Back to Main Page
Observation Date (UT) Observation Lat

Canonical Name:S4 0954+65
TeVCat Name:TeV J0958+655
Other Names:
Source Type:Blazar
R.A.:09 58 47.00 (hh mm ss)
Dec.:+65 33 55.00 (dd mm ss)
Gal Long: 145.75 (deg)
Gal Lat: 43.13 (deg)
Distance: z=0.3694
Flux: (Crab Units)
Energy Threshold:150 GeV
Spectral Index:
Extended:No
Discovery Date:2015-02
Discovered By: MAGIC
TeVCat SubCat:Default Catalog

Source Notes:

This sources was moved from the "Newly Announced" to the "Default"
catalogue on 190116 (MAGIC Collaboration, 2018).

This detection was announced in by the MAGIC Collaboration in an ATel:
- "S4 0954+65 is entered an exceptionally high state at optical and
near infrared frequencies (ATels #7057; #7055; #7046, #7001; #6996),
which triggered the MAGIC observations."

Source Position:
The position quoted is taken from MAGIC Collaboration (2015):
- R.A. (J2000): 09 58 47.00
- Dec. (J2000): +65 33 55.00

Classification - FSRQ or BL Lac?:
After a review of the literature on 160915, prompted by reading
Tavecchio (2016), this source has been re-classified in TeVCat
as a blazar of unknown type. Previously, it had been classified as a
FSRQ.
A summary on its nature is given below.
From Becerra Gonzalez et al. (2020):
- "Regarding the classification of the source, according to the EW at
rest frame measure for the Mg II line, it is < 5 Angstroms and
therefore, the source can be classified as a BL Lac object. However,
it is close to the limit of 5 Angstroms, possibly being consistent
with the existence of a weak BLR"
- "From the optical spectra presented in this work we can derive the
expected luminosity of the BLR and the torus in order to compare it
with the photon field needed to explain its MWL SED."
- The authors use the results of these calculations to "conclude that
even if the source is classified as a BL Lac object, its optical
spectrum is compatible with the existence of an IR torus powerful
enough to provide the external photon field required for the
production of gamma rays needed to successfully explain its MWL SED."
- The authors conclude by stating "With this work, we can also confirm
the classification of S4 0954+65 as a BL Lac object, but close to the
limit and therefore, it seems to confirm that the target could be a
transitional object showing characteristics from both BL Lacs and
FSRQs. From the observed Mg II emission line the luminosity of the
disk, BLR and torus have been derived under certain commonly used
assumption for the covering factors. The results demonstrate that the
observed optical spectrum is compatible with the existence of a torus
luminous enough to provide the IR external photon field needed for the
production of gamma rays as required by the high CD MWL SED."

