arXiv subjects

Robert Schwarz

Publications and source records attributed to Robert Schwarz.

10 recordsLinked to original sources

arXiv2026

BICEP/Keck XXI: Constraints on Early-Universe Parity Violation from Multipole-Dependent Birefringence

We present the first constraints on multipole-dependent cosmic birefringence using CMB polarization data from the BK18 dataset, which combines observations from BICEP2, Keck Array, and BICEP3 at frequencies of 95, 150, and 220 GHz. Photon coupling to an axion-like field leads to the rotation of CMB polarization, inducing non-zero EB cross-correlations. We show that a multipole-dependent rotation beta(l) imprints a distinct signature in the polarization spectra that can be constrained. Specifically, we consider an Early Dark Energy (EDE) scenario in which a pseudoscalar field couples to photons through a Chern-Simons interaction, generating a polarization rotation with multipole dependence. We introduce a phenomenological beta(l) as a step function, obtaining constraints on the step function size consistent with zero, with uncertainties less than 0.15 degrees (68% CL). In addition, using multi-frequency EE, BB, and EB cross-spectra, along with robust BICEP/Keck foreground treatment and likelihood framework, we derive constraints on the axion-photon coupling amplitude g for several choices of EDE parameters. For the baseline best-fit value f_{EDE} = 0.087 from the Planck 2018 analysis, we obtain g = 0.11 +/- 0.37 (68% CL), consistent with previous limits.

astro-ph.CO↗

arXiv2019

Fast-Cadence TESS Photometry and Doppler Tomography of the Asynchronous Polar CD Ind: A Revised Accretion Geometry from Newly Proposed Spin and Orbital Periods

The TESS spacecraft observed the asynchronous polar CD Ind at a two-minute cadence almost continuously for 28 days in 2018, covering parts of 5 consecutive cycles of the system's 7.3-day beat period. These observations provide the first uninterrupted photometry of a full spin-orbit beat cycle of an asynchronous polar. Twice per beat cycle, the accretion flow switched between magnetic poles on the white dwarf, causing the spin pulse of the white dwarf (WD) to alternate between two waveforms after each pole-switch. An analysis of the waveforms suggests that one accretion region is continuously visible when it is active, while the other region experiences lengthy self-eclipses by the white dwarf. We argue that the previously accepted periods for both the binary orbit and the WD spin have been misidentified, and while the cause of this misidentification is a subtle and easily overlooked effect, it has profound consequences for the interpretation of the system's accretion geometry and doubles the estimated time to resynchronization. Moreover, our timings of the photometric maxima do not agree with the quadratic ephemeris from Myers et al. (2017), and it is possible that the optical spin pulse might be an unreliable indicator of the white dwarf's rotation. Finally, we use Doppler tomography of archival time-resolved spectra from 2006 to study the accretion flow. While the accretion flow showed a wider azimuthal extent than is typical for synchronous polars, it was significantly less extended than in the three other asynchronous polars for which Doppler tomography has been reported.

astro-ph.SR↗

arXiv2019

On the Relation between the Extended Supporting Hyperplane Algorithm and Kelley's Cutting Plane Algorithm

Recently, Kronqvist et al.~\cite{KronqvistLundellWesterlund2016} rediscovered the supporting hyperplane algorithm of Veinott~\cite{Veinott1967} and demonstrated its computational benefits for solving convex mixed-integer nonlinear programs. In this paper we derive the algorithm from a geometric point of view. This enables us to show that the supporting hyperplane algorithm is equivalent to Kelley's cutting plane algorithm~\cite{J.E.Kelley1960} applied to a particular reformulation of the problem. As a result, we extend the applicability of the supporting hyperplane algorithm to convex problems represented by general, not necessarily convex, differentiable functions that satisfy a mild condition.

math.OC↗

arXiv2013

The night sky brightness at Potsdam-Babelsberg

We analyze the results of a 2 years (2011--2012) time series of night sky photometry performed at the Leibniz Institute for Astrophysics in Potsdam (AIP). This observatory is located on top of a hill ("Babelsberg"), 22\,km to the southwest of the center of Berlin. The measurements have been performed with a Unihedron Sky Quality Meter. We find night sky brightness values ranging from 16.5 to 20.3 mag$_{\rm SQM}$ arcsec$^{-2}$; the latter (best) value corresponds to 4.7 times the natural zenithal night sky brightness. We discuss the influence of clouds, of the Moon and other factors on the night sky brightness. With respect to the influence of the Moon, it turns out that Potsdam-Babelsberg, despite its proximity to Berlin, still shows a circalunar periodicity of the night sky brightness, although it is much weaker than naturally. The light-pollution-enhancing effect of clouds dominates the night sky brightness by far. Overcast nights with light pollution (up to 16.5 mag$_{\rm SQM}$ arcsec$^{-2}$) are brighter than clear full moon nights ($\approx$ 18.5 mag$_{\rm SQM}$ arcsec$^{-2}$) in roughly the same proportion as the latter compared to clear nights with light pollution (20.3 mag$_{\rm SQM}$ arcsec$^{-2}$).

