Divas Sanwal

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

Discovery of 424 Millisecond Pulsations from the Radio-quiet Neutron Star in the Supernova Remnant PKS 1209–51/52

The central source of the supernova remnant PKS 1209-52 was observed with the Advanced CCD Imaging Spectrometer aboard Chandra X-ray observatory on 2000 January 6-7. The use of the Continuos Clocking mode allowed us to perform the timing analysis of the data with time resolution of 2.85 ms and to find a period P=0.42412927+/-2.3e-7 s. The detection of this short period proves that the source is a neutron star. It may be either an active pulsar with unfavorably directed radio beam or a truly radio-silent neutron star whose X-ray pulsations are caused by a nonuniform distribution of surface temperature. To infer the actual properties of this neutron star, the period derivative should be measured.The central source of the supernova remnant PKS 1209-51/52 was observed with the Advanced CCD Imaging Spectrometer aboard Chandra X-Ray Observatory on 2000 January 6-7. The use of the continuous clocking mode allowed us to perform the timing analysis of the data with a time resolution of 2.85 ms and to find a period P = 0.42412924 s ± 0.23 μs. The detection of this short period proves that the source is a neutron star. It may be either an active pulsar with an unfavorably directed radio beam or a truly radio-silent neutron star whose X-ray pulsations are caused by a nonuniform distribution of surface temperature. To infer the actual properties of this neutron star, the period derivative should be measured.

The Compact Central Object in Cassiopeia A: A Neutron Star with Hot Polar Caps or a Black Hole?

The central pointlike X-ray source of the Cassiopeia A supernova remnant was discovered in the Chandra first light observation and found later in the archival ROSAT and Einstein images. The analysis of these data does not show statistically significant variability of the source. Because of the small number of photons detected, different spectral models can fit the observed spectrum. The power-law fit yields the photon index gamma=2.6-4.1, and luminosity L(0.1-5.0 keV&parr0;=&parl0;2-60&parr0;x1034 ergs s-1 for d=3.4 kpc. The power-law index is higher, and the luminosity lower, than those observed from very young pulsars. One can fit the spectrum equally well with a blackbody model with T=6-8 MK, R=0.2-0.5 km, and Lbol=&parl0;1.4-1.9&parr0;x1033 ergs s-1. The inferred radii are too small, and the temperatures too high, for the radiation to be interpreted as emitted from the whole surface of a uniformly heated neutron star. Fits with the neutron star atmosphere models increase the radius and reduce the temperature, but these parameters are still substantially different from those expected for a young neutron star. One cannot exclude, however, the possibility that the observed emission originates from hot spots on a cooler neutron star surface. An upper limit on the (gravitationally redshifted) surface temperature is Tinfinitys<1.9-2.3 MK, depending on the chemical composition of the surface and the stars radius. Among several possible interpretations, we favor a model of a strongly magnetized neutron star with magnetically confined hydrogen or helium polar caps (Tinfinitypc approximately 2.8 MK, Rpc approximately 1 km) on a cooler iron surface (Tinfinitys approximately 1.7 MK). Such temperatures are consistent with the standard models of neutron star cooling. Alternatively, the observed radiation may be interpreted as emitted by a compact object (more likely, a black hole) accreting from a residual disk or from a late-type dwarf in a close binary.

Variability of the Vela Pulsar Wind Nebula Observed with Chandra

The observations of the pulsar wind nebula (PWN) around the Vela pulsar with the Advanced CCD Imaging Spectrometer aboard the Chandra X-Ray Observatory, taken on 2000 April 30 and November 30, reveal its complex morphology reminiscent of that of the Crab PWN. Comparison of the two observations shows changes up to 30% in the surface brightness of the PWN features. Some of the PWN elements show appreciable shifts, up to a few arcseconds (~1016 cm), and/or spectral changes. Such variations hold the potential to provide a new diagnostics for the study of PWNe.

X-RADIATION FROM THE MILLISECOND PULSAR J0437 4715

We report on spectral and timing observations of the nearest millisecond pulsar, J0437-4715, with the Chandra X-Ray Observatory. The pulsar spectrum, detected up to 7 keV, cannot be described by a simple one-component model. We suggest that it consists of two components: a nonthermal power-law spectrum generated in the pulsar magnetosphere, with a photon index ? ? 2, and a thermal spectrum emitted by heated polar caps, with a temperature decreasing outward from 2 to 0.5 MK. The lack of spectral features in the thermal component suggests that the neutron star surface is covered by a hydrogen (or helium) atmosphere. The timing analysis shows one X-ray pulse per period, with a pulsed fraction of about 40% and the peak at the same pulse phase as the radio peak. No synchrotron pulsar-wind nebula is seen in X-rays.

