arXiv subjects

G. G. Pavlov

Publications and source records attributed to G. G. Pavlov.

At least 73 records · Page 4Linked to original sources

arXiv2007

X-ray emission from the planet pulsar B1257+12

We report the detection of the millisecond pulsar B1257+12 with the Chandra X-ray Observatory. In a 20 ks exposure we detected 25 photons from the pulsar, with energies between 0.4 and 2.0 keV, corresponding to the flux F_X=(4.4+/- 0.9)*10^{-15} ergs s^{-1} cm^{-2} in this energy range. The X-ray spectrum can be described by a power-law model with photon index Gamma = 2.8 and luminosity L_X \approx 2.5*10^{29} ergs s^{-1} in the 0.3--8 keV band, for a plausible distance of 500 pc and hydrogen column density N_H=3*10^{20} cm^{-2}. Alternatively, the spectrum can be fitted by a blackbody model with kT ~ 0.22 keV and projected emitting area ~2000 m^2. If the thermal X-rays are emitted from two symmetric polar caps, the bolometric luminosity of the two caps is 2 L_bol ~ 3*10^{29} ergs s^{-1}. We compared our results with the data on other 30 millisecond pulsars observed in X-rays and found that the apparent X-ray efficiency of PSR B1257+12, L_X/Edot ~ 3*10^{-5} for d=500 pc, is lower than those of most of millisecond pulsars. This might be explained by an unfavorable orientation of the X-ray pulsar beam if the radiation is magnetospheric, or by strong asymmetry of polar caps if the radiation is thermal (e.g., one of the polar caps is much brighter than the other and remains invisible for most part of the pulsar period). Alternatively, it could be attributed to absorption of X-rays in circumpulsar matter, such as a flaring debris disk left over after formation of the planetary system around the pulsar.

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arXiv2007

TeV source HESS J1804-216 in X-rays and other wavelengths

The field of the extended TeV source HESS J1804-216 was serendipitously observed with the Chandra ACIS detector. The data reveal several X-ray sources within the bright part of HESS J1804-216. The brightest of these objects, CXOU J180432.4-214009, which has been also detected with Swift and Suzaku, is consistent with being a point-like source, with the 0.3-7 keV flux of (1.7\pm0.2)\times10^{-13} ergs s^{-1} cm^{-2}. Its hard and strongly absorbed spectrum can be fitted by the absorbed power-law model with the best-fit photon index of 0.45 and hydrogen column density of 4\times10^{22} cm^{-2}, both with large uncertainties due to the strong correlation between these parameters. A search for pulsations resulted in a 106 s period candidate, which however has a low significance of 97.9%. We found no infrared-optical counterparts for this source. The second brightest source, CXOU J180441.9-214224, which has been detected with Suzaku, is either extended or multiple, with the flux of about 10^{-13} ergs cm^{-2} s^{-1}. We found a nearby M dwarf within the X-ray source extension, which could contribute a fraction of the observed X-ray flux. The remaining sources are very faint (<3\times 10^{-14} ergs cm^{-2} s^{-1}), and at least some of them are likely associated with nearby stars. Although one or both of the two brighter X-ray sources could be faint accreting binaries or remote pulsars with pulsar wind nebulae (hence possible TeV sources), their relation to HESS J1804-216 remains elusive. The possibility that HESS J1804-216 is powered by the relativistic wind from the young pulsar B1800-21, located at a distance of about 10 pc from the TeV source, still remains a more plausible option.