astro-ph.IM↗

arXiv2012

On the HU Aquarii planetary system hypothesis

In this work, we investigate the eclipse timing of the polar binary HU Aquarii that has been observed for almost two decades. Recently, Qian et al. attributed large (O-C) deviations between the eclipse ephemeris and observations to a compact system of two massive jovian companions. We improve the Keplerian, kinematic model of the Light Travel Time (LTT) effect and re-analyse the whole currently available data set. We add almost 60 new, yet unpublished, mostly precision light curves obtained using the time high-resolution photo-polarimeter OPTIMA, as well as photometric observations performed at the MONET/N, PIRATE and TCS telescopes. We determine new mid--egress times with a mean uncertainty at the level of 1 second or better. We claim that because the observations that currently exist in the literature are non-homogeneous with respect to spectral windows (ultraviolet, X-ray, visual, polarimetric mode) and the reported mid--egress measurements errors, they may introduce systematics that affect orbital fits. Indeed, we find that the published data, when taken literally, cannot be explained by any unique solution. Many qualitatively different and best-fit 2-planet configurations, including self-consistent, Newtonian N-body solutions may be able to explain the data. However, using high resolution, precision OPTIMA light curves, we find that the (O-C) deviations are best explained by the presence of a single circumbinary companion orbiting at a distance of ~4.5 AU with a small eccentricity and having ~7 Jupiter-masses. This object could be the next circumbinary planet detected from the ground, similar to the announced companions around close binaries HW Vir, NN Ser, UZ For, DP Leo or SZ Her, and planets of this type around Kepler-16, Kepler-34 and Kepler-35.

astro-ph.EP↗

arXiv2006

Constraining the GRB Collimation with a Survey for Orphan Afterglows

Gamma-ray bursts are believed to be produced in highly-relativistic collimated outflows. Support for this comes among others from the association of the times of detected breaks in the decay of afterglow light curves with the collimation angle of the jets. An alternative approach to estimate a limit on the collimation angle uses GRB afterglows without detected prompt-emission counterparts. Here we report on the analysis of a dedicated survey for the search of these orphan afterglows using the Wide Field Imager at the 2.2m MPI/ESO telescope at La Silla, Chile. We monitored ~12 square degrees. in up to 25 nights typically spaced by one to two nights with a limiting magnitude of R=23. Four previously unknown optical transients were discovered and three of these associated with a flare star, a cataclysmic variable and a dwarf nova. The fourth source shows indications for an extragalactic origin but the sparse sampling of the light curve prevents a reliable classification. We discuss the results in the context of the collimation of GRBs.

astro-ph↗

arXiv1998

Tomography of Polars

We are reviewing the power of Doppler tomography for detailed investigations of polars (magnetic cataclysmic binaries without accretion disk). Using high-resolution spectroscopy in combination with tomography as analytic tool, the structure and extent of the accretion flows and accretion curtains in the binaries can be uncovered and the irradiated and non-irradiated parts of the mass-donating secondary stars can be made visible. In addition we show, how basic system parameters like the binary inclination and the mass ratio can be addressed by tomography.

astro-ph↗

arXiv1998

Filling up the period gap: RX J0803.4-4748, a new 137 minute polar

We report the discovery of a new AM Herculis system as the counterpart of the bright and very soft X-ray source RX J0803.4-4748 found serendipitously in a ROSAT PSPC pointing. Optical photometry showed deep (2 mag) variations with a period of ~137 min placing RX J0803.4-4748 right into the period gap. The modulation can be interpreted in terms of cyclotron beaming and is very similar to that of other non-selfeclipsing polars like e.g. V843 Cen. Such a geometry is also supported by the X-ray light curve of a later HRI pointing which was lacking any pronounced faint-phase. Phase-resolved optical spectra are charaterized by strong Balmer and HeII 4686 Angstroem line emission superimposed onto a blue continiuum, and reveal disinct cyclotron humps at certain phases. The field strength of 39 MG as derived from the separation of the harmonics is in agreement with that of other soft X-ray polars.

astro-ph↗

arXiv1998

RX J1016.9-4103: A new soft X-ray polar in the period gap

We have discovered a new AM Her system as the optical counterpart of the ROSAT All-Sky-Survey source RX J1016.9-4103 (= 1RXS J101659.4-410332). The X-ray spectrum is very soft and the X-ray intensity is strongly modulated with the orbital period. Optical photometric and spectroscopic follow-up observations reveal a synchronously rotating binary with an orbital period of 134 min, placing RX J1016.9-4103 in the period gap. The strength of the TiO bands suggests a secondary spectral type later than M3 V and a distance of 615+/-150 pc. Based on two clearly visible broad humps in the optical spectrum (interpreted as cyclotron features) a magnetic field strength of 52 MG is deduced thus proving the polar classification.

astro-ph↗

arXiv1997

Discovery of the high-field polar RX J1724.0+4114

We report the discovery of a new AM Herculis binary (polar) as the optical counterpart of the soft X-ray source RX J1724.0+4114 detected during the ROSAT all-sky survey. The magnetic nature of this V~17 mag object is confirmed by low-resolution spectroscopy showing strong Balmer and HeII emission lines superimposed on a blue continuum which is deeply modulated by cyclotron humps. The inferred magnetic field strength is 50+/-4 MG (or possibly even ~70 MG). Photometric observations spanning ~3 years reveal a period of 119.9 min, right below the period gap. The morphology of the optical and X-ray light curves which do not show eclipses by the secondary star, suggests a self-eclipsing geometry. We derive a lower limit of d~250 pc.

astro-ph↗