M. Marelli

Eight gamma-ray pulsars discovered in blind frequency searches of Fermi LAT data

We report the discovery of eight gamma-ray pulsars in blind frequency searches using the LAT, onboard the Fermi Gamma-ray Space Telescope. Five of the eight pulsars are young (tau_c 10^36 erg/s), and located within the Galactic plane (|b|<3 deg). The remaining three are older, less energetic, and located off the plane. Five pulsars are associated with sources included in the LAT bright gamma-ray source list, but only one, PSR J1413-6205, is clearly associated with an EGRET source. PSR J1023-5746 has the smallest characteristic age (tau_c=4.6 kyr) and is the most energetic (Edot=1.1E37 erg/s) of all gamma-ray pulsars discovered so far in blind searches. PSRs J1957+5033 and J2055+25 have the largest characteristic ages (tau_c~1 Myr) and are the least energetic (Edot~5E33 erg/s) of the newly-discovered pulsars. We present the timing models, light curves, and detailed spectral parameters of the new pulsars. We used recent XMM observations to identify the counterpart of PSR J2055+25 as XMMU J205549.4+253959. In addition, publicly available archival Chandra X-ray data allowed us to identify the likely counterpart of PSR J1023-5746 as a faint, highly absorbed source, CXOU J102302.8-574606. The large X-ray absorption indicates that this could be among the most distant gamma-ray pulsars detected so far. PSR J1023-5746 is positionally coincident with the TeV source HESS J1023-575, located near the young stellar cluster Westerlund 2, while PSR J1954+2836 is coincident with a 4.3 sigma excess reported by Milagro at a median energy of 35 TeV. Deep radio follow-up observations of the eight pulsars resulted in no detections of pulsations and upper limits comparable to the faintest known radio pulsars, indicating that these can be included among the growing population of radio-quiet pulsars in our Galaxy being uncovered by the LAT, and currently numbering more than 20.

An accreting pulsar with extreme properties drives an ultraluminous x-ray source in NGC 5907

Spinning up an extragalactic neutron star Ultraluminous x-ray sources (ULXs) are strange objects in other galaxies that cannot be explained by conventional accretion onto stellar-mass objects. This has led to exotic interpretations, such as the long-sought intermediate-mass black holes. Israel et al. observed a ULX in the nearby galaxy NGC 5907 and found that it is instead a neutron star. The spinning neutron star is accreting material so fast that its spin period is quickly accelerating. The only way that it can consume enough material to explain these properties is if it has a strong multipolar magnetic field. Science, this issue p. 817 An ultraluminous x-ray source in NGC 5907 is a spinning neutron star with a complex magnetic field. Ultraluminous x-ray sources (ULXs) in nearby galaxies shine brighter than any x-ray source in our Galaxy. ULXs are usually modeled as stellar-mass black holes (BHs) accreting at very high rates or intermediate-mass BHs. We present observations showing that NGC 5907 ULX is instead an x-ray accreting neutron star (NS) with a spin period evolving from 1.43 seconds in 2003 to 1.13 seconds in 2014. It has an isotropic peak luminosity of ~1000 times the Eddington limit for a NS at 17.1 megaparsec. Standard accretion models fail to explain its luminosity, even assuming beamed emission, but a strong multipolar magnetic field can describe its properties. These findings suggest that other extreme ULXs (x-ray luminosity ≥ 1041 erg second−1) might harbor NSs.

PSR J1838-0537: Discovery of a young, energetic gamma-ray pulsar

We report the discovery of PSR J1838−0537, a gamma-ray pulsar found through a blind search of data from the Fermi Large Area Telescope (LAT). The pulsar has a spin frequency of 6.9 Hz and a frequency derivative of −2.2 × 10 −11 Hz s −1 , implying a young characteristic age of 4970 yr and a large spin-down power of 5.9 × 10 36 erg s −1 . Follow-up observations with radio telescopes detected no pulsations; thus PSR J1838−0537 appears radio-quiet as viewed from Earth. In 2009 September the pulsar suffered the largest glitch so far seen in any gamma-ray-only pulsar, causing a relative increase in spin frequency of about 5.5 × 10 −6 . After the glitch, during a putative recovery period, the timing analysis is complicated by the sparsity of the LAT photon data, the weakness of the pulsations, and the reduction in average exposure from a coincidental, contemporaneous change in LAT’s sky-survey observing pattern. The pulsar’s sky position is coincident with the spatially extended TeV source HESS J1841−055 detected by the High Energy Stereoscopic System (H.E.S.S.). The inferred energetics suggest that HESS J1841−055 contains a pulsar wind nebula powered by the pulsar.