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arXiv2006

X-ray emission from PSR B1800-21, its wind nebula, and similar systems

We detected X-ray emission from PSR B1800-21 and its synchrotron nebula with the Chandra X-ray Observatory. The pulsar's observed flux is (1.4+/-0.2) 10^{-14} ergs cm^{-2} s^{-1} in the 1-6 keV band. The spectrum can be described by a two-component PL+BB model, suggesting a mixture of thermal and magnetospheric emission. For a plausible hydrogen column density n_{H}=1.4 10^{22} cm^{-2}, the PL component has a slope Gamma=1.4+/-0.6 and a luminosity L_{psr}^{nonth}=4 10^{31}(d/4 kpc)^2 ergs s^{-1}. The properties of the thermal component (kT=0.1-0.3 keV, L^{bol}=10^{31}-10^{33} ergs s^{-1}) are very poorly constrained because of the strong interstellar absorption. The compact, 7''\times4'', inner pulsar-wind nebula (PWN), elongated perpendicular to the pulsar's proper motion, is immersed in a fainter asymmetric emission. The observed flux of the PWN is (5.5+/-0.6) 10^{-14} ergs cm^{-2} s^{-1} in the 1-8 keV band. The PWN spectrum fits by a PL model with Gamma=1.6+/-0.3, L=1.6 10^{32} (d/4 kpc})^2 ergs s^{-1}. The shape of the inner PWN suggests that the pulsar moves subsonically and X-ray emission emerges from a torus associated with the termination shock in the equatorial pulsar wind. The inferred PWN-pulsar properties (e.g., the PWN X-ray efficiency, L_{pwn}/\dot{E}~10^{-4}; the luminosity ratio, L_{pwn}/L_{psr}^{nonth}=4; the pulsar wind pressure at the termination shock, p_s=10^{-9} ergs cm^{-3}) are very similar to those of other subsonically moving Vela-like objects detected with Chandra (L_{pwn}/\dot{E}=10^{-4.5}-10^{-3.5}, L_{pwn}/L_{psr}^{nonth}~5, p_s=10^{-10}-10^{-8} ergs cm^{-1}).

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arXiv2006

The prospects for X-ray polarimetry and its potential use for understanding neutron stars

We review the state of the art for measuring the X-ray polarization of neutron stars. We discuss how valuable precision measurements of the degree and position angle of polarization as a function of energy and, where relevant, of pulse phase, would provide deeper insight into the details of the emission mechanisms. We then review the current state of instrumentation and its potential for obtaining relevant data. Finally, we conclude our discussion with some opinions as to future directions.

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arXiv2006

UV emission from young and middle-aged pulsars: Connecting X-rays with the optical

We present the UV spectroscopy and timing of three nearby pulsars (Vela, B0656+14 and Geminga) recently observed with the Space Telescope Imaging Spectrograph. We also review the optical and X-ray properties of these pulsars and establish their connection with the UV properties. We show that the multiwavelengths properties of neutron stars (NSs) vary significantly within the sample of middle-aged pulsars. Even larger differences are found between the thermal components of Ge-minga and B0656+14 as compared to those of radio-quiet isolated NSs. These differences could be attributed to different properties of the NS surface layers.

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arXiv2006

On the nature of the hard X-ray source IGR J2018+4043

We found a very likely counterpart to the recently discovered hard X-ray source IGR J2018+4043 in the multi-wavelength observations of the source field. The source, originally discovered in the 20-40 keV band, is now confidently detected also in the 40-80 keV band, with a flux of (1.4 +/- 0.4) x 10(-11) erg cm(-2) s(-1). A 5 ks Swift observation of the IGR J2018+4043 field revealed a hard point-like source with the observed 0.5-10 keV flux of 3.4(+0.7)(-0.8) x 10(-12) erg cm(-2) s(-1) (90% confidence level) at alpha = 20h18m38.55s, delta = +40d41m00.4s (with a 4.2" uncertainty). The combined Swift-INTEGRAL spectrum can be described by an absorbed power-law model with photon index gamma = 1.3 +/- 0.2 and N_H = 6.1(+3.2)(-2.2) x 10(22) cm(-2). In archival optical and infrared data we found a slightly extended and highly absorbed object at the Swift source position. There is also an extended VLA 1.4 GHz source peaked at a beam-width distance from the optical and X-ray positions. The observed morphology and multiwavelength spectra of IGR J2018+4043 are consistent with those expected for an obscured accreting object, i.e. an AGN or a Galactic X-ray binary. The identification suggests possible connection of IGR J2018+4043 to the bright gamma-ray source GEV J2020+4023 (3EG J2020+4017) detected by COS B and CGRO EGRET in the gamma-Cygni SNR field.

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arXiv2006

Chandra X-Ray Observatory Observations of Neutron Stars: An Overview

We present a brief review of Chandra X-ray Observatory observations of neutron stars. The outstanding spatial and spectral resolution of this great observatory have allowed for observations of unprecedented clarity and accuracy. Many of these observations have provided new insights into neutron star physics. We present an admittedly biased and overly brief overview of these observations, highlighting some new discoveries made possible by the Observatory's unique capabilities. We also include our analysis of recent multiwavelength observations of the putative pulsar and its pulsar-wind nebula in the IC 443 SNR.

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arXiv2006

X-ray emission from the double neutron star binary B1534+12: Powered by the pulsar wind?