IE 1207.4-520: the puzzling pulsar at the center of the supernova remnant PKS 1209-51/52

A second Chandra observation of 1E 1207.4-5209, the central source of the supernova remnant (SNR) PKS 1209-51/52, allowed us to confirm the previously detected period of 424 ms and, assuming a uniform spin-down, estimate the period derivative, ~ (0.7-3) × 10-14 s s-1. The corresponding characteristic age of the pulsar, P/2 ~ 200-900 kyr, is much larger than the estimated age of the SNR, ~7 kyr. The values of the spin-down luminosity, ~ (0.4-1.6) × 1034 ergs s-1, and conventional magnetic field, B ~ (2-4) × 1012 G, are typical for a middle-aged radio pulsar, although no manifestations of pulsar activity have been observed. If 1E 1207.4-5209 is indeed the neutron star formed in the same supernova explosion that created PKS 1209-51/52, such a discrepancy in ages could be explained either by a long initial period, close to its current value, or, less likely, by a very large braking index of the pulsar. Alternatively, the pulsar could be a foreground object unrelated to the SNR, but the probability of such a coincidence is very low.Second Chandra observation of 1E 1207.4-5209, the central source of the supernova remnant PKS 1209-51/52, allowed us to confirm the previously detected period of 424 ms and, assuming a uniform spin-down, estimate the period derivative, Pdot (0.7-3)e-14 s/s. The corresponding characteristic age of the pulsar, P/2Pdot~200-900 kyr, is much larger than the estimated age of the SNR, \~7 kyr. The values of the spin-down luminosity, Edot (0.4-1.6)e34 erg/s, and conventional magnetic field, B (2-4)e12 G, are typical for a middle-aged radio pulsar, although no manifestations of pulsar activity have been observed. If 1E 1207.4-5209 is indeed the neutron star formed in the same supernova explosion that created PKS 1209-51/52, such a discrepancy in ages could be explained either by a long initial period, close to its current value, or, less likely, by a very large braking index of the pulsar. Alternatively, the pulsar could be a foreground object unrelated to the supernova remnant, but the probability of such a coincidence is very low.

An XMM-Newton observation of the drifting pulsar B0943+10

Radio pulsar subpulse drifting has been interpreted as rotation of subbeams (sparks) of pair plasma produced by intermittent breakdowns of an inner vacuum gap above the pulsar polar cap. This model also predicts strong thermal X-ray emission from the polar cap caused by inflowing particles created in spark discharges. We have observed the best-studied drifting pulsar B0943+10 with XMM-Newton and detected a point source coincident with the radio pulsar position. Its spectrum could be fitted with a thermal blackbody model, although a power-law model is also acceptable. The thermal fit gives a bolometric luminosity Lbol ≈ 5 × 1028 ergs s-1 and a surface area A ≈ 103(T/3 MK)-4 m2, much smaller than the conventional polar cap area, 6 × 104 m2. Such thermal radiation can be interpreted as emitted from footprints of sparks drifting in an inner gap of a height h ~ (0.1-0.2)rpc, where rpc is the polar cap radius. However, the original vacuum gap model by Ruderman & Sutherland requires some modification to reconcile the X-ray and radio data.

Variations in the spin period of the radio-quiet pulsar 1E 1207.4-5209

The X-ray source 1E 1207.4-5209 is a compact central object in the G296.5+10.0 supernova remnant. Its spin period of 424 ms, discovered with Chandra, suggests that it is a neutron star. The X-ray spectrum of this radio-quiet pulsar shows at least two absorption lines, the first spectral features discovered in radiation from an isolated neutron star. Here we report the results of timing analyses of Chandra and XMM-Newton observations of this source showing a nonmonotonous behavior of its period. We discuss three hypotheses that might explain the observational result. The first assumes that 1E 1207.4-5209 is a glitching pulsar, with frequency jumps of Δf 5 μHz occurring every 1-2 yr. The second hypothesis explains the deviations from a steady spin-down as being due to accretion (with the accretion rate varying from ~1013 to 1016 g s-1) from a disk possibly formed from ejecta produced in the supernova explosion. Finally, the period variations might be explained by assuming the pulsar is in a wide binary system with a long period Porb ~ 0.2-6 yr and a low-mass companion M2 < 0.3 M☉.

CONSTRAINING THE EVOLUTION OF ZZ CETI

We report our analysis of the stability of pulsation periods in the DAV star (pulsating hydrogen atmosphere white dwarf) ZZ Ceti, also called R548. On the basis of observations that span 31 years, we conclude that the period 213.13 s observed in ZZ Ceti drifts at a rate dP/dt ≤ (5.5 ± 1.9) × 10-15 s s-1, after correcting for proper motion. Our results are consistent with previous values for this mode and an improvement over them because of the larger time base. The characteristic stability timescale implied for the pulsation period is P/ ≥ 1.2 Gyr, comparable to the theoretical cooling timescale for the star. Our current stability limit for the period 213.13 s is only slightly less than the present measurement for another DAV, G117-B15A, for the period 215.2 s, establishing this mode in ZZ Ceti as the second most stable optical clock known, comparable to atomic clocks and more stable than most pulsars. Constraining the cooling rate of ZZ Ceti aids theoretical evolutionary models and white dwarf cosmochronology. The drift rate of this clock is small enough that we can set interesting limits on reflex motion due to planetary companions.