arXiv subjects

John R. Thorstensen

Publications and source records attributed to John R. Thorstensen.

At least 19 recordsLinked to original sources

arXiv2026

Optical Spectroscopy of TeV-emitting BL Lac Candidates

TeV-emitting BL Lacertae (BL Lac) blazars have important implications for the study of jet phenomenology, particle acceleration, and ultra high-energy cosmic ray production. They also allow for indirect studies of the extragalactic background light, cosmic magnetic fields, and axion-like particles. The upcoming Cherenkov Telescope Array (CTA), a ground-based gamma-ray observatory, will expand the observed TeV-emitting BL Lac population and enhance the aforementioned fields of study. However, reliable redshifts are crucial for planning and interpreting observations with CTA and only about 50% of gamma-ray BL Lacs have spectroscopic redshifts. We performed medium resolution optical spectroscopy of 16 TeV-emitting BL Lac candidates with the SALT and MDM telescopes. We measured spectroscopic redshifts for the full sample, ranging from 0.059 to 0.4, including 5 new spectroscopic redshift measurements.

astro-ph.HE↗

arXiv2026

SRGA J115215.0$-$510656: an unusual long-period eclipsing dwarf nova with disc wind signatures

We present the first detailed optical study of the cataclysmic variable SRGA J115215.0$-$510656, based on new time-resolved photometric and spectroscopic observations complemented by long-baseline Transiting Exoplanet Survey Satellite (TESS) data. The TESS light curve reveals deep, recurring eclipses consistent with a high-inclination geometry and an orbital period of 0.43567659(9)d. The eclipse morphology during outburst is consistent with a possible 'inside-out' type outburst and supports classification of the system as a U Gem-type dwarf nova. By combining eclipse phase width and ellipsoidal modulation, we constrain the system geometry to a narrow locus in the $(q,i)$ plane, with allowed mass ratios $0.28 \lesssim q \lesssim 0.84$ and inclinations $i$ $\simeq$75$-$84$^{\circ}$. The persistence of single-peaked Balmer lines during outburst, together with strong He II emission and a flattened Balmer decrement, points towards emission arising in a disc wind or vertically extended regions above the disc. Absorption features from a late-type secondary star (approximately K3) are detected, contributing roughly 30 per cent of the red optical flux. Comparison with main-sequence expectations suggests that the donor star is moderately inflated, consistent with a mildly evolved secondary. With its long orbital period, modest outburst amplitude, and emission-line characteristics, SRGA J115215.0$-$510656 appears to be a rare and compelling example of a bright, long-period dwarf nova whose optical properties are influenced by disc-wind processes during outburst.

astro-ph.SR↗

arXiv2025

Observations of Five Southern-Hemisphere Cataclysmic Binary Stars

We present observations and analyses of five little-studied cataclysmic binary stars in the southern celestial hemisphere. Our new observations are from the South African Astronomical Observatory. The objects and salient results are as follows: (i) 6dF0752-54 is a dwarf nova with an orbital period Porb = 5.05 hr that shows a contribution from a mid-M type secondary in its mean spectrum. (ii) J0916-26 had been suspected of being a magnetic CV with an eclipse period of 3.37 hr. Our spectrum corroborates this classification. (iii) GSC 08944 is a novalike variable with Porb = 3.80 hr. Archival photometry also shows a persistent photometric period near 4.03 hr, apparently from a positive super- hump. Its emission line behavior is consistent with an SW Sextantis-type novalike. (iv) MGAB-V253, also Gaia20eys, had been identified as a short-period eclipsing novalike with Porb = 1.44 hr. Our spectrum shows shows broad emission lines consistent with this, and the extensive TESS data show a persistent modulation near 1.35 hr, evidently a negative superhump. It is less luminous than most novalikes, but significantly brighter than quiescent dwarf novae with comparably short periods. (v) Finally, DDE 45 shows a complicated variability history, cycling rapidly between high and low states for a time and more recently showing outbursts resembling a U Gem-type dwarf nova. We find a 2.07 hr radial velocity period, which also appears in archival TESS photometry. The emission lines are double-peaked, with an orbital S-wave similar to low-inclination dwarf novae. To be published in Astronomical Journal.

astro-ph.SR↗

arXiv2025

Discovery of Faint Nebulosity Around a Z Camelopardalis-type Cataclysmic Variable in Antlia: Nova Shell or Ancient Planetary Nebula?

