VLT and Suzaku observations of the Fermi pulsar PSR J1028-5819
Abstract
Context. The launch of the Fermi Gamma-ray Space Telescope in 2008 opened new perspectives in the multi-wavelength studies of neutron stars, with more than 100 γ-ray pulsars having since been detected. While most Fermi pulsars had previously been observed in the X-rays with Chandra and XMM-Newton, optical observations with 8 m-class telescopes exist for only a tiny fraction of them.
Aims: We aim to search for optical emission from the Fermi pulsar PSR J1028 - 5819 (P = 91.4 ms). With a spin-down age τ ~ 90 kyr and a rotational energy loss rate of Ė ~ 8.3 × 1035 erg s-1, PSR J1028 - 5819 can be considered a transition object between the young, Vela-like pulsars and the middle-aged ones. At a distance of ~2.3 kpc and with a relatively low hydrogen column density PSR J1028 - 5819 is a good potential target for 8 m-class telescopes.
Methods: Owing to its recent discovery, no optical observations of this pulsar have been reported so far. We used optical images taken with the Very Large Telescope (VLT) in the B and V bands to search for the optical counterpart of PSR J1028 - 5819 or constrain its optical brightness. At the same time, we used an archival Suzaku observation to confirm the preliminary identification of the pulsar's X-ray counterpart obtained by Swift.
Results: Owing to the large uncertainty in the pulsar's radio position and the presence of a bright (V = 13.2) early F-type star at ≲ 4″ (star A), we are unable to detect its counterpart down to flux limits of B ~ 25.4 and V ~ 25.3, the deepest obtained so far for PSR J1028 - 5819. From the Suzaku observations, we find that the X-ray spectrum of the pulsar's candidate counterpart is best-fit by a power-law with spectral index ΓX = 1.7 ± 0.2 and an absorption column density NH < 1021 cm-2, which would support the proposed X-ray identification. Moreover, we find possible evidence of diffuse emission around the pulsar. If real and associated with a pulsar wind nebula (PWN), its surface brightness and angular extent would be compatible with the expectations for a ~100 kyr old pulsar at the distance of PSR J1028 - 5819.
Conclusions: A far more accurate radio position for PSR J1028 - 5819 is necessary to better determine its position relative to star A. Future high-spatial resolution observations with both the HST and Chandra will be more able to distinguish the optical emission of PSR J1028 - 5819 from the halo of star A and confirm the existence of the candidate PWN.
- Publication:
-
Astronomy and Astrophysics
- Pub Date:
- July 2012
- DOI:
- 10.1051/0004-6361/201218869
- arXiv:
- arXiv:1205.1905
- Bibcode:
- 2012A&A...543A.130M
- Keywords:
-
- stars: neutron;
- pulsars: individual: PSR J1028;
- 5819;
- gamma rays: stars;
- X-rays: stars;
- Astrophysics - High Energy Astrophysical Phenomena
- E-Print:
- 10 pages, 9 figures, submitted to Astronomy and Astrophysics