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Register a new userLong-term AGILE monitoring of the puzzling gamma-ray source 3EG J1835+5918
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We present the AGILE gamma-ray observations of the field containing the puzzling gamma-ray source 3EG J1835+5918. This source is one of the most remarkable unidentified EGRET sources. An unprecedentedly long AGILE monitoring of this source yields important information on the positional error box, flux evolution, and spectrum. 3EG J1835+5918 has been in the AGILE field of view several times in 2007 and 2008 for a total observing time of 138 days from 2007 Sept 04 to 2008 June 30 encompassing several weeks of continuous coverage. With an exposure time approximately twice that of EGRET, AGILE confirms the existence of a prominent gamma-ray source (AGL J1836+5926) at a position consistent with that of EGRET, although with a remarkably lower average flux value for photon energies greater than 100 MeV. A 5-day bin temporal analysis of the whole data set of AGL J1836+5926 shows some evidence for variability of the gamma-ray flux. The source spectrum between 100 MeV and 1 GeV can be fitted with a power law with photon index in the range 1.6-1.7, fully consistent with the EGRET value. The faint X-ray source RX J1836.2+5925 that has been proposed as a possible counterpart of 3EG J1835+5918 is well within the AGILE error box. Future continuous monitoring (both by AGILE and GLAST) is needed to confirm the gamma-ray flux variability and to unveil the source origin, a subject that is currently being pursued through a multiwavelength search for counterparts.
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The EGRET telescope has repeatedly observed 3EG J1835+5918 as a bright and steady source of high-energy gamma-ray radiation which has not yet been indentified. EGRET data from CGRO observation cycle 1 to 7 have been reanalysed above 100 MeV and above 1 GeV. The gamma-ray source location represents the latest and probably final positional assessment based on EGRET data. We especially adress the question of flux variability and spectral variability. The results of a X-ray/optical identification campaign towards 3EG J1835+5918 are given. The one object which might be associated with the gamma-ray source 3EG J1835+5918 has the characteristics of an isolated neutron star and possible of a radio-quiet pulsar.
The EGRET telescope aboard NASAs Compton GRO has repeatedly detected 3EG J1835+5918, a bright and steady source of high-energy gamma-ray emission with no identification suggested until recently. The long absence of any likely counterpart for a bright gamma-ray source located 25 degrees off the Galactic plane initiated several attempts of deep observations at other wavelengths. We report on counterparts in X-rays on a basis of a 60 ksec ROSAT HRI image. In order to conclude on the plausibility of the X-ray counterparts, we reanalyzed data from EGRET at energies above 100 MeV and above 1 GeV, including data up to CGRO observation cycle 7. The gamma-ray source location represents the latest and probably the final positional assessment based on EGRET data. The X-ray counterparts were studied during follow-up optical identification campaigns, leaving only one object to be likely associated with the gamma-ray source 3EG J1835+5918. This object, RX J1836.2+5925, has the characteristics of an isolated neutron star and possibly of a radio-quiet pulsar.
We report on long-term monitoring of anomalous X-ray pulsars (AXPs) using the Rossi X-ray Timing Explorer (RXTE). Using phase-coherent timing, we find a wide variety of behaviors among the sources, ranging from high stability (in 1E 2259.1+586 in quiescence and 4U 0142+61), to instabilities so severe that phase-coherent timing is not possible (in 1E 1048.1-5937). We note a correlation in which timing stability in AXPs decreases with increasing $dot{ u}$. The timing stability of soft gamma repeaters (SGRs) in quiescence is consistent with this trend, which is similar to one seen in radio pulsars. We find no significant pulse morphology variations in any AXP in quiescence. We considered high signal-to-noise average pulse profiles for each AXP as a function of energy. We show that, as in the timing properties, there is a variety of different behaviors for the energy dependence. We also used the monitoring and archival data to obtain pulsed flux time series for each source. We have found no large changes in pulsed flux for any source in quiescence, and have set $1sigma$ upper limits on variations ~20-30% depending on the source. We have recently discovered bursts from the direction of two AXPs: 1E 1048.1-5937 the most SGR-like AXP, and 1E 2259.1+586 the most rotationally stable AXP. We compare the temporal, spectral and flux properties of these events to those of SGR bursts, and show that the two phenomena are very similar. These results imply a close relationship between AXPs and SGRs, with both being magnetars.
We have made radio continuum, HI and X-ray observations in the direction of the unidentified EGRET source 3EG J1410-6147, using the Australia Telescope Compact Array and the Chandra X-ray Observatory. The observations encompass the supernova remnant (SNR) G312.4-0.4 and the two young pulsars PSRs J1412-6145 and J1413-6141. We derive a lower distance limit of 6 kpc to the SNR, although interpretation of positive velocity features in the HI spectrum may imply the SNR is more distant than 14 kpc. PSR J1412-6145, with an age of 50 kyr, is the pulsar most likely associated with SNR G312.4-0.4. X-rays are not detected from either pulsar and diffuse X-ray emission near the bright western edge of the SNR is weak. Although there is circumstantial evidence that this western region is a pulsar wind nebula (PWN), the embedded pulsar PSR J1412-6145 is apparently not sufficiently powerful to explain the radio enhancement. The origin of the electron acceleration in this region and of the gamma-rays remain unidentified, unless the distance to PSR J1413-6141 is at least a factor of 3 lower than its dispersion measure distance.
We present the AGILE-GRID monitoring of Cygnus X-3, during the period between November 2007 and July 2009. We report here the whole AGILE-GRID monitoring of Cygnus X-3 in the AGILE pointing mode data-taking, to confirm that the gamma-ray activity coincides with the same repetitive pattern of multiwavelength emission and to analyze in depth the overall gamma-ray spectrum by assuming both leptonic and hadronic scenarios. Seven intense gamma-ray events were detected in this period, with a typical event lasting one or two days. These durations are longer than the likely cooling times of the gamma-ray emitting particles, implying we see continuous acceleration rather than the result of an impulsive event such as the ejection of a single plasmoid which then cools as it propagates outwards. Cross-correlating the AGILE-GRID light curve with X-ray and radio monitoring data, we find that the main events of gamma-ray activity have been detected while the system was in soft spectral X-ray states (RXTE/ASM count rate > 3 counts/s), that coincide with local and often sharp minima of the hard X-ray flux (Swift/BAT count rate < 0.02 counts/cm^2/s), a few days before intense radio outbursts. [...] These gamma-ray events may thus reflect a sharp transition in the structure of the accretion disk and its corona, which leads to a rebirth of the microquasar jet and subsequent enhanced radio activity. [...] Finally, we examine leptonic and hadronic emission models for the gamma-ray events and find that both scenarios are valid. In the leptonic model - based on inverse Compton scatterings of mildly relativistic electrons on soft photons from the Wolf-Rayet companion star and from the accretion disk - the emitting particles may also contribute to the overall hard X-ray spectrum, possibly explaining the hard non-thermal power-law tail sometimes seen during special soft X-ray states in Cygnus X-3.
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