Cyclotron Features Research Articles

Luminosity‐related Changes in the Cyclotron Resonance Structure of the Binary X‐Ray Pulsar 4U 0115+63

The X-ray binary pulsar 4U 0115+63 was observed twice with the Ginga Large Area Counter, in 1990 February and in 1991 March, when the luminosities in 2-60 keV were 1.4 × 1038 and 2.0 × 1037 ergs s-1, respectively. In 1990, two harmonic cyclotron features were detected at energies of 11.3 and 22.1 keV, in agreement with previous observations. In 1991, the structure changed to a single absorption at an energy of 15.6 keV, which is interpreted as the fundamental resonance. All features were observed in absorption. The higher resonance energy observed in 1991 may be attributed to a decrease of the height of the accretion column, in response to the factor of ~6.8 reduction in the mass accretion rate.

First evidence of a cyclotron feature in an anomalous X-ray pulsar

Abstract We report on the results of the longest uninterrupted observation of the Anomalous X-ray Pulsar 1RXS J170849–400910 obtained with the BeppoSAX satellite in August 2001. The best-fit phase-averaged spectrum was found to be an absorbed power law plus blackbody model, with photon index Γ∼2.4 and a black body temperature of kTbb∼ 0.4 keV. We confirm the presence of significant spectral variations with the rotational phase of the pulsar. In the spectrum corresponding to the rising part of the pulse we found an absorption-like feature at ∼8.1 keV (significance level of 4ς), most likely due to cyclotron resonant scattering. The centroid energy converts to a magnetic field of 9 × 1011G and 1.6 × 1015G in the case of electrons and protons, respectively. If confirmed, this would be the first detection of a cyclotron feature in the spectrum of an anomalous X-ray pulsar.

Spectral Features in the Thermal Emission from Isolated Neutron Stars: Dependence on Magnetic Field Strengths

We study several effects that influence the strength of the proton cyclotron and atomic features in the thermal spectra of magnetic neutron stars. We show that it is possible for vacuum polarization to strongly suppress the spectral lines when the magnetic field B 1014 G. For weaker fields (B 7 × 1013 G), the surface spectrum is unaffected by vacuum polarization; thus, the proton cyclotron absorption line can have a large equivalent width. Using an approximate calculation of the synthetic spectra, we show that variation of magnetic fields over the neutron star surface leads to broadening of the line. The recently detected absorption features from the isolated neutron stars RX J1308.6+2127, RX J1605.3+3249, and RX J0720.4-3125 can plausibly be explained by the proton cyclotron resonance, with possible blending due to atomic transitions, in the atmosphere of the star.

Evidence of a Cyclotron Feature in the Spectrum of the Anomalous X-Ray Pulsar 1RXS J170849−400910

We report the results of a long observation of the anomalous X-ray pulsar 1RXS J170849-400910 obtained with the BeppoSAX satellite in August 2001. The best-fit phase-averaged spectrum was an absorbed power law plus blackbody model, with photon index Gamma~2.4 and a blackbody temperature of kT_bb~0.4 keV. We confirm the presence of significant spectral variations with the rotational phase of the pulsar. In the spectrum corresponding to the rising part of the pulse we found an absorption-like feature at ~8.1 keV (a significance of 4 sigma), most likely due to cyclotron resonant scattering. The centroid energy converts to a magnetic field of 9x10^11 G and 1.6x10^15 G in the case of electrons and protons, respectively. If confirmed, this would be the first detection of a cyclotron feature in the spectrum of an anomalous X-ray pulsar.

Two Rare Magnetic Cataclysmic Variables with Extreme Cyclotron Features Identified in the Sloan Digital Sky Survey

Two newly identified magnetic cataclysmic variables discovered in the Sloan Digital Sky Survey (SDSS), SDSS J155331.12+551614.5 and SDSS J132411.57+032050.5, have spectra showing highly prominent, narrow, strongly polarized cyclotron humps with amplitudes that vary on orbital periods of 4.39 and 2.6 hr, respectively. In the former, the spacing of the humps indicates the third and fourth harmonics in a magnetic field of ~60 MG. The narrowness of the cyclotron features and the lack of strong emission lines imply very low temperature plasmas and very low accretion rates, so that the accreting area is heated by particle collisions rather than accretion shocks. The detection of rare systems like these exemplifies the ability of the SDSS to find the lowest accretion rate close binaries.