X-ray pulsations from the radio-quiet gamma-ray pulsar in CTA 1

Prompted by the Fermi-LAT discovery of a radio-quiet gamma-ray pulsar inside the CTA 1 supernova remnant, we obtained a 130 ks XMM-Newton observation to assess the timing behavior of this pulsar. Exploiting both the unprecedented photon harvest and the contemporary Fermi-LAT timing measurements, a 4.7σ single-peak pulsation is detected, making PSR J0007+7303 the second example, after Geminga, of a radio-quiet gamma-ray pulsar also seen to pulsate in X-rays. Phase-resolved spectroscopy shows that the off-pulse portion of the light curve is dominated by a power-law, non-thermal spectrum, while the X-ray peak emission appears to be mainly of thermal origin, probably from a polar cap heated by magnetospheric return currents, pointing to a hot spot varying throughout the pulsar rotation.

RADIO-QUIET AND RADIO-LOUD PULSARS: SIMILAR IN GAMMA-RAYS BUT DIFFERENT IN X-RAYS

We present new Chandra and XMM-Newton observations of a sample of eight radio-quiet Gamma-ray pulsars detected by the Fermi Large Area Telescope. For all eight pulsars we identify the X-ray counterpart, based on the X-ray source localization and the best position obtained from Gamma-ray pulsar timing. For PSR J2030+4415 we found evidence for an about 10 arcsec-long pulsar wind nebula. Our new results consolidate the work from Marelli et al. 2011 and confirm that, on average, the Gamma-ray--to--X-ray flux ratios (Fgamma/Fx) of radio-quiet pulsars are higher than for the radio-loud ones. Furthermore, while the Fgamma/Fx distribution features a single peak for the radio-quiet pulsars, the distribution is more dispersed for the radio-loud ones, possibly showing two peaks. We discuss possible implications of these different distributions based on current models for pulsar X-ray emission.

CLASSIFICATION AND RANKING OF FERMI LAT GAMMA-RAY SOURCES FROM THE 3FGL CATALOG USING MACHINE LEARNING TECHNIQUES

We apply a number of statistical and machine learning techniques to classify and rank gamma-ray sources from the Third Fermi Large Area Telescope (LAT) Source Catalog (3FGL), according to their likelihood of falling into the two major classes of gamma-ray emitters: pulsars (PSR) or Active Galactic Nuclei (AGN). Using 1904 3FGL sources that have been identified/associated with AGN (1738) and PSR (166), we train (using 70% of our sample) and test (using 30%) our algorithms and find that the best overall accuracy (>96%) is obtained with the Random Forest (RF) technique, while using a logistic regression (LR) algorithm results in only marginally lower accuracy. We apply the same techniques on a sub-sample of 142 known gamma-ray pulsars to classify them into two major subcategories: young (YNG) and millisecond pulsars (MSP). Once more, the RF algorithm has the best overall accuracy (~90%), while a boosted LR analysis comes a close second. We apply our two best models (RF and LR) to the entire 3FGL catalog, providing predictions on the likely nature of {\it unassociated} sources, including the likely type of pulsar (YNG or MSP). We also use our predictions to shed light on the possible nature of some gamma-ray sources with known associations (e.g. binaries, SNR/PWN). Finally, we provide a list of plausible X-ray counterparts for some pulsar candidates, obtained using Swift, Chandra, and XMM. The results of our study will be of interest for both in-depth follow-up searches (e.g. pulsar) at various wavelengths, as well as for broader population studies.

ON THE PUZZLING HIGH-ENERGY PULSATIONS OF THE ENERGETIC RADIO-QUIET γ-RAY PULSAR J1813–1246

We have analyzed the new deep XMM-Newton and Chandra observations of the energetic, radio-quiet pulsar J18131246. The X-ray spectrum is nonthermal, very hard, and absorbed. Based on spectral considerations, we propose that J1813 is located at a distance further than 2.5 kpc. J1813 is highly pulsed in the X-ray domain, with a light curve characterized by two sharp, asymmetrical peaks, separated by 0.5 in phase. We detected no significant X-ray spectral changes during the pulsar phase.We extended the available Fermi ephemeris to five years.We found two glitches. The gamma-ray light curve is characterized by two peaks, separated by 0.5 in phase, with a bridge in between and no off-pulse emission. The spectrum shows clear evolution in phase, being softer at the peaks and hardening toward the bridge. Surprisingly, both X-ray peaks lag behind the gamma-ray ones by a quarter of phase. We found a hint of detection in the 30-500 keV band with INTEGRAL, which is consistent with the extrapolation of both the soft X-ray and gamma-ray emission of J1813. The unique X-ray and gamma-ray phasing suggests a singular emission geometry. We discuss some possibilities within the current pulsar emission models. Finally, we develop an alternative geometrical model where the X-ray emission comes from polar cap pair cascades.