We report our discovery of a faint nebula surrounding a previously little-studied 15th-mag variable star, ASASSN-19ds, in the southern-hemisphere constellation Antlia. Spectra verify that the star is a cataclysmic variable (CV). Using new and archival photometry, we confirm that ASASSN-19ds is an eclipsing binary with an orbital period of 0.139 days (3.34 hr). Moreover, its out-of-eclipse brightness shows a "sawtooth" light curve with an amplitude of $\sim$1~mag and an interval between peaks that varies between about three to nearly five weeks. Its mean absolute magnitude in the Gaia system is $M_G=+6.5$. These combined properties lead to a classification of ASASSN-19ds as a Z Camelopardalis-type CV. We obtained deep narrow-band images of the nebulosity, using modest-sized telescopes and extremely long exposure times. Our imagery reveals a bipolar morphology, with thin arcs at the ends of the major axis, likely indicating an interaction with the interstellar medium. We consider several scenarios for the origin of the nebula, but from the existing information we are unable to distinguish between it being ejecta from an unobserved classical-nova outburst several centuries ago, or an ancient planetary nebula. Future observations should be able to decide between these possibilities. At the star's distance of only $\sim$557 pc, a nova eruption would have been a spectacular naked-eye event.

astro-ph.SR↗

arXiv2025

ASASSN-14dx: A cataclysmic variable harbouring a massive pulsating white dwarf

We present the results of our study of ASASSN-14dx, a previously known but poorly characterised cataclysmic variable (CV). The source was observed as part of an ongoing high-time-resolution photometric survey of CVs, which revealed that, in addition to the known 82.8min orbital period, it also exhibits other transient periods, the strongest of which around 4 and 14 min. Here, we report our findings resulting from a multifaceted follow-up programme consisting of optical spectroscopy, spectropolarimetry, imaging polarimetry, and multicolour fast photometry. We find that the source displays complex optical variability, which is best explained by the presence of a massive white dwarf exhibiting non-radial pulsations. An intermediate polar-like scenario involving a spinning magnetic white dwarf can be ruled out based on the detected changes in the observed periods. Based on our optical spectroscopy, we can constrain the mass and effective temperature of the white dwarf to be ~1.1 Msol and 16 100 K, respectively. The overall intrinsic flux level of the source is unusually high, suggesting that there remains significant residual emission from the accretion disc and/or the white dwarf even ten years after the 2014 outburst. Finally, we cannot detect any spectroscopic signatures from the donor star, making ASASSN-14dx a possible period bouncer system evolving towards a longer orbital period.

astro-ph.SR↗

arXiv2024

Cataclysmic variables from Sloan Digital Sky Survey -- V (2020-2023) identified using machine learning

SDSS-V is carrying out a dedicated survey for white dwarfs, single and in binaries, and we report the analysis of the spectroscopy of 504 cataclysmic variables (CVs) and CV candidates obtained during the first 34 months of observations of SDSS-V. We developed a convolutional neural network (CNN) to aid with the identification of CV candidates among the over 2 million SDSS-V spectra obtained with the BOSS spectrograph. The CNN reduced the number of spectra that required visual inspection to $\simeq2$ per cent of the total. We identified 776 CV spectra among the CNN-selected candidates, plus an additional 27 CV spectra that the CNN misclassified, but that were found serendipitously by human inspection of the data. Analysing the SDSS-V spectroscopy and ancillary data of the 504 CVs in our sample, we report 61 new CVs, spectroscopically confirm 248 and refute 13 published CV candidates, and we report 82 new or improved orbital periods. We discuss the completeness and possible selection biases of the machine learning methodology, as well as the effectiveness of targeting CV candidates within SDSS-V. Finally, we re-assess the space density of CVs, and find $1.2\times 10^{-5}\,\mathrm{pc^{-3}}$.