We report the detection of the double neutron star binary (DNSB) B1534+12 (= J1537+1155) with the Chandra X-ray Observatory. This DNSB (orbital period 10.1 hr) consists of the millisecond (recycled) pulsar J1537+1155A (P_A=37.9 ms) and a neutron star not detected in the radio. After the remarkable double pulsar binary J0737-3039, it is the only other DNSB detected in X-rays. We measured the flux of (2.2\pm 0.6)\times10^{-15} ergs s^{-1} cm^{-2} in the 0.3-6 keV band. The small number of collected counts allows only crude estimates of spectral parameters. The power-law fit yields the photon index of 3.2\pm 0.5 and the unabsorbed 0.2-10 keV luminosity L_X=6\times10^{29} ergs s^{-1} = 3\times 10^{-4}Edot_A, where Edot_A is the spin-down power of J1537+1155A. Alternatively, the spectrum can be fitted by a blackbody model with T = 2.2 MK and the projected emitting area of ~ 5\times 10^3 m^2. The distribution of photon arrival times over binary orbital phase shows a deficit of X-ray emission around apastron, which suggests that the emission is caused by interaction of the relativistic wind from J1537+1155A with its neutron star companion. We also reanalyzed the Chandra and XMM-Newton observations of J0737-3039 and found that its X-ray spectrum is similar to the spectrum of B1534+12, and its X-ray luminosity is about the same fraction of Edot_A, which suggests similar X-ray emission mechanisms. However, the X-ray emission from J0737-3039 does not show orbital phase dependence. This difference can be explained by the smaller eccentricity of J0737-3039 or a smaller misalignment between the equatorial plane of the millisecond pulsar and the orbital plane of the binary.

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arXiv2006

The pulsar wind nebula of the Geminga pulsar

The superb spatial resolution of Chandra has allowed us to detect a 20''-long tail behind the Geminga pulsar, with a hard spectrum (photon index 1.0+/-0.2) and a luminosity (1.3+/-0.2) 10^{29} ergs/s in the 0.5 - 8 keV band, for an assumed distance of 200 pc. The tail could be either a pulsar jet, confined by a toroidal magnetic field of about 100 microGauss, or it can be associated with the shocked relativistic wind behind the supersonically moving pulsar confined by the ram pressure of the oncoming interstellar medium. We also detected an arc-like structure 5'' - 7'' ahead of the pulsar, extended perpendicular to the tail, with a factor of 3 lower luminosity. We see a 3-sigma enhancement in the Chandra image apparently connecting the arc with the southern outer tail that has been possibly detected with XMM-Newton. The observed structures imply that the Geminga's pulsar wind is intrinsically anisotropic.

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arXiv2006

A Chandra/ACIS Study of 30 Doradus I. Superbubbles and Supernova Remnants

We present an X-ray tour of diffuse emission in the 30 Doradus star-forming complex in the Large Magellanic Cloud using high-spatial-resolution X-ray images and spatially-resolved spectra obtained with the Advanced CCD Imaging Spectrometer aboard the Chandra X-ray Observatory. The dominant X-ray feature of the 30 Doradus nebula is the intricate network of diffuse emission generated by interacting stellar winds and supernovae working together to create vast superbubbles filled with hot plasma. We construct maps of the region showing variations in plasma temperature (T = 3--9 million degrees), absorption (N_H = 1--6 x 10^{21} cm^{-2}), and absorption-corrected X-ray surface brightness (S_X = 3--126 x 10^{31} ergs s^{-1} pc^{-2}). Enhanced images reveal the pulsar wind nebula in the composite supernova remnant N157B and the Chandra data show spectral evolution from non-thermal synchrotron emission in the N157B core to a thermal plasma in its outer regions. In a companion paper we show that R136, the central massive star cluster, is resolved at the arcsecond level into almost 100 X-ray sources. Through X-ray studies of 30 Doradus the complete life cycle of such a massive stellar cluster can be revealed.

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arXiv2005

X-ray Emission from the nearby PSR B1133+16

We detected a nearby (d=360 pc), old (5 Myr) pulsar B1133+16 with Chandra. The observed pulsar's flux is $(0.8\pm 0.2)\times 10^{-14}$ ergs cm$^{-2}$ s$^{-1}$ in the 0.5-8 keV band. Because of the small number of counts detected, the spectrum can be described by various models. A power-law fit of the spectrum gives a photon index $Γ= 2.5$ and an isotropic luminosity of $1.4\times 10^{29}$ ergs s$^{-1}$ in the 0.5-8 keV band, which is about $1.6\times 10^{-3}$ of the spin-down power $\dot{E}$. The spectrum can also be fitted by a blackbody model with a temperature of $\approx 2.8$ MK and a projected emitting area of $\sim 500$ m$^2$, possibly a hot polar cap. The X-ray properties of PSR B1133+16 are similar to those of other old pulsars observed in X-rays, particularly the drifting pulsar B0943+10.

