Using archival as well as freshly acquired data, we assess the X-ray behavior of the Fermi/Large Area Telescope Delta *g-ray pulsars listed in the First Fermi source catalog. After revisiting the ...relationships between the pulsars' rotational energy losses and their X-ray and Delta *g-ray luminosities, we focus on the distance-independent Delta *g-to-X-ray flux ratios. When plotting our F Delta *g/F X values as a function of the pulsars' rotational energy losses, one immediately sees that pulsars with similar energetics have F Delta *g/F X spanning three decades. Such spread, most probably stemming from vastly different geometrical configurations of the X-ray and Delta *g-ray emitting regions, defies any straightforward interpretation of the plot. Indeed, while energetic pulsars do have low F Delta *g/F X values, little can be said for the bulk of the Fermi neutron stars. Dividing our pulsar sample into radio-loud and radio-quiet subsamples, we find that, on average, radio-quiet pulsars do have higher values of F Delta *g/F X , implying an intrinsic faintness of their X-ray emission and/or a different geometrical configuration. Moreover, despite the large spread mentioned above, statistical tests show a lower scatter in the radio-quiet data set with respect to the radio-loud one, pointing to a somewhat more constrained geometry for the radio-quiet objects with respect to the radio-loud ones.
The future of ground-based gamma-ray astronomy Caraveo, Patrizia A.
Atti della Accademia nazionale dei Lincei. Rendiconti Lincei. Scienze fisiche e naturali,
12/2019, Volume:
30, Issue:
Suppl 1
Journal Article
Peer reviewed
Ground-based gamma-ray astronomy is a young branch of high-energy astrophysics, addressing the highest energies in the electromagnetic spectrum. TeV photons cannot be produced by “ordinary” stars ...since they require quite extreme conditions, thus they allow the study of extreme astrophysical objects such as supernova remnants, neutron stars, black holes, both stellar and supermassive, as well as the elusive dark matter. To make it possible to go a step further in this exciting field, a world-wide collaboration is building the Cherenkov telescope array, an ambitious global observatory, to which the Italian community is strongly committed. The foreseen contribution of the Istituto Nazionale di Astrofisca (INAF) will be briefly outlined.
Full text
Available for:
EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OBVAL, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
ABSTRACT A particularly intriguing recent result from γ-ray astronomy missions is the detection of powerful flares from the Crab Nebula, which challenges the current understanding of pulsar wind ...nebulae and acceleration mechanisms. To search for the production site(s) of these flares, we conducted a multi-wavelength observing campaign using Keck, the Hubble Space Telescope (HST), and the Chandra X-ray Observatory. As the short timescales of the γ-ray flares ( day) suggest a small emitting region, the Crab's inner knot (about 0.6 arcsec from the pulsar) is a candidate site for such flaring. This paper describes observations of the inner knot, seeking to understand its nature and possible relationship with γ-ray flares. Using singular-value decomposition, analysis of the HST images yielded results consistent with traditional methods while substantially reducing some uncertainties. These analyses show that the knot's intrinsic properties (especially size and brightness) are correlated with its (projected) separation from the pulsar. This characterization of the inner knot helps in constraining standard shock model parameters, under the assumption that the knot lies near the shocked surface. While the standard shock model gives good agreement in several respects, two puzzles persist: (a) the observed angular size of the knot relative to the pulsar-knot separation is much smaller than expected; and (b) the variable high degree of polarization (reported by others) is difficult to reconcile with a highly relativistic downstream flow. However, the IR-optical flux of the inner knot is marginally consistent with the shock accelerating most of the Nebula's optical-emitting particles.