astro-ph.SR↗

arXiv2023

Optical Studies of Seven Bright Southern Cataclysmic Variable Stars

We report spectroscopic observations of seven bright southern cataclysmic variable stars, collected on a single two-week observing run using the 1.9-m Radcliffe telescope at the South African Astronomical Observatory. We used radial velocity time series, in some cases in combination with other data, to determine or clarify orbital periods for five of them, namely ATO J061.1478-31.0634, BMAM-V547, MGAB-V202, NSV 4202, and V1147 Cen. For BMAM-V547, we use data from the Transiting Exoplanet Survey Satellite (TESS) to corroborate and sharpen the orbital period; the TESS data also show a photometric period near 3.93 d, likely indicating precession of the accretion disk. Also, we find a periodic modulation in the radial velocities of the SU UMa-type dwarf nova Var Ret2005, but are unable to specify a unique cycle count. Finally, we show a spectrum of ASASSN-V J061528.41-412007.3 that appears typical of a luminous novalike variable.

astro-ph.SR↗

arXiv2023

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

CSS1603+19 is a cataclysmic variable (CV) with an orbital period of 81.96 min, near the minimal period of cataclysmic variables. It is unusual in having a strong mid-infrared excess inconsistent with thermal emission from a brown dwarf companion. Here we present time-resolved multi-wavelength 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 5MG. 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 onto 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 $M_1>0.24 M_{\odot}$ for the white dwarf accretor and $M_2=0.0644\pm0.0074 M_\odot$ 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.

astro-ph.SR↗

arXiv2021

YY Hya and its interstellar environment

During a search for previously unknown Galactic emission nebulae, we discovered a faint 36' diameter Halpha emission nebula centered around the periodic variable YY Hya. Although this star has been classified as RR-Lyr variable, such a classification is inconsistent Gaia distance of ~450 pc. GALEX image data also shows YY Hya to have a strong UV excess, suggesting the existence of a hot, compact binary companion. In addition to our discovery image data, we obtained image of the region with CHILESCOPE time-series spectroscopy at MDM observatory. Also, we used data from various space missions to derive an exact orbital period and a SED. We find that YY Hya is a compact binary system containing a K dwarf star which is strongly irradiated by a hot WD companion. The spectral characteristics of the emission lines, visible only during maximum light of the perfectly sinusoidal optical}light curve shows signatures much like members of the BE UMa variable family. These are post common envelope pre-cataclysmic variables. However the companion star here is more massive than found in other group members and the progenitor of the white dwarf must have been a 3 to 4 Mo star. The nebula seems to be an ejected common envelope shell with a mass in the order of one Mo and an age of 500000 years. This makes it to be the biggest hitherto known such shell. Alignment of neighboring nebulosities some 45' to the northeast and southwest of YY Hya suggests that the system had strong bipolar outflows. We briefly speculate it might be related to the 1065 BP "guest-star" reported in ancient Chinese records as well.

astro-ph.SR↗

arXiv2021

Spectroscopy of the Proposed White Dwarf Pulsar ASASSN-V J205543.90+240033.5

We obtained spectra of ASASSN-V J205543.90+240033.5 (J2055), a system that shows photometric variations similar to the white dwarf (WD) pulsar AR Scorpii (Kato et al. arXiv:2109.03979). Our spectra display a continuum rising steeply toward the blue as well as an array of emission lines. Resolved Balmer and Paschen lines are seen with H$α$ and H$β$ having central absorption features. The strongest lines are unresolved CII, CIII, and NIII as well as doubly ionized helium. The spectra are similar to that of YY Hya (Kimeswenger et al. arXiv:2110.03935), and suggest that J2055 is a post-common envelope binary consisting of a hot compact star irradiating the face of a secondary of unknown spectral type. Velocity variations detected from the emission lines confirm the binary nature of J2055. The origin of the 10 minute photometric variation remains uncertain.