The cyclotron spectrum of anisotropic ultrarelativistic electrons: Interpretation of X-ray pulsar spectra

The spectrum of cyclotron radiation produced by electrons with a strongly anisotropic velocity distribution is calculated taking into account higher harmonics. The motion of the electrons is assumed to be ultrarelativistic along the magnetic field and nonrelativistic across the field. One characteristic feature of the resulting spectrum is that harmonics of various orders are not equally spaced. The physical properties and observed spectra of four X-ray pulsars displaying higher cyclotron harmonics are analyzed. It was shown that at least in one of them, the cyclotron feature can apparently be only an emission line. Moreover, the observed harmonics are not equidistant, and display certain other properties characteristic of emission by strongly anisotropic ultrarelativistic electrons. In addition, there are indirect theoretical arguments that the electrons giving rise to cyclotron features in the spectra of X-ray pulsars are ultrarelativistic and characterized by strongly anisotropic distributions. As a result, estimates of the magnetic fields of X-ray pulsars (which are usually derived from the energies of cyclotron lines) and certain other physical parameters require substantial revision.

Proton Cyclotron Features in Thermal Spectra of Ultramagnetized Neutron Stars

A great deal of interest has been recently raised in connection with the possibility that soft γ-ray repeaters (SGRs) and anomalous X-ray pulsars (AXPs) contain magnetars, young neutron stars endowed with magnetic fields ≳ 1014 G. In this paper we calculate thermal spectra from ultramagnetized neutron stars for values of the luminosity and magnetic field believed to be relevant to SGRs and AXPs. Emergent spectra are found to be very close to a blackbody at the star effective temperature and exhibit a distinctive absorption feature at the proton cyclotron energy Ec,p ≃ 0.63(B/1014 G) keV. The proton cyclotron features (PCFs) are conspicuous (equivalent width of up to many hundreds eV) and relatively broad (ΔE/E ~ 0.05-0.2). The detection of the PCFs is well within the capabilities of present X-ray spectrometers, like the HETGS and METGS on board Chandra. Their observation might provide decisive evidence in favor of the existence of magnetars.

Formation of Annihilation-Cyclotron Lines in Strong Magnetic Fields Near Neutron Stars

We study cyclotron and polarization spectral features in magnetized vacuum at frequencies near the one-photon annihilation resonance in a subcritical magnetic field. On the basis of this study, we propose a new method for determination of the neutron-star magnetic field. Using the dipole-field model, we show that detection of the annihilation line in absorption and an additional line in emission at MeV energies is evidence of the existence of a magnetic field (2.8-3.6)· 1012G on the surface of a neutron star.

The broad band spectral properties of binary X-ray pulsars

The X-ray telescopes on board BeppoSAX are an optimal set of instruments to observe bright galactic binary pulsars. These sources emit very hard and quite complex X-ray spectra that can be accurately measured with BeppoSAX between 0.1 and 200 keV. A prototype of this complexity, the source Her X−1, shows at least seven different components in its spectrum. A broad band measure is therefore of paramount importance to have a thorough insight into the physics of the emitting region. Moreover the detection of cyclotron features, when present, allows a direct and highly significant measure of the magnetic field intensity in the emission region. In this paper we briefly report the results obtained with BeppoSAX on this class of sources, with emphasis on the detection and on the measured properties of the cyclotron lines.

The broad band spectral properties of galactic X-ray binary pulsars

BeppoSAX observed several galactic binary X-ray pulsars during the Science Verification Phase and in the first year of the regular program. The complex emission spectra of these sources are an ideal target for the BeppoSAX instrumentation, that can measure the emission spectra in an unprecedented broad energy band. Using this capability of BeppoSAX a detailed observational work can be done on the galactic X-ray pulsars. In particular the 0.1–200 keV energy band allows the shape of the continuum emission to be tightly constrained. A better determination of the underlying continuum allows an easier detection of features superimposed onto it, both at low energy (Fe K and L, Ne lines) and at high energies (cyclotron features). We report on the spectral properties of a sample of X-ray pulsars observed with BeppoSAX comparing the obtained results. Some ideas of common properties are also discussed and compared with our present understanding of the emission mechanisms and processes.

