Multi-wavelength studies of the gamma-ray pulsar PSR J1907+0602

PSR J1907+0602 is a radio-faint, 107-ms GeV gamma-ray pulsar that was discovered with the Fermi LAT in a blind pulsar search. PSR J1907+0602 is located near the bright, extended TeV gamma-ray source MGRO J1908+06 which may be an associated pulsar wind nebula. We present an analysis of XMM-Newton X-ray data and EVLA radio data of the pulsar. We detect a faint X-ray source coincident with the gamma-ray pulsar and investigate its spectral and timing properties. We also find marginal evidence for a bow shock in the X-ray images. The pulsar was not detected with the EVLA, and we derive upper limits on the time-averaged radio flux in multiple frequency bands.Comment: 4 pages, 2 figures. Published in HIGH ENERGY GAMMA-RAY ASTRONOMY: 5th International Meeting on High Energy Gamma-Ray Astronom

Unidentified Fermi LAT Transients Near The Galactic Plane

The Fermi LAT has detected numerous transient gamma-ray sources near the Galactic plane, several of which have been shown to be located within our Galaxy. We present an analysis of LAT Pass 8 data of seven previously reported, but still unidentified transient gamma-ray sources located within 10{\deg} of the Galactic plane. We detect significant gamma-ray emission lasting several days for three of these sources: Fermi J0035+6131, Fermi J0905-3527, and Fermi J0910-5041. However, we were not able to detect the increase in gamma-ray emission that has previously been reported in the other four cases. We also review available multiwavelength data for the transients and discuss potential counterparts.Comment: 6 pages, Published in the Proceedings of the 6th International Symposium on High-Energy Gamma-Ray Astronomy (Gamma2016), July 11-15, 2016, Heidelberg, German

XMM-Newton observes flaring in the polar UZ For during a low state

During an XMM-Newton observation, the eclipsing polar UZ For was found in a peculiar state with an extremely low X-ray luminosity and occasional X-ray and UV flaring. For most of the observation, UZ For was only barely detected in X-rays and about 800 times fainter than during a high state previously observed with ROSAT. A transient event, which lasted about 900 s, was detected simultaneously by the X-ray instruments and, in the UV, by the Optical Monitor. The transient was likely caused by the impact of 10^17-10^18 g of gas on the main accretion region of the white dwarf. The X-ray spectrum of the transient is consistent with 7 keV thermal bremsstrahlung from the shock-heated gas in the accretion column. A soft blackbody component due to reprocessing of X-rays in the white dwarf atmosphere is not seen. The likely origin of the UV emission during the transient is cyclotron radiation from the accretion column. We conclude from our analysis that the unusual flaring during the low state of UZ For was caused by intermittent increases of the mass transfer rate due to stellar activity on the secondary.Comment: To be published in MNRAS, 8 pages, 4 figure

First XMM-Newton observations of strongly magnetic cataclysmic variables - II. Timing studies of DP Leo and WW Hor

XMM-Newton was used to observe two eclipsing, magnetic cataclysmic variables, DP Leo and WW Hor, continuously for three orbital cycles each. Both systems were in an intermediate state of accretion. For WW Hor we also obtained optical light curves with the XMM-Newton Optical Monitor and from ground-based observations. Our analysis of the X-ray and optical light curves allows us to constrain physical and geometrical parameters of the accretion regions and derive orbital parameters and eclipse ephemerides of the systems. For WW Hor we directly measure horizontal and vertical temperature variations in the accretion column. From comparisons with previous observations we find that changes in the accretion spot longitude are correlated with the accretion rate. For DP Leo the shape of the hard X-ray light curve is not as expected for optically thin emission, showing the importance of optical depth effects in the post-shock region. We find that the spin period of the white dwarf is slightly shorter than the orbital period and that the orbital period is decreasing faster than expected for energy loss by gravitational radiation alone.Comment: Accepted for publication in MNRAS, 12 pages, 6 figure

Relativistic Iron Line Emission from the Neutron Star Low-mass X-ray Binary 4U 1636-536

We present an analysis of XMM-Newton and RXTE data from three observations of the neutron star LMXB 4U 1636-536. The X-ray spectra show clear evidence of a broad, asymmetric iron emission line extending over the energy range 4-9 keV. The line profile is consistent with relativistically broadened Fe K-alpha emission from the inner accretion disk. The Fe K-alpha line in 4U 1636-536 is considerably broader than the asymmetric iron lines recently found in other neutron star LMXBs, which indicates a high disk inclination. We find evidence that the broad iron line feature is a combination of several K-alpha lines from iron in different ionization states.Comment: 7 pages, 2 figures, Published in the Astrophysical Journa

First XMM-Newton observations of strongly magnetic cataclysmic variables I: spectral studies of DP Leo and WW Hor

We present an analysis of the X-ray spectra of two strongly magnetic cataclysmic variables, DP Leo and WW Hor, made using XMM-Newton. Both systems were in intermediate levels of accretion. Hard optically thin X-ray emission from the shocked accreting gas was detected from both systems, while a soft blackbody X-ray component from the heated surface was detected only in DP Leo. We suggest that the lack of a soft X-ray component in WW Hor is due to the fact that the accretion area is larger than in previous observations with a resulting lower temperature for the re-processed hard X-rays. Using a multi-temperature model of the post-shock flow, we estimate that the white dwarf in both systems has a mass greater than 1 Msun. The implications of this result are discussed. We demonstrate that the `soft X-ray excess' observed in many magnetic cataclysmic variables can be partially attributed to using an inappropriate model for the hard X-ray emission.Comment: Accepted by MNRAS as a letter, 5 pages, 2 figure

Irregular Mass Transfer in the Polars VV Puppis and V393 Pavonis during the Low State

The polars VV Pup and V393 Pav were observed with XMM-Newton during states of low accretion rate with peak X-ray luminosities of ~1 x 10^30 and ~1 x 10^31 erg/s, respectively. In both polars, accretion onto the white dwarf was extremely irregular, and the accretion rate varied by more than 1 order of magnitude on timescales of ~1 hr. Our observations suggest that this type of irregular accretion is a common phenomenon in polars during the low state. The likely cause of the accretion rate fluctuations are coronal mass ejections or solar flares on the companion star that intermittently increase the mass transfer into the accretion stream. Our findings demonstrate that the companion stars in cataclysmic variables possess highly active atmospheres.Comment: Accepted for publication in ApJ, 16 pages, 3 figure

XMM-Newton Observations of the Dark Accelerator MGRO J1908+06

MGRO J1908+06 is one of the brightest TeV gamma-ray sources in the Galactic disk. It was originally discovered with Milagro and later confirmed with H.E.S.S. and VERITAS as an extended TeV source. A nearby GeV gamma-ray pulsar, PSR J1907+06, was recently discovered with the Fermi LAT suggesting that MGRO J1908+06 is an asymmetric pulsar wind nebula. While counterparts to the pulsar have been found at X-ray and radio wavelengths, no emission other than TeV gamma rays has so far been detected from MGRO J1908+06. The pulsar wind nebula appears to have an extremely low X-ray to TeV gamma-ray flux ratio. We have obtained XMM-Newton data for the region near MGRO J1908+06 and present the results of our search for extended X-ray emission from the pulsar wind nebula