Observations of the Archetypical Supernova Remnant W28 (G6.4-0.1)

We present a joint analysis of optical emission-line and X-ray observations of the archetypical Galactic mixed-morphology supernova remnant (MMSNR) W28 (G6.4–0.1). MMSNRs comprise a class of sources whose shell-like radio morphology contrasts with a filled center in X-rays; the origin of these contrasting morphologies remains uncertain. Our CTIO images reveal enhanced [S ii] emission relative to Hα along the northern and eastern rims of W28. Hydroxyl (OH) masers are detected along these same rims, supporting prior studies suggesting that W28 is interacting with molecular clouds at these locations, as observed for several other MMSNRs. Our ROSAT HRI mosaic of W28 provides almost complete coverage of the supernova remnant (SNR). The X-ray and radio emission is generally anti-correlated, except for the luminous northeastern rim, which is prominent in both bands. Our Chandra observation sampled the X-ray-luminous central diffuse emission. Spectra extracted from the bright central peak and from nearby annular regions are best fit with two overionized recombining plasma models. We also find that while the X-ray emission from the central peak is dominated by swept-up material, that from the surrounding regions shows evidence for oxygen-rich ejecta, suggesting that W28 was produced by a massive progenitor. We also analyze the X-ray properties of two X-ray sources (CXOU J175857.55‑233400.3 and 3XMM J180058.5–232735) projected into the interior of W28 and conclude that neither is a neutron star associated with the SNR. The former is likely to be a foreground cataclysmic variable or a quiescent low-mass X-ray-binary, while the latter is likely to be a coronally active main-sequence star. Read more here.

The Millisecond Pulsar J2124-3358 and its Far-ultraviolet Bow Shock Nebula

We observed the nearby millisecond pulsar J2124-3358 with the Hubble Space Telescope in broad far-UV (FUV) and optical filters. The pulsar is detected in both bands with fluxes F(1250-2000 Å) = (2.5 ± 0.3) × 10-16 erg s-1 cm-2 and F(3800-6000 Å) = (6.4 ± 0.4) × 10-17 erg s-1 cm-2, which corresponds to luminosities of ≈5.8 × 1027 and 1.4 × 1027 erg s-1, for d = 410 pc and E(B – V) = 0.03. The optical-FUV spectrum can be described by a power-law model, fν ~ ν α , with slope α = 0.18-0.48 for a conservative range of color excess, E(B – V) = 0.01-0.08. Since a spectral flux rising with frequency is unusual for pulsar magnetospheric emission in this frequency range, it is possible that the spectrum is predominantly magnetospheric (power law with α < 0) in the optical, while it is dominated by thermal emission from the neutron star surface in the FUV. For a neutron star radius of 12 km, the surface temperature would be between 0.5 × 105 and 2.1 × 105 K for α ranging from -1 to 0, E(B – V) = 0.01-0.08, and d = 340-500 pc. In addition to the pulsar, the FUV images reveal extended emission that is spatially coincident with the known Hα bow shock, making PSR J2124-3358 the second pulsar (after PSR J0437-4715) with a bow shock detected in the FUV. Read more here.

Chandra Observations of the Pulsar Wind Nebula Created by PSR B0355+54

We report on Chandra X-ray Observatory (Chandra) observations of the pulsar wind nebula (PWN) associated with PSR B0355+54 (eight observations with a 395 ks total exposure, performed over an eight month period). We investigated the spatial and spectral properties of the emission coincident with the pulsar, compact nebula (CN), and extended tail. We find that the CN morphology can be interpreted in a way that suggests a small angle between the pulsar spin axis and our line of sight, as inferred from the radio data. On larger scales, emission from the 7′ (≈ 2 pc) tail is clearly seen. We also found hints of two faint extensions nearly orthogonal to the direction of the pulsar’s proper motion. The spectrum extracted at the pulsar position can be described with an absorbed power-law + blackbody model. The nonthermal component can be attributed to magnetospheric emission, while the thermal component can be attributed to emission from either a hot spot (e.g., a polar cap) or the entire neutron star surface. Surprisingly, the spectrum of the tail shows only a slight hint of cooling with increasing distance from the pulsar. This implies either a low magnetic field with fast flow speed, or particle reacceleration within the tail. We estimate physical properties of the PWN and compare the morphologies of the CN and the extended tail with those of other bow shock PWNe observed with long Chandra exposures. Read more here.

