Neglected X-ray discovered polars Beuermann, K.; Burwitz, V.; Reinsch, K. ...
Astronomy and astrophysics (Berlin),
07/2017, Letnik:
603
Journal Article
Recenzirano
Odprti dostop
We report photometric and spectroscopic observations of the polar V358 Aqr (=RX J2316-0527) collected over 25 yr. It was discovered as a bright very soft X-ray source in the ROSAT All Sky Survey, but ...had lapsed into a low state when reobserved in 1993. We have obtained an optical photometric ephemeris, which is free of cycle count errors and allows to correctly phase events around the orbit even for observations that lie decades apart. V358 Aqr possesses an accreting pole in the upper hemisphere of the white dwarf that is visible over the entire orbital period of 209.45 min. The magnetic field strength derived from cyclotron line emission is 31.8 MG. The orbital motion of the cyclotron lines yields an inclination of 60° and an offset of the magnetic pole from the rotational axis of about 10°. The secondary star is of spectral type dM4.0 ± 0.5 and the distance is 540 ± 100 pc. V358 Aqr is peculiar in showing giant optical outbursts that bear all of the characteristics of stellar flares. With two flares observed in 72 h on source, the flare frequency may be high. The total energy radiated in the flare of 28 November 2010 exceeded 1036 erg. While the flares clearly occur on the secondary star, it seems that they may or may not be connected with coronal mass ejection and subsequent accretion onto the white dwarf.
We present a multi-epoch spectral and timing analysis of the isolated neutron star RBS1223. New XMM-Newton data obtained in January 2004 confirm the spin period to be twice as long as previously ...thought, $P_{\rm spin} = 10.31$ s. The combined ROSAT, Chandra, and XMM-Newton data (6 epochs) give, contrary to earlier findings, no clear indication of a spin evolution of the neutron star. The X-ray light curves are double-humped with pronounced hardness ratio variations suggesting an inhomogeneous surface temperature with two spots separated by about ~160°. The sharpness of the two humps suggests a mildly relativistic star with a ratio between Rns, the neutron star radius at source, and rS, the Schwarzschild-radius, of $R_{\rm ns}/r_{\rm S} > 3.5$. Assuming Planckian energy distributions as local radiation sources, light curves were synthesized which were found to be in overall qualitative agreement with observed light curves in two different energy bands. The temperature distribution used was based on the crustal field models by Geppert et al. (2004) for a central temperature of $T_{\rm c} = 10^8$ K and an external dipolar field of $B \sim 10^{13}$ G. This gives a mean atmospheric temperature of 55 eV. A much simpler model with two homogeneous spots with $T_\infty = 92$ eV and 84 eV, and a cold rest star, $T_{\rm star,\infty} < 45$ eV, invisible at X-ray wavelengths, was found to be similarly successful. The new temperature determination and the new $\dot{P}_{\rm spin}$ suggest that the star is older than previously thought, $T \simeq 10^{5\dots6}$ yr. The model-dependent distance to RBS1223 is estimated between 76 pc and 380 pc (for $R_{\rm ns} = 12$ km).
We present follow-up spectroscopy and photometry of 11 post-common envelope binary (PCEB) candidates identified from multiple Sloan Digital Sky Survey (SDSS) spectroscopy in an earlier paper. Radial ...velocity measurements using the Na i λλ8183.27, 8194.81 absorption doublet were performed for nine of these systems and provided measurements of six orbital periods in the range Porb= 2.7– 17.4 h. Three PCEB candidates did not show significant radial velocity variations in the follow-up data, and we discuss the implications for the use of SDSS spectroscopy alone to identify PCEBs. Differential photometry confirmed one of our spectroscopic orbital periods and provided one additional Porb measurement. Binary parameters are estimated for the seven objects for which we have measured the orbital period and the radial velocity amplitude of the low-mass companion star, Ksec. So far, we have published nine SDSS PCEBs orbital periods, all of them Porb < 1 d. We perform Monte Carlo simulations and show that 3σ SDSS radial velocity variations should still be detectable for systems in the orbital period range of Porb∼ 1– 10 d. Consequently, our results suggest that the number of PCEBs decreases considerably for Porb > 1 d, and that during the CE phase the orbital energy of the binary star is may be less efficiently used to expel the envelope than frequently assumed.