astro-ph.SR↗

arXiv2021

Confirmation of a Second Propeller: A High-Inclination Twin of AE~Aquarii

For decades, AE Aquarii (AE Aqr) has been the only cataclysmic variable star known to contain a magnetic propeller: a persistent outflow whose expulsion from the binary is powered by the spin-down of the rapidly rotating, magnetized white dwarf. In 2020, LAMOST-J024048.51+195226.9 (J0240) was identified as a candidate eclipsing AE Aqr object, and we present three epochs of time-series spectroscopy that strongly support this hypothesis. We show that during the photometric flares noted by Thorstensen (2020) (arXiv:2007.09285), the half-width-at-zero-intensity of the Balmer and HeI lines routinely reaches a maximum of ~3000 km/s, well in excess of what is observed in normal cataclysmic variables. This is, however, consistent with the high-velocity emission seen in flares from AE Aqr. Additionally, we confirm beyond doubt that J0240 is a deeply eclipsing system. The flaring continuum, HeI and much of the Balmer emission likely originate close to the WD because they disappear during the eclipse that is centered on inferior conjunction of the secondary star. The fraction of the Balmer emission remaining visible during eclipse has a steep decrement and it is likely produced in the extended outflow. Most enticingly of all, this outflow produces a narrow P-Cyg absorption component for nearly half of the orbit, and we demonstrate that this scenario closely matches the outflow kinematics predicted by Wynn, King, & Horne (1997). While an important piece of evidence for the magnetic-propeller hypothesis -- a rapid WD spin period -- remains elusive, our spectra provide compelling support for the existence of a propeller-driven outflow viewed nearly edge-on, enabling a new means of rigorously testing theories of the propeller phenomenon.

astro-ph.SR↗

arXiv2021

Chandra, NuSTAR, and Optical Observations of the Cataclysmic Variables IGR J17528-2022 and IGR J20063+3641

We report on Chandra, NuSTAR, and MDM observations of two INTEGRAL sources, namely IGR J17528-2022 and IGR J20063+3641. IGR J17528-2022 is an unidentified INTEGRAL source, while IGR J20063+3641 was recently identified as a magnetic cataclysmic variable (mCV) by Halpern et al. (2018). The Chandra observation of IGR J17528-2022 has allowed us to locate the optical counterpart to the source and to obtain its optical spectrum, which shows a strong H$α$ emission line. The optical spectrum and flickering observed in the optical time-series photometry in combination with the X-ray spectrum, which is well fit by an absorbed partially covered thermal bremsstrahlung model, suggests that this source is a strong mCV candidate. The X-ray observations of IGR J20063+3641 reveal a clear modulation with a period of 172.46$\pm0.01$ s, which we attribute to the white dwarf spin period. Additional MDM spectroscopy of the source has also allowed for a clear determination of the orbital period at 0.731$\pm0.015$ d. The X-ray spectrum of this source is also well fit by an absorbed partially covered thermal bremsstrahlung model. The X-ray spectrum, spin periodicity, and orbital periodicity allow this source to be further classified as an intermediate polar.

astro-ph.HE↗

arXiv2021

ASASSN-18aan: An Eclipsing SU UMa-type Cataclysmic Variable with a 3.6-hour Orbital Period and a Late G-type Secondary Star

