Letter to the Editor

On the distance of PG 1553+11

A lineless BL Lacertae object active in the TeV band

¹ University of Insubria, via Valleggio 11, 22100 Como, Italy e-mail: aldo.treves@uninsubria.it
² INAF – Osservatorio Astronomico di Padova, Vicolo dell'Osservatorio 5, 35122 Padova, Italy e-mail: renato.falomo@oapd.inaf.it
³ INAF – IASF, via Bassini 15, 20133 Milano, Italy e-mail: uslenghi@iasf-milano.inaf.it

Received: 16 July 2007
Accepted: 14 August 2007

Abstract

Context.The redshift of PG 1553+11, a bright BL Lac object (), is still unknown. It has been recently observed in the TeV band, a fact that offers an upper limit for the redshift .

Aims.We intend to provide a lower limit for the distance of the object.

Methods.We used a procedure to constrain the apparent magnitude of the host galaxy in archived HST images. Supposing that the host galaxy is typical of BL Lac objects (M_R -22.8), a lower limit to the distance can be obtained from the limit on the apparent magnitude of the host galaxy.

Results.Using the limit on the host galaxy magnitude, the redshift is found to be ≥0.25.

Conclusions.The redshift of PG 1553+11 is probably in the range 0.3-0.4, making this object the most distant extragalactic source so far detected in the TeV energies. We suggest that other bright BL Lac objects of unknown redshift and similar spectroscopic characteristics may be interesting targets for TeV observations.