We report on the emission properties of PSR B1929+10 and its putative trail from a multiwavelength study performed using optical, X-ray, and radio data. XMM-Newton observations confirm the existence ...of the diffuse emission with a trail morphology lying in a direction opposite to the transverse motion of the pulsar. The trail spectrum is nonthermal and produced by electron-synchrotron emission in the shock between the pulsar wind and the surrounding medium. Radio data from the Effelsberg 11 cm radio continuum survey show an elongated feature that roughly coincides with the X-ray trail. Three not fully resolved radio sources seen in the NVSS survey data at 1.4 GHz match with part of the elongated radio feature seen at 11 cm. The emission properties observed from PSR B1929+10 are in excellent agreement with a nonthermal, and thus magnetospheric-radiation-dominated, emission scenario. The pulsar's X-ray spectrum is best described by a single power-law model with a photon index of 2.72 super(+) sub(-) super(0) sub(0) super(.) sub(.) super(1) sub(0) super(2) sub(9). A flux contribution from the thermal emission of heated polar caps of at most 67% is inferred from a best-fitting composite Planckian and power-law spectral model. A pure thermal emission spectrum consisting of two Planckian spectra is regarded as unlikely. A broken power-law spectral model with E sub(break) = 0.83 super(+) sub(-) super(0) sub(0) super(.) sub(.) super(0) sub(0) super(5) sub(3) keV and the photon indexes a sub(1) = 1.12 super(+) sub(-) super(0) sub(0) super(.) sub(.) super(0) sub(0) super(2) sub(3) and a sub(2) = 2.48 super(+) sub(-) super(0) sub(0) super(.) sub(.) super(0) sub(0) super(8) sub(7) can describe the optical and X-ray data entirely in terms of a nonthermal magnetospheric origin. The X-ray pulse profile observed in the 0.2-10 keV band is found to be markedly different from the broad sinusoidal pulse profile seen in the low statistic Roentgensatellit (ROSAT) data. Fitting Gaussians to the X-ray light curve indicates the possible existence of three pulse components. A small narrow pulse, characterized by energies greater than 1 keV, is found to lead the radio main pulse by 620. The fraction of pulsed photons in the 0.2-10 keV band is 32% c 4%. For the subbands 0.2-1.0 and 1.0-2.1 keV the pulsed fraction is 24% c 5% and 44% c 6%, respectively, indicating a mild energy dependence at a 62 s level. Simulations in the framework of an outer gap emission model are able to reproduce the observed X-ray pulse profile and its phase shift relative to the radio pulse.
Neutron Stars as Particle Accelerators Caraveo, Patrizia A.
Nuclear and particle physics proceedings,
April-June 2018, 2018-04-00, Volume:
297-299
Journal Article
Peer reviewed
According to standard theoretical interpretations, in neutron stars' magnetospheres particles are accelerated along the magnetic field lines where the highly-magnetized surrounding offers the ideal ...conditions to make them radiate high-energy gamma-rays (E ≥ 100 MeV) that bear the timing signature of their parent neutron star. Moreover, the accelerated particles (mostly electrons and positrons) can either move outward, to propagate into space, or be funnelled back, towards the star surface. While particles impinging on the neutron star surface generate hot spots, detectable in X-rays, outgoing ones could light-up the neutron star surroundings giving rise to extended features, visible both in X-and in Very-High-Energy (VHE) gamma-rays (E ≥ 100 GeV).
By combining gamma-ray light curves and spectra with the X-ray emission, both thermal (from the hot spots) and non thermal (from somewhere in the magnetosphere) we can try to map the emission geography within the light cylinder. Moreover, we can trace the particles' propagation outside the neutron stars' magnetospheres through their synchrotron emission, responsible for X-ray extended features, and their VHE gamma-rays inverse Compton emission, which give rise to extended sources, whose shapes, however, appear different from that of the corresponding X-ray ones since they are produced by particles of different energies.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
ABSTRACT We analyzed a deep XMM-Newton observation of the radio-quiet γ-ray PSR J2055+2539. The spectrum of the X-ray counterpart is nonthermal, with a photon index of Γ = 2.36 0.14 (1 confidence). ...We detected X-ray pulsations with a pulsed fraction of 25% 3% and a sinusoidal shape. Taking into account considerations on the γ-ray efficiency of the pulsar and on its X-ray spectrum, we can infer a pulsar distance ranging from 450 to 750 pc. We found two different nebular features associated with PSR J2055+2539 and protruding from it. The angle between the two nebular main axes is ∼162 8 0 7. The main, brighter feature is 12′ long and <20″ thick, characterized by an asymmetry with respect to the main axis that evolves with the distance from the pulsar, possibly forming a helical pattern. The secondary feature is 250″ × 30″. Both nebulae present an almost flat brightness profile with a sudden decrease at the end. The nebulae can be fitted by either a power-law model or a thermal bremsstrahlung model. A plausible interpretation of the brighter nebula is in terms of a collimated ballistic jet. The secondary nebula is most likely a classical synchrotron-emitting tail.
PDF