We report photometric and spectroscopic observations of the eclipsing SU UMa-type dwarf nova ASASSN-18aan. We observed the 2018 superoutburst with 2.3 mag brightening and found the orbital period ($P_{\rm orb}$) to be 0.149454(3) d, or 3.59 hr. This is longward of the period gap, establishing ASASSN-18aan as one of a small number of long-$P_{\rm orb}$ SU UMa-type dwarf novae. The estimated mass ratio, ($q=M_2/M_1 = 0.278(1)$), is almost identical to the upper limit of tidal instability by the 3:1 resonance. From eclipses, we found that the accretion disk at the onset of the superoutburst may reach the 3:1 resonance radius, suggesting that the superoutburst of ASASSN-18aan results from the tidal instability. Considering the case of long-$P_{\rm orb}$ WZ Sge-type dwarf novae, we suggest that the tidal dissipation at the tidal truncation radius is enough to induce SU UMa-like behavior in relatively high-$q$ systems such as SU UMa-type dwarf novae, but that this is no longer effective in low-$q$ systems such as WZ Sge-type dwarf novae. The unusual nature of the system extends to the secondary star, for which we find a spectral type of G9, much earlier than typical for the orbital period, and a secondary mass $M_2$ of around 0.18 M$_{\odot}$, smaller than expected for the orbital period and the secondary's spectral type. We also see indications of enhanced sodium abundance in the secondary's spectrum. Anomalously hot secondaries are seen in a modest number of other CVs and related objects. These systems evidently underwent significant nuclear evolution before the onset of mass transfer. In the case of ASASSN-18aan, this apparently resulted in a mass ratio lower than typically found at the system's $P_{\rm orb}$, which may account for the occurrence of a superoutburst at this relatively long period.

astro-ph.SR↗

arXiv2020

V1460 Her: A fast spinning white dwarf accreting from an evolved donor star

We present time-resolved optical and ultraviolet spectroscopy and photometry of V1460~Her, an eclipsing cataclysmic variable with a 4.99\,h orbital period and an overluminous K5-type donor star. The optical spectra show emission lines from an accretion disc along with absorption lines from the donor. We use these to measure radial velocities, which, together with constraints upon the orbital inclination from photometry, imply masses of $M_1=0.869\pm0.006\,\mathrm{M}_\odot$ and $M_2=0.295\pm0.004\,\mathrm{M}_\odot$ for the white dwarf and the donor. The radius of the donor, $R_2=0.43\pm0.002\,\mathrm{R}_\odot$, is $\approx 50$ per cent larger than expected given its mass, while its spectral type is much earlier than the M3.5 type that would be expected from a main sequence star with a similar mass. HST spectra show strong $\mathrm{N{\small V}}$ 1240 A emission but no $\mathrm{C{\small IV}}$ 1550 A emission, evidence for CNO-processed material. The donor is therefore a bloated, over-luminous remnant of a thermal-timescale stage of high mass transfer and has yet to re-establish thermal equilibrium. Remarkably, the HST ultraviolet data also show a strong 30 per cent peak-to-peak, $38.9\,$s pulsation that we explain as being due to the spin of the white dwarf, potentially putting V1460 Her in a similar category to the propeller system AE Aqr in terms of its spin frequency and evolutionary path. AE Aqr also features a post-thermal timescale mass donor, and V1460 Her may therefore be its weak magnetic field analogue since the accretion disc is still present, with the white dwarf spin-up a result of a recent high accretion rate.

astro-ph.SR↗

arXiv2020

Follow-up Studies of Five Cataclysmic Variable Candidates Discovered by LAMOST

We report follow-up observations of five cataclysmic variable candidates from LAMOST published by Hou et al. (2020). LAMOST J024048.51+195226.9 is the most unusual of the five; an early-M type secondary star contributes strongly to its spectrum, and its spectral and photometric behavior are strikingly reminiscent of the hitherto-unique propeller system AE Aqr. We confirm that a 7.34-hr period discovered in the Catalina survey data (Drake et al. 2014) is orbital. Another object, LAMOST J204305.95+341340.6 appears to be a near twin of the novalike variable V795 Her, with an orbital period in the so-called 2-3 hour "gap". LAMOST J035913.61+405035.0 is evidently an eclipsing, weakly-outbursting dwarf nova with a 5.48-hr period. Our spectrum of LAMOST J090150.09+375444.3 is dominated by a late-type secondary and shows weak, narrow Balmer emission moving in phase with the absorption lines, but at lower amplitude; we do not see the HeII 4686 emission evident in the published discovery spectrum. We again confirm that a period from the Catalina data, in this case 6.80 hr, is orbital. LAMOST J033940.98+414805.7 yields a radial-velocity period of 3.54 hr, and its spectrum appears to be typical of novalike variables in this period range. The spectroscopically-selected sample from LAMOST evidently includes some interesting cataclysmic variables that have been unrecognized until now, apparently because of the relatively modest range of their photometric variations.