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arXiv2005

Near-Infrared and Optical Limits for the Central X-ray Point Source in the Cassiopeia A Supernova Remnant

Near-infrared images of the center of the Cas A supernova remnant taken with the NICMOS 2 camera on HST using the F110W and F160W filters (~J & ~H bands) have magnitude limits of 26.2 and 24.6 respectively, but reveal no sources within a 1.2" radius of the Chandra detected central X-ray point source (XPS). The NICMOS data, taken together with broadband optical magnitude limits (R ~ 28 mag) obtained from a deep STIS CCD exposure taken with a clear filter (50CCD), indicate that the XPS luminosities are very low in the optical/NIR bands (e.g., L_H < 3 x E29 erg/s) with no optical, J, or H band counterpart to the XPS easily detectable by HST. We discuss the nature of the Cas A central compact object based upon these new near-infrared and optical flux limits.

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arXiv2005

Hard Extended X-ray Source in the IC 443 SNR Resolved by Chandra: A Fast Ejecta Fragment or a New Pulsar Wind Nebula?

A Chandra observation of the isolated hard X-ray source XMMU J061804.3+222732, located in the region of apparent interaction of the supernova remnant IC 443 with a molecular cloud, resolved the complex structure of the source in a few bright clumps embedded in an extended emission of a ~ 30 arcsec size. The X-ray spectra of the clumps and the extended emission are dominated by a hard power-law component with a photon index of 1.2--1.4. In addition, we see some indications of an optically thin thermal plasma of a ~ 0.3 keV temperature. The observed X-ray morphology and spectra are consistent with those expected for an isolated supernova ejecta fragment interacting with a dense ambient medium. A possible alternative interpretation is a pulsar wind nebula associated with either IC 443 or another SNR, G189.6+3.3.

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arXiv2005

Ultraviolet, X-ray, and Optical Radiation from the Geminga Pulsar

We observed the gamma-ray pulsar Geminga with the HST STIS/MAMA detectors to measure the Geminga's UV spectrum and pulsations. The slope of the far-UV (FUV) spectrum is close to that of a Rayleigh-Jeans spectrum, suggesting that the FUV radiation is dominated by thermal emission from the neutron star (NS) surface. The measured FUV flux, F_FUV=(3.7+/-0.2)x10^-15 ergs cm^-2 s^-1 in 1155-1702 A band, corresponds to a brightness temperature T_RJ=(0.3-0.4)(d_200/R_13)^2 MK. The soft thermal component of the Geminga's X-ray spectrum measured with the XMM-Newton observatory corresponds to a temperature T_s=0.49+/-0.01 MK and radius R_s = (12.9+/-1.0)d_200 km. Unlike other NSs detected in the UV-optical, the FUV spectrum of Geminga lies below the extrapolation of the soft thermal component, which might be associated with the Geminga's very low temperature. Surprisingly, the thermal FUV radiation is strongly pulsed, showing a narrow dip at a phase close to that of a broader minimum of the soft X-ray light curve. The strong pulsations might be attributed to partial occultations of the thermal UV radiation by regions of the magnetosphere filled with electron/positron plasma. In contrast with the FUV spectrum, the near-IR (NIR) through near-UV (NUV) spectrum of Geminga is clearly nonthermal. It can be described by a power-law model, F_nu \propto nu^{-alpha} with a slope alpha = 0.43+/-0.15, close to alpha = 0.56+/-0.24 of the hard X-ray (E>2.5 keV) magnetospheric component. The observed NIR-optical-NUV spectrum is marginally consistent with the extrapolation of the X-ray magnetospheric spectrum. The NUV pulsations, however, do not show a clear correlation with the hard X-ray pulsations.

