Cyclotron Emission Line Research Articles

The high-field polar RX J1007.5–2017

We report optical and X-ray observations of the high-field polar RXJ1007.5-2017 performed between 1990 and 2012. It has an orbital period of 208.60 min determined from the ellipsoidal modulation of the secondary star in an extended low state. The spectral flux of the dM3- secondary star yields a distance of 790+-105 pc. At low accretion levels, \RX{} exhibits pronounced cyclotron emission lines. The second and third harmonic fall in the optical regime and yield a field strength in the accretion spot of 94 MG. The source is highly variable on a year-to-year basis and was encountered at visual magnitudes between V \sim 20 and V \sim 16. In the intermediate state of 1992 and 2000, the soft X-ray luminosity exceeds the sum of the luminosities of the cyclotron source, the hard X-ray source, and the accretion stream by an order of magnitude. An X-ray high state, corresponding to the brightest optical level, has apparently not been observed so far.

XMM-Newton and INTEGRAL study of the SFXT IGR J18483-0311 in quiescence: hint of a cyclotron emission feature?

Abstract We report the results from archival XMM–Newton and INTEGRAL observations of the Supergiant Fast X-ray Transient (SFXT) IGR J18483−0311 in quiescence. The 18–60 keV hard X-ray behaviour of the source is presented here for the first time; it is characterized by a spectral shape (Γ∼ 2.5) similar to that during outburst activity, and the lowest measured luminosity level is ∼1034 erg s−1. The 0.5–10 keV luminosity state, measured by XMM–Newton during the apastron passage, is about one order of magnitude lower and it is reasonably fitted by an absorbed blackbody model yielding parameters consistent with previous measurements. In addition, we find evidence (∼3.5σ significance) of an emission-like feature at ∼3.3 keV in the quiescent 0.5–10 keV source spectrum. The absence of any known or found systematic effects, which could artificially introduce the observed feature, gives us confidence about its non-instrumental nature. We show that its physical explanation in terms of atomic emission line appears unlikely, and conversely we attempt to ascribe it to an electron cyclotron emission line which would imply a neutron star magnetic field of the order of ∼3 × 1011 G. Importantly, such direct estimation is in very good agreement with that independently inferred by us in the framework of accretion from a spherically symmetric stellar wind. If firmly confirmed by future longer X-ray observations, this would be the first detection ever of a cyclotron feature in the X-ray spectrum of an SFXT, with important implications on theoretical models.

The nature of the close magnetic white dwarf + probable brown dwarf binary SDSS J121209.31+013627.7*

Optical time series photometry of the short period magnetic white dwarf + probable brown dwarf binary SDSS 121209.31+013627.7 reveals pulse-like variability in all bands from i' to u', peaking at u'. These modulations are most likely due to a self-eclipsing accretion hot spot on the white dwarf, rotating into view every 88.43 minutes. This period is commensurate with the radial velocity period determined by Schmidt et al. 2005 of ~90 minutes, and consistent with the rotation period of the accretor being equal to the binary orbital period. We combine our observations with those recently published by Koen and Maxted 2006 to provide an accurate ephemeris. We also detect the system in X-rays with Swift, and estimate the accretion rate at ~1x10^-13Msun per year. We suggest that SDSS1212 is most likely a magnetic cataclysmic variable in an extended state of very low accretion, similar to the well-studied Polar EF Eri. Alternatively, the putative brown dwarf is not filling its Roche Lobe and the system is a detached binary in which the white dwarf is efficiently accreting from the wind of the secondary. Six such post-common envelope, ``pre-Polar'' systems - termed ``low accretion rate Polars (LARPs)'' by Schwope et al. 2002 - have previously been identified through optical cyclotron emission lines. Cyclotron emission from SDSS1212 has recently been detected in the near-IR Debes et al. 2006 but, if detached, it would be the first ``LARP'' with a probably sub-stellar secondary. It is unclear whether an L-dwarf wind is strong enough to provide the measured accretion rate. We suggest further observations to distinguish between the Roche Lobe over-flow and wind accretion scenarios.

Cyclotron Emission Features in the Spectra of Polars with Low Accretion Rates

AbstractWe calculate non-LTE transfer of cyclotron-harmonics radiation in the hot spot of an accreting magnetic white dwarf (WD) in the case where the contribution of the accretion column to the observed radiation is negligible. The observed cyclotron spectra of a polar with a low accretion rate are found. Based on the theory developed, we interpret the cyclotron emission line spectra of the systems and determine physical conditions in the emission regions of HS 1023+3900 and HS 0922+1333.

