arXiv subjects

J. Thorstensen

Publications and source records attributed to J. Thorstensen.

11 recordsLinked to original sources

arXiv2017

IGR J19552+0044: A new asynchronous short period polar: "Filling the gap between intermediate and ordinary polars"

Based on XMM--Newton X-ray observations IGR J19552+0044 appears to be either a pre-polar or an asynchronous polar. We conducted follow-up optical observations to identify the sources and periods of variability precisely and to classify this X-ray source correctly. Extensive multicolor photometric and medium- to high-resolution spectroscopy observations were performed and period search codes were applied to sort out the complex variability of the object. We found firm evidence of discording spectroscopic (81.29+/-0.01m) and photometric (83.599+/-0.002m) periods that we ascribe to the white dwarf (WD)\ spin period and binary orbital period, respectively. This confirms that IGR J19552+0044 is an asynchronous polar. Wavelength-dependent variability and its continuously changing shape point at a cyclotron emission from a magnetic WD with a relatively low magnetic field below 20 MG. The difference between the WD spin period and the binary orbital period proves that IGR J19552+0044 is a polar with the largest known degree of asynchronism (0.97 or 3%).

astro-ph.SR↗

arXiv2016

The Young and Bright Type Ia Supernova ASASSN-14lp: Discovery, Early-Time Observations, First-Light Time, Distance to NGC 4666, and Progenitor Constraints

On 2014 Dec. 9.61, the All-Sky Automated Survey for SuperNovae (ASAS-SN or "Assassin") discovered ASASSN-14lp just $\sim2$ days after first light using a global array of 14-cm diameter telescopes. ASASSN-14lp went on to become a bright supernova ($V = 11.94$ mag), second only to SN 2014J for the year. We present prediscovery photometry (with a detection less than a day after first light) and ultraviolet through near-infrared photometric and spectroscopic data covering the rise and fall of ASASSN-14lp for more than 100 days. We find that ASASSN-14lp had a broad light curve ($Δm_{15}(B) = 0.80 \pm 0.05$), a $B$-band maximum at $2457015.82 \pm 0.03$, a rise time of $16.94^{+ 0.11 }_{- 0.10 }$ days, and moderate host--galaxy extinction ($E(B-V)_{\textrm{host}} = 0.33 \pm 0.06$). Using ASASSN-14lp we derive a distance modulus for NGC 4666 of $μ= 30.8 \pm 0.2$ corresponding to a distance of $14.7 \pm 1.5$ Mpc. However, adding ASASSN-14lp to the calibrating sample of Type Ia supernovae still requires an independent distance to the host galaxy. Finally, using our early-time photometric and spectroscopic observations, we rule out red giant secondaries and, assuming a favorable viewing angle and explosion time, any non-degenerate companion larger than $0.34 R_{\textrm{sun}}$.

astro-ph.CO↗

arXiv2013

Orbital, Superhump, and Superorbital Periods in the Cataclysmic Variables AQ Mensae and IM Eridani

We report photometric detections of orbital and superorbital signals, and negative orbital sidebands, in the light curves of the nova-like cataclysmic variables AQ Mensae and IM Eridani. The frequencies of the orbital, superorbital, and sideband signals are 7.0686 (3), 0.263 (3), and 7.332 (3) cycles per day (c/d) in AQ Mensae, and 6.870 (1), 0.354 (7), and 7.226 (1) c/d in IM Eridani. We also find a spectroscopic orbital frequency in IM Eridani of 6.86649 (2) c/d. These observations can be reproduced by invoking an accretion disc that is tilted with respect to the orbital plane. This model works well for X-ray binaries, in which irradiation by a primary neutron star can account for the disc's tilt. A likely tilt mechanism has yet to be identified in CVs, yet the growing collection of observational evidence indicates that the phenomenon of tilt is indeed at work in this class of object. The results presented in this paper bring the number of CVs known to display signals associated with retrograde disc precession to twelve. We also find AQ Mensae to be an eclipsing system. The eclipse depths are highly variable, which suggests that the eclipses are grazing. This finding raises the possibility of probing variations in disc tilt by studying systematic variations in the eclipse profile.

astro-ph.SR↗

arXiv2011

The Discovery and Nature of Optical Transient CSS100217:102913+404220

We report on the discovery and observations of the extremely luminous optical transient CSS100217:102913+404220 (CSS100217 hereafter). Spectroscopic observations show this transient was coincident with a galaxy at redshift z=0.147, and reached an apparent magnitude of V ~ 16.3. After correcting for foreground Galactic extinction we determine the absolute magnitude to be M_V =-22.7 approximately 45 days after maximum light. Based on our unfiltered optical photometry the peak optical emission was L = 1.3 x 10^45 erg s^-1, and over a period of 287 rest-frame days had an integrated bolometric luminosity of 1.2 x 10^52 erg. Analysis of the pre-outburst SDSS spectrum of the source shows features consistent with a Narrow-line Seyfert1 (NLS1) galaxy. High-resolution HST and Keck followup observations show the event occurred within 150pc of nucleus of the galaxy, suggesting a possible link to the active nuclear region. However, the rapid outburst along with photometric and spectroscopic evolution are much more consistent with a luminous supernova. Line diagnostics suggest that the host galaxy is undergoing significant star formation. We use extensive follow-up of the event along with archival CSS and SDSS data to investigate the three most likely sources of such an event; 1) an extremely luminous supernova; 2) the tidal disruption of a star by the massive nuclear black hole; 3) variability of the central AGN. We find that CSS100217 was likely an extremely luminous type IIn supernova that occurred within range of the narrow-line region of an AGN. We discuss how similar events may have been missed in past supernova surveys because of confusion with AGN activity.

