The Discovery of 8.7s Pulsations from the Ultrasoft X-Ray Source 4u0142+614

We discovered a periodicity at about 8.7s from the X--ray sources 4U0142+61, previously considered a possible black hole candidate on the basis of its ultrasoft spectrum. The pulsations are visible only in the 1--4 keV energy range, during an observation obtained with the EXOSAT satellite in August 1984. A search for delays in the pulse arrival times caused by orbital motion gave negative results. In the same data, periodic oscillations at 25 minutes had been previously found in an additional hard spectral component dominating above 4 keV which arises from the X-ray transient RX J0146.9+6121, discovered with ROSAT and identified with a Be star. Though the very high (>10^4) X--ray to optical flux ratio of 4U0142+61 is compatible with models based on an isolated neutron star, the simplest explanation involves a low mass X--ray binary with a very faint companion, similar to 4U1626--67. The discovery of periodic pulsations from 4U0142+61 weakens the phenomenological criterion that an ultrasoft spectral component is a signature of accreting black holes.Comment: plain LaTeX v3.1, 14 pages + 2 PostScript figures available upon request to [email protected] . To appear on The Astrophysical Journal, Letters. SISSA ref.: 106/94/

Recent Results on the Anomalous X-ray Pulsars

The ''Anomalous X-ray Pulsars'' (AXPs) are a small group of X-ray pulsars characterized by periods in the 5-10 s range and by the absence of massive companion stars. There are now 7 possible members of this class of objects. We review recent observational results on their X-ray spectra, spin period evolution, and searches for orbital motion and discuss the implications for possible models.Comment: 4 pages, 1 figures. To appear in The Active X-ray Sky: Results from BeppoSAX and Rossi-XTE, Nuclear Physics B Proceedings Supplements, L. Scarsi, H. Bradt, P. Giommi and F. Fiore (eds.), Elsevier Science B.

The broad band X-ray spectrum of the black hole candidate GRS1758-258

We present the results of a BeppoSAX observation of the black hole candidate GRS1758-258 carried out in 1997, while the source was in its low/hard state. The X-ray spectrum, simultaneously observed over the broad energy range from 0.1 to 200 keV, can be well described by a Comptonized emission model with electron temperature kT_e = 31.4 keV and optical depth of 4.0 (spherical geometry), although a cut-off power-law and a reflection model cannot be excluded. Additionally, a broad iron line at 6.4 keV with equivalent width EW = 67 eV has been marginally detected. The 0.1-200 keV luminosity is 1.4x10^37 erg/s for an assumed distance of 8 kpc. The soft and hard luminosities are such that the source falls inside the so-called "burster box". No evidence for a soft excess is present.Comment: accepted for publication in Astronomy and Astrophysic

Ten years of INTEGRAL observations of the hard X-ray emission from SGR 1900+14

We exploited the high sensitivity of the INTEGRAL IBIS/ISGRI instrument to study the persistent hard X-ray emission of the soft gamma-ray repeater SGR 1900+14, based on ~11.6 Ms of archival data. The 22-150 keV INTEGRAL spectrum can be well fit by a power law with photon index 1.9 +/- 0.3 and flux F_x = (1.11 +/- 0.17)E-11 erg/cm^2/s (20-100 keV). A comparison with the 20-100 keV flux measured in 1997 with BeppoSAX, and possibly associated with SGR 1900+14, shows a luminosity decrease by a factor of ~5. The slope of the power law above 20 keV is consistent within the uncertainties with that of SGR 1806-20, the other persistent soft gamma-ray repeater for which a hard X-ray emission extending up to 150 keV has been reported.Comment: Accepted for publication in Astronomy & Astrophysics. 4 page

The detection of variability from the candidate IR counterpart to the Anomalous X-ray Pulsar 1E1048.1-5937

We report on the detection of variability from the proposed IR counterpart to the Anomalous X-ray Pulsar (AXP) 1E1048.1-5937 based on Chandra and ESO optical/IR deep observations carried out in 2001-2002. Within the narrow Chandra uncertainty region for 1E1048.1-5937 we found only one relatively faint (J=22.1+/-0.3, J-Ks=2.4) source, while the recently proposed IR counterpart was not detected down to a limiting Ks magnitude of about 20.7 (3sigma confidence level). This implies a remarkable IR brightening of this object, Delta Ks>1.3, on a timescale of about 50 days. Although our knowledge of the IR properties of AXPs is rather limited (there is only another source, 1E2259+586, for which IR variability has been detected), the observed IR variability of the proposed counterpart strengthens its association with 1E1048.1-5937. Our results make the IR (and presumably optical) variability a likely common characteristic of AXPs, and provide new constraints on this class of objects.Comment: 4 pages. Accepted for publication on ApJ Letters. emulateapj5.sty macro use