From MAGIC Collaboration (2018):
- "The broadband spectrum can be modeled with an emission mechanism
commonly invoked for flat spectrum radio quasars, i.e. inverse Compton
scattering on an external soft photon field from the dust torus, also
known as external Compton. The light curve and SED phenomenology is
consistent with an interpretation of a blob propagating through a
helical structured magnetic field and eventually crossing a standing
shock in the jet, a scenario typically applied to flat spectrum radio
quasars (FSRQs) and low-frequency peaked BL Lac objects (LBL)."
In Tavecchio (2016), when tallying the number of TeV FSRQs, the following comment is made:
"I do not include in this list the blazar S4 0954+65, often considered
a FSRQ, since it is likely a BL Lac of unknown redshift (Landoni et al. (2015))."
From Tanaka et al. (2016):
"This object clearly meets the formal definition of a BL Lac (see
Table 35 and Fig. 8 of Lawrence et al. 1996). However, its archival
(non-simultaneous) multi-wavelength spectral energy distribution (SED)
hints at the presence of a “blue bump” more typical of a FSRQ. Past
X-ray observation by ROSAT (e.g., Comastri et al. 1997) shows a
flatter energy distribution than typical for a radio-selected BL Lac
leading to the suggestion that S4 0954+65 may be a transition object
with properties that lie in between the BL Lac and FSRQ classes. This
idea has also been explored by Ghisellini et al. (2011), who, however,
conclude that it should be classified as a LBL (a “low-peaked” BL Lac
object) based on the luminosity of the broad-line region in Eddington
units, rather than the emission lines’ equivalent width."
In their TeV discovery ATel, the MAGIC Collaboration referred to this
object as a FSRQ and hence it was classified thusly in TeVCat:
From MAGIC Collaboration (2015):
- "The MAGIC collaboration reports the discovery of very high energy
(VHE; E>100 GeV) gamma-ray emission from the FSRQ S4 0954+65"
It has also been referred to as a FSRQ in many other ATels:
- ATel 6709
- ATel 6996
- ATel 7001
- ATel 7007
- ATel 7046
- ATel 7093
- ATel 7750
- ATel 8445
In all of the Fermi catalogues, however, it has been classified as a
BL Lac based on its spectral properties. When further subclassified it
was listed as an LSP in 3LAC and as an ISP in 1LAC:
- 3FGL
- 2FGL
- 1FGL
- 3LAC
- 2LAC
- 1LAC
- 2FHL
- 1FHL
It is also classified as a BL Lac in CGRaBS by Healey et al. (2008)
It is also classified as a BL Lac in Veron-Cetty & Veron (2006)
From Landoni et al. (2015):
- "This source was classified as a BL Lac object by Walsh et al. (1984)
and exhibits all the properties of its class"
They also note the following:
"On the basis of a number of observable quantities such as the
apparent magnitude, point like images, featureless spectrum and their
gamma-ray emission, S4 0954+65 appears similar to few other bright BL Lacs
objects such as PG 1553+113 (R ∼ 14) and H 1722+119 (R ∼ 15,
Landoni et al. (2014))."

Distance:
The distance of z=0.368 that had been quoted here in TeVCat has been
called into question by Landoni et al. (2015). We have
edited the redshift entry for this source accordingly (151012).
Update 25 November 2020: the redshift has been updated to z = 0.3694 +/- 0.0011
following the results of Becerra Gonzalez et al. (2020) described below.

From Becerra Gonzalez et al. (2020):
- "The observations of S4 0954+65 were carried out using the DOLORES
(Device Optimized for the LOw RESolution) spectrograph at TNG in long
slit configuration. Six observations were carried out using the LRB
grism."
- "We obtained consistent redshift estimates based on the strongest
emission lines detected on the spectra taken on 2015-05-21. Thus, the
estimated redshift of S4 0954+65 is z = 0.3694 +/- 0.0011 is derived
from the Mg II line, and the values z = 0.3667 +/- 0.0003 and
z = 0.3671 +/- 0.0003 are derived from the lines [OIII] and [OII],
respectively"
- "Therefore, our results confirm the previous redshift estimates from
Lawrence et al. (1986, 1996); Stickel et al. (1993). Hence, the redshift
lower limit of z >= 0.45 derived more recently by Landoni et al. (2015)
is ruled out based on the results on this work."
- the authors conclude by stating "a solid redshift determination of
z = 0.3694 +/- 0.0011 is established for this target"

From Landoni et al. (2015):
- "Contrary to previous reports on the redshift, we found that the
optical spectrum is featureless at an equivalent width limit of ∼ 0.1
ÌŠAngstromsA. A critical analysis of the existing observations indicates
that the redshift is still unknown. Based on the new data we estimate
a lower limit to the redshift at z ≥ 0.45."
- "The first attempt to determine the redshift of this source was done
by Lawrence et al. (1986) who proposed z = 0.367. Further optical
spectroscopy was then obtained by Stickel et al. (1993) confirming
this redshift. However, the two determinations of the redshift are
mainly based on different spectral features casting some doubts on the
soundness of the proposed redshift. Moreover, we note that, in spite
of the low z, both ground based imaging and HST observations
(Scarpa et al. 2000; Urry et al. 2000) failed to detect the underlying
diffuse emission from the host galaxy, suggesting a higher redshift
for this source.

Spectral Information:
From MAGIC Collaboration (2018):
- Spectral index: 4.58 +/- 0.66(stat) +/- 0.15(syst)


Seen by: MAGIC
Want a reference added? Send a bibtex entry to the TeVCat Team