A search for cyclotron lines in the bursting pulsar, GRO J1744-28

We report a search for cyclotron line absorption in the pulsed spectrum of the “Bursting Pulsar”, GRO J1744-28 using the Proportional Counter Array (PCA) and the High Energy X-ray Timing Experiment (HEXTE) on the Rossi X-ray Timing Explorer ( RXTE). We focus on data obtained around the peak of the 1996–1997 outburst. Improvements made in these analyses include the use of more accurate PCA and HEXTE detector response matrices, data with a larger overlapping energy band, and a continuum model especially well suited to cyclotron line searches. We find that the spectrum can be adequately fit with a smooth continuum model typical of other X-ray pulsars, with no modification for cyclotron absorption required. In fact, the data are inconsistent with cyclotron features with depths greater than a few percent of the continuum.

Spectroscopic observations of the eclipsing polar MN Hya (RX J0929-24)

We present low--medium resolution optical spectroscopy of the eclipsing AM Her system MN Hya (RX J0929--24). We determine the magnetic field strength at the primary accretion region of the white dwarf to be 42MG from the spacing of cyclotron features visible during ~0.4--0.7. From spectra taken during the eclipse we find that the secondary has a M3--4 spectral type. Combined with the eclipse photometry of Sekiguchi, Nakada & Bassett and an estimate of the interstellar extinction we find a distance of ~300--700pc. We find unusual line variations at phase ~0.9: Halpha is seen in absorption and emission. This is at the same point in the orbital phase that a prominent absorption dip is seen in soft X-rays.

Multiwavelength Data Suggest A Cyclotron Feature on the Hot Thermal Continuum of Geminga

A new set of ground-based optical, HST, UV, and EUVE data on Geminga are presented. The object, identified with a high proper motion isolated neutron star (INS), is seen to emit thermal radiation with a temperature in the range (2.2-2.8) × 105 K. This is compatible with the global thermal emission from the surface of a standard neutron star of Geminga's spin-down age located at the distance of about 160 pc, recently measured through its annual parallax. However, in the (3400-8000 A) range, combined ground-based and HST data suggest the presence of a spectral feature (either in emission or possibly in absorption) superimposed on the Rayleigh-Jeans continuum. Its simplest interpretation could be related to an ion (H or He) cyclotron frequency for the object's canonical magnetic field of 1.5 × 1012 G. This is the first observational evidence for an optical spectral feature in an INS. A search for a similar effect should also be feasible for PSR 0656+14, the only other INS possibly showing optical thermal emission from its surface.

Gamma-ray burst energy spectra: theoretical models, old and new

The modelling of gamma-ray burst (GRB) spectra has considerable potential for increasing the understanding of these enigmatic sources. A diversity of ideas and analyses has been generated over the last two decades to explain line features and continuum shapes, encompassing both older galactic neutron star and “new age” cosmological source models. This paper reviews some of the highlights of these studies, discussing the merits and limitations of various ideas, and in particular their compatibility with the observational data. The first focus will be on continuum models for GRBs, which include optically thin synchrotron emission and resonant Compton upscattering near galactic neutron stars, while the synchrotron and non-magnetic inverse Compton scattering mechanisms are prominent in the less well-developed cosmological scenarios. Line formation scenarios will then be discussed, in particular the scattering model for producing cyclotron features, which remains the only viable explanation for the Ginga observations of double lines. Absorption-like line production in cosmological burst models is generally difficult, though interesting notions such as femtolensing interference patterns have been proffered.

Discovery of a cyclotron resonance feature at 30 keV from the transient X-ray pulsar Cepheus X-4

From Ginga observations of the transient X-ray pulsar Cep X-4, a spectral line feature attributable to electron cyclotron resonance was discovered at about 30 keV in the 1.2−37 keV X-ray spectrum. The detection, a fifth firm example of cyclotron resonance from X-ray pulsars, implies a surface magnetic field of about 2.6×10 12 (1+z) G for this pulsar, where z is the gravitational redshift. Throughout the 66.25 s pulses, the cyclotron feature appears in absorption, with at most ±5% variation in the resonance center energy around the mean value of 30.5±0.4 keV

The inability of the resonant Compton upscattering model of gamma-ray bursts to produce a third cyclotron harmonic

The inability of the resonant Compton upscattering model to produce a third cyclotron resonance, which provides a strong observational test of this model, is discussed. The relevant features of the Compton upscattering model and the thermal cross sections responsible for the cyclotron lines are examined. The impossibility of producing a third cyclotron feature for all realistic temperatures of the line-forming plasma is addressed.