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arXiv2004

Search for the elusive optical counterpart of PSR J0537-6910 with the HST Advanced Camera for Surveys

We present the results of deep, high-resolution, multi-band optical observations of the field of the young (~ 5,000 yrs) 16 ms X-ray pulsar PSR J0537$-$6910 performed with the Advanced Camera for Surveys (ACS) aboard the Hubble Space Telescope (HST). Although a few new potential counterparts have been detected within or close to the revised Chandra X-ray error circle (~1 arcsec) of the pulsar, only two of them (with magnitudes m_{814W} ~ 23.9 and m_{814W} ~ 24.2) show indications of a peculiar spectrum which could be related to optical emission from the pulsar. This might be true also for a third, fainter, candidate detected only in one filter (with magnitude m_{814W} ~ 26.7). If either of the two brighter candidates is indeed the actual counterpart, the optical output of PSR J0537-6910 would make it similar to young Crab-like pulsars. If not, it would mean that PSR J0537-6910 is significantly underluminous with respect to all pulsars detected in the optical

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arXiv2004

X-ray emission from the old pulsar B0950+08

We present the timing and spectral analyses of theXMM-newton data on the 17-Myr-old, nearby radio pulsar B0950+08. This observation revealed pulsations of the X-ray flux of the pulsar at its radio period. The pulse shape and pulsed fraction are apparently different at lower and higher energies of the observed 0.2-10 keV energy range, which suggests that the radiation cannot be explained by a single emission mechanism. The X-ray spectrum of the pulsar can be fitted with a power-law model with a photon index about 1.75 and an (isotropic) luminosity about 9.8e29 erg/s in the 0.2-10 keV. Better fits are obtained with two-component, power-law plus thermal, models with index of 1.30 and 9.7e29 erg/s for the power-law component that presumably originates from the pulsar's magnetosphere. The thermal component, dominating at E>0.7 keV, can be interpreted as radiation from heated polar caps on the neutron star surface covered with a hydrogen atmosphere. The inferred effective temperature, radius, and bolometric luminosity of the polar caps are about 1 MK, 250 m, and 3e29 erg/s. Optical through X-ray nonthermal spectrum of the pulsar can be described as a single power-law with index 1.3-1.4 for the two-component X-ray fit. The ratio of the nonthermal X-ray (1-10 keV) luminosity to the nonthermal optical (4000-9000 Å) luminosity is within the range of 1e2-1e3 observed for younger pulsars, which suggests that the magnetospheric X-ray and optical emissions are powered by the same mechanism in all pulsars. An upper limit on the temperature of the bulk of the neutron star surface, inferred from the optical and X-ray data, is about 0.15 MK. We also analyze X-ray observations of several other old pulsars, B2224+65, J2043+2740, B0628-28, B1813-36, B1929+10, and B0823+26.

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arXiv2004

Spatial Variation of the X-ray spectrum of the Crab Nebula

We present spectral analysis of the Crab Nebula obtained with the {\it Chandra} X-ray observatory. The X-ray spectrum is characterized by a power-law whose index varies across the nebula. The variation can be discussed in terms of the particle injection from the pulsar in two different directions: the equatorial plane containing the torus and the symmetry axis along the jet. In the equatorial plane, spectra within the torus are the hardest, with photon index $α\approx 1.9$, and are almost independent of the surface brightness. At the periphery of the torus, the spectrum gradually softens in the outer, lower surface brightness regions, up to $α\approx 3.0$. This indicates that synchrotron losses become significant to X-ray emitting particles at the outer boundary of the torus. We discuss the nature of the torus, incorporating information from observations at other wavelengths. Spectral variations are also seen within the southern jet. The core of the jet is the hardest with $α\approx$ 2.0, and the outer sheath surrounding the core becomes softer with $α$ up to 2.5 at the outermost part. Based on the similarity between the spectra of the jet core and the torus, we suggest that the electron spectra of the particles injected from the pulsar are also similar in these two different directions. The brightness ratio between the near and far sides of the torus can be explained by Doppler boosting and relativistic aberration; however, the observed ratio cannot be derived from the standard weakly magnetized pulsar wind model. We also found a site where an optical filament comprised of supernova ejecta is absorbing the soft X-ray emission ($<$ 2 keV).

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arXiv2003

Year-scale morphological variation of the X-ray Crab Nebula

We present year-scale morphological variations of the Crab Nebula revealed by Chandra X-ray observatory. Observations have been performed about every 1.7 years over the 3 years from its launch. The variations are clearly recognized at two sites: the torus and the southern jet. The torus, which had been steadily expanding until 1.7 years ago, now appears to have shrunk in the latest observation. Additionally, the circular structures seen to the northeast of the torus have decayed into several arcs. On the other hand, the southern jet shows the growth of its overall kinked-structure. We discuss the nature of these variations in terms of the pulsar wind nebula (PWN) mechanism.

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