astro-ph.SR↗

arXiv2020

Optical Studies of 8 AM Herculis-Type Cataclysmic Variable Stars

We report detailed follow-up observations of 8 cataclysmic variable stars (CVs) that are apparently AM Her stars, also called polars. For all, we either determine orbital periods for the first time, or improve on existing determinations. The seven for which we have spectra show the high-amplitude radial velocity curves and prominent HeII 4686 emission lines characteristic of strongly magnetic CVs, and their periods, which range from 81 to 219 minutes, are also typical for AM Her stars. Two objects from the Gaia-alerts index, Gaia18aot and Gaia18aya, are newly identified as CVs. Another, RX J0636.3+6554, eclipses deeply, while CSS080228:081210+040352 shows a sharp dip that is apparently a partial eclipse. The spectrum of Gaia18aya has a cyclotron harmonic near 5500 Angstroms that constrains the surface field to about 49 Megagauss or greater.

astro-ph.SR↗

arXiv2020

Spectroscopic Studies of 30 Short-period Cataclysmic Variable Stars, and Remarks on the Evolution and Population of Similar Objects

We present spectroscopy and orbital periods Porb for 30 apparently non-magnetic cataclysmic binaries with periods below about 3 hours, nearly all of which are dwarf novae, mostly of the SU Ursae Majoris subclass. We then turn to the evidence supporting the prediction that short-period dwarf novae evolve toward longer periods after passing through a minimum period -- the "period bounce" phenomenon. Plotting data from the literature reveals that for superhump period excess $ε= (P_{\rm sh} - P_{\rm orb} )/P_{\rm orb}$ below about 0.015, the period appears to increase with decreasing epsilon, agreeing at least qualitatively with the predicted behavior. Next, motivated by the long (decadal) outburst intervals of the WZ Sagittae subclass of short-period dwarf novae, we ask whether there could be a sizable population of "lurkers" -- systems that resemble dwarf novae at minimum light, but do not outburst over accessible timescales (or at all), and therefore do not draw attention to themselves. By examining the outburst history of the Sloan Digital Sky Survey sample of CVs, which were selected by color and not by outburst, we find that a large majority of the color-selected dwarf-nova-like objects have been observed to outburst, and conclude that "lurkers", if they exist, are a relatively minor part of the CV population.

astro-ph.SR↗

arXiv2020

Followup ground-based observations of the dwarf nova KZ Gem

We present spectroscopy of stars in the immediate vicinity of the dwarf nova (DN) KZ Gem to confirm its identification, which had been ambiguous in the literature. Analysis of 73 radial velocities spanning from 2014 to 2019 provides a high-precision orbital period of 0.2224628(2)\,d ($\sim5.34$\,hr) and shows KZ\,Gem to be a double-lined DN. Time series photometry taken from 2016 to 2018 shows a variable double-hump modulation with a full amplitude of $\sim0.3$\,mag, along with five Gaussian-like transient events lasting $\sim30$\,min or more. Using the light curve code XRBinary and nonlinear fitting code NMfit, we obtain an optimized binary model of the dwarf nova (DN) KZ Gem, from time series photometry, consisting of a Roche-lobe-filling K type dwarf with a mass transfer rate of $2.7\,-\,7.9\times10^{-10}\,{\rm M}_{\odot}\,{\rm yr}^{-1}$ to a large, cool and thick disk surrounding a white dwarf, in an orbit with an inclination of $51^{\circ}.6(\pm1^{\circ}.4)$. Two hotspots on the disk are demonstrated to cause the observed variations in the ellipsoidal modulations from the secondary star. This physical model is compatible with the Gaia distance of KZ\,Gem.

astro-ph.SR↗