X- AND γ-RAY PULSATIONS OF THE NEARBY RADIO-FAINT PSR J1741–2054

We report the results of a deep XMM-Newton observation of the radio-faint γ -ray pulsar J1741−2054 and its nebula together with the analysis of five years ofFermi Large Area Telescope (LAT) data. The X-ray spectrum of the pulsar is consistent with an absorbed power law plus a blackbody, originating at least partly from the neutron star cooling. The nebular emission is consistent with that of a synchrotron pulsar wind nebula, with hints of spatial spectral variation. We extended the available Fermi LAT ephemeris and folded the γ -ray and X-ray data. We detected X-ray pulsations from the neutron star: both the thermal and non-thermal components are ∼35%–40% pulsed, with phase-aligned maxima. A sinusoid fits the thermal-folded profile well. A 10 bin phase-resolved analysis of the X-ray emission shows softening of the non-thermal spectrum during the on-pulse phases. The radio, X-ray, and γ -ray light curves are single-peaked, not phase-aligned, with the X-ray peak trailing the γ -ray peak by more than half a rotation. Spectral considerations suggest that the most probable pulsar distance is in the 0.3–1.0 kpc range, in agreement with the radio dispersion measure.

New VLT observations of the Fermi pulsar PSR J1048−5832

PSR J1048-5832 is a Vela-like (P=123.6 ms; tau~20.3 kyr) gamma-ray pulsar detected by Fermi, at a distance of ~2.7 kpc and with a rotational energy loss rate dot{E}_{SD} ~2 x 10^{36} erg/s. The PSR J1048-5832 field has been observed with the VLT in the V and R bands. We used these data to determine the colour of the object detected closest to the Chandra position (Star D) and confirm that it is not associated with the pulsar. For the estimated extinction along the line of sight, inferred from a re-analysis of the Chandra and XMM-Newton spectra, the fluxes of Star D (V~26.7; R~25.8) imply a -0.13 26.4; V>27.6) with the extrapolation of the X and gamma-ray spectra and constrained the pulsar spectrum at low-energies. In particular, the VLT upper limits suggest that the pulsar spectrum could be consistent with a single power-law, stretching from the gamma-rays to the optical.

Observations of three young γ-ray pulsars with the Gran Telescopio Canarias

We report the analysis of the first deep optical observations of three isolated γ-ray pulsars detected by the Fermi Gamma-ray Space Telescope: the radio-loud PSR J0248+6021 and PSR J0631+1036, and the radio-quiet PSR J0633+0632. The latter has also been detected in the X-rays. The pulsars are very similar in their spin-down age (τ ∼ 40–60 kyr), spin-down energy (⁠E˙∼1035 erg s−1), and dipolar surface magnetic field (B ∼ 3–5 × 1012 G). These pulsars are promising targets for multiwavelength observations, since they have been already detected in γ-rays and in radio or X-rays. None of them has been detected yet in the optical band. We observed the three pulsar fields in 2014 with the Spanish 10.4 m Gran Telescopio Canarias (GTC). We could not find any candidate optical counterpart to the three pulsars close to their most recent radio or Chandra positions down to 3σ limits of g′ ∼ 27.3, g′ ∼ 27, g′ ∼ 27.3 for PSR J0248+6021, J0631+1036, and J0633+0632, respectively. From the inferred optical upper limits and estimated distance and interstellar extinction, we derived limits on the pulsar optical luminosity. We also searched for the X-ray counterpart to PSR J0248+6021 with Chandra but we did not detect the pulsar down to a 3σ flux limit of 5 × 10−14 erg cm−2 s−1 (0.3–10 keV). For all these pulsars, we compared the optical flux upper limits with the extrapolations in the optical domain of the γ-ray spectra and compared their multiwavelength properties with those of other γ-ray pulsars of comparable age.