First Detection of a Pulsar Bow Shock Nebula in Far-UV: PSR J0437-4715

Pulsars traveling at supersonic speeds are often accompanied by cometary bow shocks seen in Hα. We report on the first detection of a pulsar bow shock in the far-ultraviolet (FUV). We detected it in FUV images of the nearest millisecond pulsar J0437-4715 obtained with the Hubble Space Telescope. The images reveal a bow-like structure positionally coincident with part of the previously detected Hα bow shock, with an apex at 10″ ahead of the moving pulsar. Its FUV luminosity, L(1250-2000 A )≈ 5 × 1028 erg s-1, exceeds the Hα luminosity from the same area by a factor of 10. The FUV emission could be produced by the shocked interstellar medium matter or, less likely, by relativistic pulsar wind electrons confined by strong magnetic field fluctuations in the bow shock. In addition, in the FUV images we found a puzzling extended (≃3″ in size) structure overlapping with the limb of the bow shock. If related to the bow shock, it could be produced by an inhomogeneity in the ambient medium or an instability in the bow shock. We also report on a previously undetected X-ray emission extending for about 5″ ahead of the pulsar, possibly a pulsar wind nebula created by shocked pulsar wind, with a luminosity L(0.5-8 keV) ~ 3 × 1028 erg s-1. Read more here.

Outflows from the Pulsar J1509-5850

PSR J1509-5850 is a middle-aged pulsar with a period of P ≈ 89 ms and spin-down power of E_dot=5.1× 1035 erg s-1, at a distance of about 3.8 kpc. We report on deep Chandra X-ray Observatory observations of this pulsar and its pulsar wind nebula (PWN). In addition to the previously detected tail extending up to 7‧ southwest from the pulsar (the southern outflow), the deep images reveal similarly long, faint, diffuse emission stretched toward the north (the northern outflow) and the fine structure of the compact nebula (CN) in the pulsar vicinity. The CN is resolved into two lateral tails and one axial tail pointing southwest (a morphology remarkably similar to that of the Geminga PWN), which supports the assumption that the pulsar moves toward the northeast. The luminosities of the southern and northern outflows are about 1× 1033 and 4× 1032 erg s-1, respectively. The spectra extracted from four regions of the southern outflow do not show any softening with increasing distance from the pulsar. The lack of synchrotron cooling suggests a high flow speed or in situ acceleration of particles. The spectra extracted from two regions of the northern outflow show a hint of softening with distance from the pulsar, which may indicate slower particle propagation. We speculate that the northern outflow is associated with particle leakage from the bow-shock apex into the ISM, while the southern outflow represents the tail of the shocked pulsar wind behind the moving pulsar. We estimate the physical parameters of the observed outflows and compare the J1509-5850 PWN with PWNe of other supersonically moving pulsars. Read more here.


X-Ray Sources in the Dwarf Spheroidal Galaxy Draco

We present the spectral analysis of an 87 ks XMM-Newton observation of Draco, a nearby dwarf spheroidal galaxy. Of the approximately 35 robust X-ray source detections, we focus our attention on the brightest of these sources, for which we report X-ray and multiwavelength parameters. While most of the sources exhibit properties consistent with active galactic nuclei, few of them possess the characteristics of low-mass X-ray binaries (LMXBs) and cataclysmic variable (CVs). Our analysis places constraints on the population of X-ray sources with LX > 3 × 1033 erg s-1 in Draco, suggesting that there are no actively accreting black hole and neutron star binaries. However, we find four sources that could be quiescent state LMXBs/CVs associated with Draco. We also place constraints on the central black hole luminosity and on a dark matter decay signal around 3.5 keV. Read more here.

Color-coded (Blue: u, Green: g, and Red: r) SDSS image of Draco. The X-ray sources are shown with green crosses.

Multi-wavelength Study of HESS J1741-302

We present the results of two Chandra X-ray Observatory (Chandra) observations of TeV γ-ray source HESS J1741-302. We investigate whether there is any connection between HESS J1741-302 and the sources seen at lower energies. One of the brightest X-ray sources in the HESS J1741-302 field, CXOU J174112.1-302908, appears to be associated with a low-mass star (possibly representing a quiescent low-mass X-ray binary or cataclysmic variable (CV)), hence, it is unlikely to be a source of TeV γ-rays. In the same field we have potentially detected X-rays from WR 98a, which is likely to be a colliding wind binary with massive stars. No TeV emission has been reported so far from such systems although predictions have been made. Finally, we found that the previously reported Suzaku source, Suzaku J1740.5-3014 (which is not covered by the Chandra observations), appears to be a hard X-ray source detected by INTERGAL ISGRI, which supports the magnetized CV classification but makes its association with the TeV emission unlikely. The young pulsar PSR B1737-30, so far undetected in X-rays and projected on the sky near the CV, may be the contributor of relativistic particles responsible for the TeV emission. Read more here.

Spitzer (Fadda et al. 2006) image (8.0 μm) of the HESS region. The green and magenta squares represent the Chandra and Suzaku observations, respectively (with the corresponding ObsIDs on top). The yellow circles show the locations and extension of the two bright regions within the HESS source (smaller circle: J1741A, larger circle: J1741B). The three pulsars (PSR B1737−30, PSR J1741−3016, and PSR J1739−3023) are shown with filled white circles.