We report on the discovery of a very distant galaxy cluster serendipitously detected in the archive of the XMM-Newton mission, within the scope of the XMM-Newton Distant Cluster Project (XDCP). ...XMMUJ0044.0-2033 was detected at a high significance level (5σ) as a compact, but significantly extended source in the X-ray data, with a soft-band flux f(r < 40″) = (1.5 ± 0.3) × 10-14 erg s-1 cm2. Optical/NIR follow-up observations confirmed the presence of an overdensity of red galaxies matching the X-ray emission. The cluster was spectroscopically confirmed to be at z = 1.579 using ground-based VLT/FORS2 spectroscopy. The analysis of the I − H colour − magnitude diagram shows a sequence of red galaxies with a colour range 3.7 < I − H < 4.6 within 1′ from the cluster X-ray emission peak. However, the three spectroscopic members (all with complex morphology) have significantly bluer colours relative to the observed red-sequence. In addition, two of the three cluster members have OII emission, indicative of on-going star formation. Using the spectroscopic redshift we estimated the X-ray bolometric luminosity, Lbol,40″ ~ 5.8 × 1044 erg s-1, implying a massive galaxy cluster. This places XMMU J0044.0-2033 at the forefront of massive distant clusters, closing the gap between lower redshift systems and recently discovered proto- and low-mass clusters at z > 1.6.
Planets orbiting post-common envelope binaries provide fundamental information on planet formation and evolution, especially for the yet nearly unexplored class of circumbinary planets. We searched ...for such planets in DP Leo, an eclipsing short-period binary, which shows long-term eclipse-time variations. Using published, reanalysed, and new mid-eclipse times of the white dwarf in DP Leo, obtained between 1979 and 2010, we find agreement with the light-travel-time effect produced by a third body in an elliptical orbit. In particular, the measured binary period in 2009/2010 and the implied radial velocity coincide with the values predicted for the motion of the binary and the third body around the common center of mass. The orbital period, semi-major axis, and eccentricity of the third body are Pc = 28.0 ± 2.0 yrs, ac = 8.2 ± 0.4 AU, and ec = 0.39 ± 0.13. Its mass of sini c Mc = 6.1 ± 0.5 MJup qualifies it as a giant planet. It formed either as a first generation object in a protoplanetary disk around the original binary or as a second generation object in a disk formed in the common envelope shed by the progenitor of the white dwarf. Even a third generation origin in matter lost from the present accreting binary can not be entirely excluded. We searched for, but found no evidence for a fourth body.
High resolution spectral observations of the polar EF Eridani obtained in a low state with X-Shooter revealed narrow emission lines from the irradiated secondary. The lines were most prominent in the ...near-IR Caii-triplet, the more intensive Hα line had additional emission likely originating from an accretion stream. The lines with a radial velocity amplitude, K2’ = 385 ± 4 km s-1, serve as tracer of the otherwise unseen companion. The mass function implies a massive white dwarf with Mwd >0.65 $M_\odot$ at 3σ confidence, and a short distance to the binary, d ~ 111 pc (<145 pc at 3σ confidence). The spectral energy distribution from the UV to the IR together with the high mass ratio gives further strong evidence of EF Eri being a post period-minimum object with M2 < 0.06 $M_\odot$.
We report on the analysis of new X-ray data obtained with XMM-Newton and Chandra from two ROSAT-discovered X-ray dim isolated neutron stars (XDINs). was observed with XMM-Newton in April 2003, 2.5 ...years after the first observation. The EPIC-pn data confirm that this object is an X-ray pulsar with 11.371 s neutron star spin period. The X-ray spectrum is consistent with absorbed black-body emission with a temperature $kT = 96$ eV and NH = 4$\,\times \,10^{19}$ cm-2 without significant changes between the two observations. Four XMM-Newton observations of between December 2002 and July 2003 did not confirm the 22.7 s pulsations originally indicated in ROSAT data, but clearly reveal a 3.453 s period. A fit to the X-ray spectrum using an absorbed black-body model yields $kT = 45$ eV, the lowest value found from the small group of XDINs and NH = 1.0$\,\times \,10^{20}$ cm-2. Including a broad absorption line improves the quality of the spectral fits considerably for both objects and may indicate the presence of absorption features similar to those reported from , and . For both targets we derive accurate X-ray positions from the Chandra data and present an optical counterpart candidate for with $B = 26.6\pm0.3$ mag from VLT imaging.
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