Marcus Alton Teter

Discovery of absorption features in the X-ray spectrum of an isolated neutron star

We observed 1E 1207.4-5209, a neutron star in the center of the supernova remnant PKS 1209-51/52, with the ACIS detector aboard the Chandra X-Ray Observatory and detected two absorption features in the source spectrum. The features are centered near 0.7 and 1.4 keV; their equivalent widths are about 0.1 keV. We discuss various possible interpretations of the absorption features and exclude some of them. A likely interpretation is that the features are associated with atomic transitions of once-ionized helium in the neutron star atmosphere with a strong magnetic field. The first clear detection of absorption features in the spectrum of an isolated neutron star provides an opportunity to measure the mass-to-radius ratio and to constrain the equation of state of the superdense matter.

The Variable Jet of the Vela Pulsar

Observations of the Vela pulsar-wind nebula (PWN) with the Chandra X-ray Observatory have revealed a complex, variable PWN structure, including inner and outer arcs, a jet in the direction of the pulsar’s proper motion, and a counter-jet in the opposite direction, embedded in diffuse nebular emission. The jet consists of a bright, 8 ′′ -long inner jet, between the pulsar and the outer arc, and a dim, curved outer jet that extends up to ∼ 100 ′′ in approximately the same direction. From the analysis of thirteen Chandra observations spread over ≈ 2.5 years we found that this outer jet shows particularly stron g variability, changing its shape and brightness. We observed bright blobs in the outer jet moving away from the pulsar with apparent speeds (0.3‐0.6) c and fading on time-scales of days to weeks. If the blobs are carried away by a flow along the jet, the observed variations suggest mildly relativistic flow velocities, ab out (0.3‐0.7) c. The spectrum of the outer jet fits a power-law model with a photon index = 1.3 ± 0.1. For a distance of 300 pc, the apparent average luminosity of the outer jet in the 1‐8 keV band is about 3 × 10 30 erg s -1 , compared to 6 × 10 32 from the whole PWN within 42 ′′ from the pulsar. The X-ray emission of the outer jet can be interpreted as synchrotron radiation of ultrarelativistic electrons/positrons. This interpreta tion allows one to estimate the magnetic field, ∼ 100 µG, maximum energy of X-ray emitting electrons, ∼ 2 × 10 14 eV, and energy injection rate, ∼ 8 × 10 33 erg s -1 , for the outer jet. In the summed PWN image, we see a faint, strongly bent, extension of the outer jet. This bending could be caused by combined action of a wind within the supernova remnant, with a velocity of a few ×10 km s -1 , along with the ram pressure due to the pulsar’s proper motio n. The more extreme bends closer to the pulsar, as well as the apparent side motions of the outer j et, can be associated with kink instabilities of a magnetically confined, pinched jet flow. Another feature fou nd in the summed image is a dim, ∼ 2 ′ -long outer counter-jet, which also shows a power-law spectrum with ≈ 1.2‐1.5. Southwest of the jet/counter-jet (i.e., approximately perpendicular to the direction of pulsar’s p roper motion), an extended region of diffuse emission is seen. Relativistic particles responsible for this radia tion are apparently supplied by the outer jet. Subject headings: ISM: jets and outflows — pulsars: individual (Vela) — stars: n eutron — stars: winds, outflows — supernova remnants: individual (Vela) — X-rays: s tars

Flight spectral response of the ACIS instrument

We discuss the flight calibration of the spectral response of the Advanced CCD Imaging Spectrometer (ACIS) on-board the Chandra X-ray Observatory (CXO). The spectral resolution and sensitivity of the ACIS instrument have both been evolving over the course of the mission. The spectral resolution of the frontside-illuminated (FI) CCDs changed dramatically in the first month of the mission due to radiation damage. Since that time, the spectral resolution of the FI CCDs and the Backside-illuminated (BI) CCDs have evolved gradually with time. We demonstrate the efficacy of charge-transfer inefficiency (CTI) correction algorithms which recover some of the lost performance. The detection efficiency of the ACIS instrument has been declining throughout the mission, presumably due to a layer of contamination building up on the filter and/or CCDs. We present a characterization of the energy dependence of the excess absorption and demonstrate software which models the time dependence of the absorption from energies of 0.4 keV and up. The spectral redistribution function and the detection efficiency are well-characterized at energies from 1.5 to 8.0~keV primarily due to the existence of strong lines in the ACIS calibration source in that energy range. The calibration at energies below 1.5 keV is challenging because of the lack of strong lines in the calibration source and also because of the inherent non-linear dependence with energy of the CTI and the absorption by the contamination layer. We have been using data from celestial sources with relatively simple spectra to determine the quality of the calibration below 1.5 keV. We have used observations of 1E0102.2-7219 (the brightest supernova remnant in the SMC), PKS2155-304 (a bright blazar), and the pulsar PSR~0656+14 (nearby pulsar with a soft spectrum), since the spectra of these objects have been well-characterized by the gratings on the CXO. The analysis of these observations demonstrate that the CTI correction recovers a significant fraction of the spectral resolution of the FI CCDs and the models of the time-dependent absorption result in consistent measurements of the flux at low energies for data from a BI (S3) CCD.

Central Compact Objects in Supernova Remnants

There are point-like sources in central regions of several supernova remnants which have not been detected outside the X-ray range. The X-ray spectra of these Central Compact Objects (CCOs) have thermal components with blackbody temperatures of 0.2-0.5 keV and characteristic sizes of 0.3-3 km. Most likely, the CCOs are neutron stars born in supernova explosions. We overview their observational properties, emphasizing the Chandra data, and compare them with magnetars.

The jets of the Vela pulsar

Abstract Chandra observations of the Vela pulsar-wind nebula (PWN) have revealed a jet in the direction of the pulsar’s proper motion, and a counter-jet in the opposite direction, embedded in diffuse nebular emission. The jet consists of a bright, 8″-long inner jet, between the pulsar and the outer arc, and a dim, curved outer jet that extends up to ∼100″ in approximately the same direction. From the analysis of thirteen Chandra observations spread over ≈2.5 years we found that this outer jet shows particularly strong variability, changing its shape and brightness. We observed bright blobs in the outer jet moving away from the pulsar with apparent speeds (0.3– 0.6) c and fading on time-scales of days to weeks. The spectrum of the outer jet fits a power-law model with a photon index Γ=1.3±0.1. For a distance of 300 pc, the apparent average luminosity of the outer jet in the 1–8 keV band is about 3×1030 erg s−1, compared to 6×1032 from the whole PWN within 42″ from the pulsar. The X-ray emission of the outer jet can be interpreted as synchrotron radiation of ultrarelativistic electrons/positrons. This interpretation allows one to estimate the magnetic field, ∼100 μG, maximum energy of X-ray emitting electrons, ∼2×1014 eV, and energy injection rate, ∼8×1033 erg s−1, for the outer jet. In the summed PWN image we see a dim, ∼2′-long outer counter-jet, which also shows a power-law spectrum with Γ≈1.2–1.5. Southwest of the jet/counter-jet (i.e., approximately perpendicular to the direction of pulsar’s proper motion), an extended region of diffuse emission is seen. Relativistic particles responsible for this radiation are apparently supplied by the outer jet.