977 to 2011, demonstrate 16 new primary minimum light times in the high state. Furthermore, seven newly found low-state transient events from 2006 to 2009 were discovered, consisting of five Gaussian-shaped events and two events with an exponential form with decay timescales of $<$0.005 days; these timescales are one order of magnitude shorter than those of previous X-ray flare events. In the state transition, two special events were detected: a "disrupted event" with an amplitude of $\sim$ 2 mag and a duration of $\sim$ 72 minutes and continuing R-band twin events larger than all known R-band flares detected in M-type red dwarfs. All 45 available high-state data points spanning over 35 yr were used to construct an updated O-C diagram of \objectname{AM Herculis}, which clearly shows a significant sine-like variation with a period of 12-15 yr and an amplitude of 6-9 minutes. Using the inspected physical parameters of the donor star, the secular variation in the O-C diagram cannot be interpreted by any decided angular momentum loss mechanism, but can satisfy the condition $\tau_{\dot{\rm M}_{2}}\simeq\tau_{\rm KH}>>\tau_{\dot{\rm R}_{\rm 2}}$, which is required by numerical calculations of the secular evolution of cataclysmic variables. In order to explain the prominent periodic modulation, three plausible mechanisms - spot motion, the light travel-time effect, and magnetic active cycles - are discussed in detail.

DETECTION OF X-RAY PERIODICITY FROM A NEW ECLIPSING POLAR CANDIDATE XGPS-I J183251-100106

We report the results from a detailed analysis of an archival XMM-Newton observation of the X-ray source XGPS-I J183251-100106, which has been suggested as a promising magnetic cataclysmic variable candidate based on its optical properties. A single periodic signal of 1.5 hrs is detected from all EPIC cameras on board XMM-Newton. The phase-averaged X-ray spectrum can be well-modeled with a thermal bremsstrahlung of a temperature kT~50 keV. Both X-ray spectral and temporal behavior of this system suggest it as a eclipsing cataclysmic variable of AM Herculis (or polar) type.

CSS091109:035759+102943: A candidate polar

AbstractWe report optical time‐resolved photometry of the CRTS transient CSS091109:035759+102943. Pronounced orbital variability with a 114 min period, large X‐ray variability and the IR to X‐ray spectral energy distribution suggest a classification as a magnetic cataclysmic binary, a likely AM Herculis star or polar (© 2012 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

A new soft X-ray spectral model for polars with an application to AM Herculis

We present a simple heuristic model for the time-averaged soft X-ray temperature distribution in the accretion spot on the white dwarf in polars. The model is based on the analysis of the Chandra LETG spectrum of the prototype polar AM Her and involves an exponential distribution of the emitting area vs. blackbody temperature a(T) = a0 exp(-T/T0). With one free parameter besides the normalization, it is mathematically as simple as the single blackbody, but is physically more plausible and fits the soft X-ray and far-ultraviolet spectral fluxes much better. The model yields more reliable values of the wavelength-integrated flux of the soft X-ray component and the implied accretion rate than reported previously.

A detection of an anti-correlated hard X-ray lag in AM Herculis

Context {Earlier cross-correlation studies for AM Her were performed in various energy range from optical to X-ray and suggested that it mostly shows a high level of correlation but on occasion it shows a low level of correlation or uncorrelation.} Aims {To investigate the degree of correlation between soft (2-4 keV) and hard (9-20 keV) X-rays, we perform the cross-correlation study of the X-ray data sets of AM Her obtained with {\it RXTE}.} Methods {We cross-correlate the background-subtracted soft and hard X-ray light curves using the XRONOS program crosscor and fit a model to the obtained cross-correlation functions.} Results {We detect a hard X-ray lag of $192\pm33$ s in a specific section of energy-dependent light curve, where the soft X-ray (2-4 keV) intensity decreases but the hard X-ray (9-20 keV) intensity increases. From a spectral analysis, we find that the X-ray emission temperature increases during the anti-correlated intensity variation. In two other observations, the cross-correlation functions show a low level of correlation, which is consistent with the earlier results performed in a different energy range.} Conclusions {We report a detection of an anti-correlated hard X-ray lag of $\sim$190 s from the proto-type polar AM Her. The hard X-ray lag is detected for the first time in the given energy range, and it is the longest lag among those reported in magnetic cataclysmic variables. We discuss the implications of our findings regarding the origin of the hard X-ray lag and the anti-correlated intensity variation.}

Magnetospherically induced torques in asynchronous AM Herculis binaries

The AM Herculis binaries are an important class of cataclysmic variables in which the accreting white dwarf primary is observed to rotate synchronously with the orbit. The torque needed to spin the primary towards synchronism is believed to be generated by the interaction of the primary's magnetic field with the tidally synchronized secondary star. It is shown here that such a torque is generated by the interaction of the magnetosphere of an asynchronous primary with the turbulent secondary. The effects of magnetic diffusion, shearing and field advection are accounted for and the resulting magnetic torques are calculated for a range of degrees of asynchronism. Non-central magnetic forces enable the primary to exchange angular momentum with the orbit, while a torque acts on the star which spins it towards synchronism. This torque has the form of a resonance curve, with a maximum value at moderate asynchronism.

Discovery of an Unusual High State in the Long-Term Light Curve of AM Herculis

ABSTRACTThe magnetic cataclysmic variable archetype AM Herculis is known to display high and low photometric states, in which it switches from Roche lobe-overflow accretion (high) to wind accretion (low). The origin of this behavior is still unknown (although it is believed to be linked to the presence of starspots located near the L1 point on the secondary star), as are the specifics of duration and frequency of the two states. During the recent (2007) transition from an exceptionally long period of persistent low state behavior back to the high state, AM Her entered a short (∼2 month) intermediate state, in which its brightness remained at ∼0.5 mag fainter than the normal high-state level. Within the recorded history of AM Her, this was the first time the system displayed such behavior. Using data from the American Association of Variable Star Observers, we compare the characteristics of this faint high state to the subsequent normal high state, and a prior “failed high state,” in which the system briefly reached a brightness level near that of the normal high state before rapidly returning to the low state. The distribution of magnitude values attained during each state, along with the shapes of the orbital light curve in each state, support a scenario in which the specific configuration of the accretion flow between the secondary star and the white dwarf must be rapidly established at the end of a low state, even before the mass transfer rate has reached the normal high-state level.