A potential cyclotron line signature in low-luminosity X-ray sources

Estimates indicate there may be greater than or approximately equal to 10(exp 3) low-luminosity X-ray pulsars (L less than or approximately equal to 10(exp 34) ergs/s) in the Galaxy undergoing 'low-state' wind accretion in Be/X-ray binary systems, and approximately 10(exp 8)-10(exp 9) isolated neutron stars which may be accreting directly from the interstellar medium. Despite their low effective temperatures (kT(sub e) less than or approximately equal to 300 eV), low-luminosity accreting neutron stars with magnetic fields B approximately (0.7-7) x 10(exp 12) G could emit a substantial fraction (0.5%-5%) of their total luminosity in a moderately broadened (Epsilon/delta Epsilon approximately 2-4) cyclotron emission line which peaks in the energy range approximately 5-20 keV. The bulk of the thermal emission from these stars will be in the extreme ultraviolet/soft X-ray regime. In sharp contrast, the nonthermal cyclotron component predicted here will not be strongly absorbed, and consequently it may be the only distinguishing signature for the bulk of these low-luminosity sources. We propose a search for this cyclotron emission feature in long pointed observations of the newly discovered candidate isolated neutron star MS 0317.7-6477, and the Be/X-ray transient pulsar 4U 0115+63 in its quiescent state. We note that an emission-like feature similar to the one we predict here has been reported in the energy spectrum of the unusual X-ray pulsar 1E 2259+586.

Baloon-borne observations using a large area xenon detector telescope above 20 keV

Since 1983, we have been continuing a programme of balloon-borne hard X-ray observations in the 20–100 keV region using large area multi-wire proportional counters filled with xenon+methane mixture. The instrument consists of two such detectors, each having an effective area of ≈1200 cm 2 with a passive collimator of 5° × 5° FWHM. The detectors are mounted on a servo-controlled orientation platform. The energy resolution of the xenon detectors is 10 % FWHM at 22 keV and it is well suited for the detection of cyclotron emission lines in the spectra of X-ray sources. The X-ray light curves and energy spectra of several X-ray sources observed during two recent balloon flights are described.

Hard X-ray studies with a balloon-borne large area xenon detector telescope

Large area multi-layer proportional chambers filled with Xenon have been developed for the study of cosmic X-ray sources in 20 – 100 keV range. The X-ray telescope consists of two such detectors, each having an effective area of 1200 cm 2 and equipped with a 5° x 5° FWHM collimator, mounted on a servo-controlled orientation platform for making pointed mode observations. Superior energy resolution of the Xenon detectors, being 10 % FWHM at 22 keV, makes them well suited for detection of cyclotron emission lines in the spectra of X-ray pulsars. The X-ray light curves and energy spectrum of the 4.8 hour binary Cyg X-3, observed in a balloon flight in March 1986 is described.

The Phoswich Detection System PDS on board the SAX satellite

The Phoswich Detection System (PDS) is one of the four narrow-field instruments on board the SAX satellite. The experiment will be dedicated to temporal and spectral studies of celestial X-ray sources in the 15÷300 keV energy band. The PDS detector is composed of 4 actively shielded NaI(TI)/CsI(Na) phoswich scintillators and has a field of view of 1.4o (FWHM). The total geometric area is 800 cm2. The instrument is designed with the goal to achieve an energy resolution better than 17% at 60 keV and a 3σ flux sensitivity for an observation time of 105 s of about 1×10−6 photons cm−2 s−1 in an energy band 40 keV wide centred at 100 keV. This sensitivity corresponds to about 2 milliCrab flux units in the same energy range. The expected 3σ sensitivity of the PDS to cyclotron emission lines is about 3×10−5 photons cm−2 s−1 at 60 keV in 105 s. The CsI(Na) lateral shields of the PDS will also be used as a gamma-ray burst monitor. In this report we give a detailed description of the experiment design and we discuss the expected in-flight performances of the PDS.

Detection of photospheric Zeeman features and cyclotron emission lines in V834 Cen in a low state

Copyright Royal Astronomical Society [Full text of this article is not available in the UHRA]

Two-Photon Absorption and the Cyclotron Line in Her X-1

The cyclotron line in Her X-1 is a hard X-ray feature at ≈ 58 keV discovered by Trümper et al. (1977). A cyclotron emission line at about this energy had been predicted by Gnedin and Sunyaev (1974) and Basko and Sunyaev (1975). They developed a model for the infall of accreted matter onto a magnetized neutron star’s surface, and concluded that hot spots would form at the polar caps and radiate X-rays. They predicted that the optional depth for bremsstrahlung would be less than unity, that for Compton scattering greater than unity and that for cyclotron absorption much greater than unity. A cyclotron line is then implied due to the emission spectrum lying below the black-body spectrum except near the cyclotron energy where it rises up to the black-body limit. More recent developments of these background ideas have been reviewed by Börner (1980).

Evidence for strong cyclotron line emission in the hard X-ray spectrum of Hercules X-1

We present further results of our Hercules X-1 balloon observation on 1976 May 3 which confirm the existence of a strong line feature at approx.58 keV in the pulsed (1.24s) X-ray spectrum we reported earlier. The spectral excess in the line region over the extrapolated continuum is 5-6 sigma. Our best estimates of the line flux and line width are 3 x 10/sup -3/ photons cm/sup -2/ s/sup -1/ and less than approx.12 keV, respectively. The most likely interpretation of this line is electron cyclotron emission at the basic frequency from the hot polar plasma of the rotating neutron star. The corresponding magnetic field strength is 5.3 x 10/sup 12/ gauss. We have searched for the second-harmonic cyclotron emission line in that part of our data showing the highest signal-to-noise ratio and find a 3.3 sigma spectral enhancement near the predicted energy (110 keV). We discuss implications of the line width and the line intensity ratio for the physical conditions of the emitting plasma and the beaming geometry.