astro-ph.CO↗

arXiv2004

HS2331+3905: The cataclysmic variable that has it all

We report detailed follow-up observations of the cataclysmic variable HS2331+3905, identified as an emission-line object in the Hamburg Quasar Survey. An orbital period of 81.08min is unambiguously determined from the detection of eclipses in the light curves of HS2331+3905. A second photometric period is consistently detected at P~83.38min, ~2.8% longer than the orbital period, which we tentatively relate to the presence of permanent superhumps. High time resolution photometry exhibits short-timescale variability on time scales of 5-6min which we interpret as non-radial white dwarf pulsations, as well as a coherent signal at 1.12min, which is likely to be the white dwarf spin period. A large-amplitude quasi-sinusoidal radial velocity modulation of the Balmer and Helium lines with a period ~3.5h is persistently detected throughout three seasons of time-resolved spectroscopy. However, this spectroscopic period, which is in no way related to the orbital period, is not strictly coherent but drifts in period and/or phase on time scales of a few days. Modeling the far-ultraviolet to infrared spectral energy distribution of HS2331+3905, we determine a white dwarf temperature of Teff=10500K (assuming Mwd=0.6Msun), close to the ZZ Ceti instability strip of single white dwarfs. The spectral model implies a distance of d=90+-15pc, and a low value for the distance is supported by the large proper motion of the system, mu=0.14"/yr. The non-detection of molecular bands and the low J, H, and K fluxes of HS2331+3905 make this object a very likely candidate for a brown-dwarf donor.

astro-ph↗

arXiv2004

A 150MG magnetic white dwarf in the cataclysmic variable RX J1554.2+2721

We report the detection of Zeeman-split Lalpha absorption pi and sigma+ lines in the far-ultraviolet Hubble Space Telescope/Space Telescope Imaging Spectrograph spectrum of the magnetic cataclysmic variable RX J1554.2+2721. Fitting the STIS data with magnetic white dwarf model spectra, we derive a field strength of B~144MG and an effective temperature of 17000K<Teff<23000K. This measurement makes RX J1554.2+2721 only the third cataclysmic variable containing a white dwarf with a field exceeding 100MG. Similar to the other high-field polar AR UMa, RX J1554.2+2721 is often found in a state of feeble mass transfer, which suggests that a considerable number of high-field polars may still remain undiscovered.

astro-ph↗

arXiv2004

The mysterious nature of HS2331+3905

We report the discovery of one unique cataclysmic variable drawn from the Hamburg Quasar Survey, HS2331+3905. Follow-up observations obtained over three years unveiled a very unusual picture. The large amplitude 3.5 h radial velocity variations obtained from our optical spectroscopy is not the orbital period of the system, as one would normally expect. Instead, extensive CCD photometry strongly suggests that HS2331+3905 is a short orbital period cataclysmic variable with Porb = 81.09 min, containing a cold white dwarf which appears to exhibit ZZ Ceti pulsations.

astro-ph↗

arXiv2003

Superhumps in Cataclysmic Binaries. XXIV. Twenty More Dwarf Novae

We report precise measures of the orbital and superhump period in twenty more dwarf novae. For ten stars, we report new and confirmed spectroscopic periods - signifying the orbital period P_o - as well as the superhump period P_sh. These are GX Cas, HO Del, HS Vir, BC UMa, RZ Leo, KV Dra, KS UMa, TU Crt, QW Ser, and RZ Sge. For the remaining ten, we report a medley of P_o and P_sh measurements from photometry; most are new, with some confirmations of previous values. These are KV And, LL And, WX Cet, MM Hya, AO Oct, V2051 Oph, NY Ser, KK Tel, HV Vir, and RX J1155.4-5641. Periods, as usual, can be measured to high accuracy, and these are of special interest since they carry dynamical information about the binary. We still have not quite learned how to read the music, but a few things are clear. The fractional superhump excess epsilon [=(P_sh-P_o)/P_o] varies smoothly with P_o. The scatter of the points about that smooth curve is quite low, and can be used to limit the intrinsic scatter in M_1, the white dwarf mass, and the mass-radius relation of the secondary. The dispersion in M_1 does not exceed 24%, and the secondary-star radii scatter by no more than 11% from a fixed mass-radius relation. For the well-behaved part of epsilon(P_o) space, we estimate from superhump theory that the secondaries are 18+-6% larger than theoretical ZAMS stars. This affects some other testable predictions about the secondaries: at a fixed P_o, it suggests that the secondaries are (compared with ZAMS predictions) 40+-14% less massive, 12+-4% smaller, 19+-6% cooler, and less luminous by a factor 2.5(7). The presence of a well-defined mass-radius relation, reflected in a well-defined epsilon(P_o) relation, strongly limits effects of nuclear evolution in the secondaries.