Cyclotron features in the infrared spectrum of AM Herculis

Spectroscopic observations of the magnetic cataclysmic variable AM Herculis covering the wavelength range 1.0-2.5 μm are presented. The observations were obtained during the faint state of the system's long-term light variations. During the faint phase of the binary cycle the spectrum appears to be dominated by the radiation from the M-dwarf secondary, showing deep absorptions due to H 2 O at around 1.4 and 1.9 μm. The spectrum can be well fitted with an M-dwarf model atmosphere for an effective temperature of 3250 K. During the bright phase of the binary cycle there is excess flux which is attributed to cyclotron radiation

A theory of gamma-ray bursts based on resonant Compton scattering

view Abstract Citations (12) References (61) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A Theory of Gamma-Ray Bursts Based on Resonant Compton Scattering Vitello, Peter ; Dermer, Charles D. Abstract Photon spectra produced from the resonant Compton scattering of blackbody photons by a beam of energetic electrons and positrons are calculated. A range of surface temperatures near 1 keV and magnetic field strengths near 10 to the 12th G are studied for monoenergetic and power-law electron and positron source functions. Photon spectra similar to observed gamma-ray burst spectra are obtained for power-law beam injection functions with low-energy cutoffs. It is proposed that gamma-ray bursts are the resonant scattered emission of electron-positron pairs injected very close to the surface of a neutron star. Depending on the temperature and magnetic field, the mean free path for energy loss can be a fraction of the neutron star radius. The cyclotron features observed in gamma-ray burst spectra are due to complicated radiative transfer effects of photons with energies near the cyclotron fundamental and higher harmonics. Publication: The Astrophysical Journal Pub Date: June 1991 DOI: 10.1086/170152 Bibcode: 1991ApJ...374..668V Keywords: Black Body Radiation; Compton Effect; Gamma Ray Bursts; Magnetic Stars; Neutron Stars; Photons; Cyclotron Resonance; Electron Beams; Electron-Positron Pairs; Positrons; Radiative Transfer; Surface Temperature; Space Radiation; GAMMA RAYS: BURSTS; RADIATION MECHANISMS; STARS: MAGNETIC; STARS: NEUTRON full text sources ADS |

Dynamic effects on cyclotron scattering in pulsar accretion columns

A resonant scattering model for photon reprocessing in a pulsar accretion column is presented. The accretion column is optically thin to Thomson scattering and optically thick to resonant scattering at the cyclotron frequency. Radiation from the neutron star surface propagates freely through the column until the photon energy equals the local cyclotron frequency, at which point the radiation is scattered, much of it back toward the star. The radiation pressure in this regime is insufficient to stop the infall. Some of the scattered radiation heats the stellar surface around the base of the column, which adds a softer component to the spectrum. The partial blocking by the accretion column of X-rays from the surface produces a fan beam emission pattern. X-rays above the surface cyclotron frequency freely escape and are characterized by a pencil beam. Gravitational light bending produces a pencil beam pattern of column-scattered radiation in the antipodal direction, resulting in a strongly angle-dependent cyclotron feature.

New observations of the cyclotron absorption feature in Hercules X–1

ALTHOUGH neutron stars are generally believed to be born with intense (1011–1013 G) magnetic fields1,2, which then gradually decay3, measurements of their field strengths remain uncertain. In the special case of X-ray-emitting binary pulsars, a direct estimate of the field strength can be obtained by measuring the energy of spectral features that are due to electron cyclotron resonance4–13. With the Ginga satellite observatory14,15, we have measured a cyclotron feature in the hard X-ray spectrum of the 1.24-s binary pulsar Hercules X–1 with a much greater energy resolution than in previous observations4–9. The spectrum from 10–60 keV can be described with a simple analytical formula12,16,17, which indicates an absorption feature at ∼34 keV rather than an emission feature at ∼50keV. From this we estimate the surface magnetic field strength of Her X–1 to be (2.9±0.3) × 1012 G.