Magnesium-rich Ejecta in the SNR G284.3-1.8

We present results from two Chandra observations of the 16.6 day X-ray/γ-ray high-mass binary 1FGL J1018.6-5856 located at the center of the supernova remnant (SNR) G284.3-1.8. The binary spectra, separated by 0.25 in binary phase, are fit with an absorbed power-law model with Γ ≈ 1.7-1.8 for both observations (the flux during the second observation is a factor of 1.7 smaller). In the high-resolution ACIS-I image we found a hint of extended emission ≈ 2-3″ southeast of the binary, significant at the 3σ level. Binary evolution codes reproduce the system’s observed properties with two massive stars with an initial 18 day period, undergoing mass transfer and leaving behind a heavy ≈ 2 Mneutron star. The initial mass of the progenitor star in this scenario is 27 ± 4 M. Chandra and XMM-Newton images of the remnant show it has a relatively low X-ray surface brightness. The two brightest regions of extended X-ray emission, with luminosities ~1033 erg s-1 for d = 5 kpc, lie in the northern and western portions and show significantly different spectra. The northern patch is consistent with shocked ISM, with a low temperature and long ionization timescale. However, the western patch is dominated by ejecta, and shows significantly enhanced Mg content relative to other ejecta products. The abundance ratios inferred resemble those from the Large Magellanic Cloud remnant N49B. To our knowledge, this is only the second case of such Mg-rich ejecta found in an SNR. Nucleosynthesis models for core-collapse supernovae predict Mg-rich ejecta from very massive progenitors of > 25 M. Read more here.

X-ray spectra from the “west” region of G284, with Chandra ACIS-I data shown in black, and XMM-Newton MOS 1 and 2 shown in red and green, respectively. Overlaid is an absorbed vpshock model fit.

Extended X-Ray Object Ejected from γ-ray Binary

We present the analysis of Chandra X-ray Observatory observations of the eccentric γ-ray binary PSR B1259-63/LS 2883. The analysis shows that the extended X-ray feature seen in previous observations is still moving away from the binary with an average projected velocity of ≈ 0.07c and shows a hint of acceleration. The spectrum of the feature appears to be hard (photon index Γ ≈ 0.8) with no sign of softening compared to previously measured values. We interpret it as a clump of plasma ejected from the binary through the interaction of the pulsar with the decretion disk of the O-star around periastron passage. We suggest that the clump is moving in the unshocked relativistic pulsar wind (PW), which can accelerate the clump. Its X-ray emission can be interpreted as synchrotron radiation of the PW shocked by the collision with the clump. Read more here.

The image shows the false color X-ray image (different colors correspond to different observation epochs) demonstrating the motion and changes in morphology. The dashed line connects the binary and the centroid of the feature in the new data image.

Multiwavelength Study of HESS J1809-193

HESS J1809-193 is an extended TeV gamma-ray source in the Galactic Plane. Multiwavelength observations of the HESS J1809-193 field reveal a complex picture. We present results from three CXO and two Suzaku observations of a region in the northeastern outskirts of HESS J1809-193, where enhanced TeV emission has been reported. Our analysis also includes GeV gamma-ray and radio data. One of the X-ray sources in the field is the X-ray binary XTE J1810-189, for which we present the outburst history from multiple observatories and confirm that XTE J1810-189 is a strongly variable type I X-ray burster, which can hardly produce TeV emission. We investigate whether there is any connection between the possible TeV extension of HESS J1809-193 and the sources seen at lower energies. We find that another X-ray binary candidate, Suzaku J1811-1900, and a radio supernova remnant, SNR G11.4-0.1, can hardly be responsible for the putative TeV emission. Our multiwavelength classification of fainter X-ray point sources also does not produce a plausible candidate. We conclude that the northeast extension of HESS J1809-193, if confirmed by deeper observations, can be considered as a dark accelerator – a TeV source without visible counterpart at lower energies. Read the full text here.

On the right is the MW coverage of the Suzaku X-ray source J1811. All the images are 15′ × 13′ and aligned. Upper Left: Suzaku 0.5 to 10 keV smoothed image, with contour levels spaced linearly in brightness. Darker color corresponds to brighter X-ray emis- sion. The contours are overlaid on the other images. The red cross marks the nominal position of the compact source. Upper Right: Hα image from the SuperCOSMOS Hα Survey (Parker et al. 2005). Darker color corresponds to brighter Hα emission. Lower Left: 1.4 GHz VLA radio image from the Multi-Array Galactic Plane Imaging Survey (White et al. 2005). Lighter colors emphasize brighter radio emission. Lower Right: Spitzer MIPS 24 μm image of the source region. Lighter colors emphasize brighter IR emission.