astro-ph↗

arXiv2000

GRB 991216 Joins the Jet Set: Discovery and Monitoring of its Optical Afterglow

The optical light curve of the energetic gamma-ray burst GRB 991216 is consistent with jet-like behavior in which a power-law decay steepens from t**(-1.22 +/- 0.04) at early times to t**(-1.53 +/- 0.05) in a gradual transition at around 2 d. The derivation of the late-time decay slope takes into account the constant contribution of a host or intervening galaxy which was measured 110 d after the event at R = 24.56 +/- 0.14, although the light curve deviates from a single power law whether or not a constant term is included. The early-time spectral energy distribution of the afterglow can be described as F_nu ~ nu**(-0.74 +/- 0.05) or flatter between optical and X-ray, which, together with the slow initial decay, is characteristic of standard adiabatic evolution in a uniformly dense medium. Assuming that a reported absorption-line redshift of 1.02 is correct, the apparent isotropic energy of 6.7 x 10**53 erg is reduced by a factor of ~ 200 in the jet model, and the initial half-opening angle is ~ 6 deg. GRB 991216 is the third good example of a jet-like afterglow (following GRB 990123 and GRB 990510), supporting a trend in which the apparently most energetic gamma-ray events have the narrowest collimation and a uniform ISM environment. This, plus the absence of evidence for supernovae associated with jet-like afterglows, suggests that these events may originate from a progenitor in which angular momentum plays an important role but a massive stellar envelope or wind does not, e.g., the coalescence of a compact binary.

astro-ph↗

arXiv2000

The Peculiar Type Ic Supernova 1997ef: Another Hypernova

SN 1997ef has been recognized as a peculiar supernova from its light curve and spectral properties. The object was classified as a Type Ic supernova (SN Ic) because its spectra are dominated by broad absorption lines of oxygen and iron, lacking any clear signs of hydrogen or helium line features. The light curve is very different from that of previously known SNe Ic, showing a very broad peak and a slow tail. The strikingly broad line features in the spectra of SN 1997ef, which were also seen in the hypernova SN 1998bw, suggest the interesting possibility that SN 1997ef may also be a hypernova. The light curve and spectra of SN 1997ef were modeled first with a standard SN~Ic model assuming an ordinary kinetic energy of explosion $E_{\rm K} = 10^{51}$ erg. The explosion of a CO star of mass $M_{\rm CO} \approx 6 M_\odot$ gives a reasonably good fit to the light curve but clearly fails to reproduce the broad spectral features. Then, models with larger masses and energies were explored. Both the light curve and the spectra of SN 1997ef are much better reproduced by a C+O star model with $E_{\rm K} =$ 8 \e{51} erg and $M_{\rm CO} = 10 M_\odot$. Therefore, we conclude that SN 1997ef is very likely a hypernova on the basis of its kinetic energy of explosion. Finally, implications for the deviation from spherical symmetry are discussed in an effort to improve the light curve and spectral fits.

astro-ph↗

arXiv1996

An Optical/X--ray Study of A576, a Galaxy Cluster with a Cold Core

We analyze the galaxy population and dynamics of the galaxy cluster Abell 576 using 281 redshifts (230 new), R band CCD galaxy photometry over a 2h^{-1} Mpc by 2h^{-1} Mpc region centered on the cluster, an Einstein IPC image, and an Einstein MPC spectrum. The cluster galaxies with emission lines in their spectra have a larger velocity dispersion and are significantly less clustered on this 2h^{-1} Mpc scale than galaxies without emission lines. We show that excluding the emission line galaxies decreases the velocity dispersion by 18% and the virial mass estimate by a factor of two. The central cluster region contains a non--emission galaxy population and an intracluster medium which are significantly cooler (sigma_{core}=387^{+250}_{-105}~km/s and T_X=1.6^{+0.4}_{-0.3}~keV at 90\% confidence) than the global populations (sigma=977^{+124}_{-96}~km/s for the non--emission population and T_X>4~keV at 90% confidence). Because (1) the low dispersion population is no more luminous than the global population and (2) the evidence for a cooling flow is weak, we suggest that the cluster core may contain the remnants of a lower mass subcluster. We examine the cluster mass, baryon fraction and luminosity function. The cluster virial mass varies significantly depending on the galaxy sample used. Consistency between the hydrostatic and virial estimators can be achieved if (1) the gas temperature at r\sim1h^{-1}~Mpc is T_X\sim8keV (the best fit value) and (2) several velocity outliers are excluded from the virial calculation. Although the best fit Schechter function parameters and the ratio of galaxy to gas mass are typical of other clusters, the baryon fraction is relatively low. Using a lower limit to the binding mass, we show that the gas mass fraction is \le3h^{-3/2}% and the baryon fraction is \le6%.

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