Failed Supernovae & Relativistic Jets In The SN 2010jp (PTF10aaxi) Event

Type II supernovae events occur due to the irreversible gravitational collapse of the central Iron core of a massive star. To produce the enormous energetic release the outer material needs to be expelled outwards. Many theories, both corroboratory and contradictory, are presented on this core-bounce stage (Mueller & Hillebrandt 1981; Woosley & Weaver 1986; Ruben & Tuominen 1988; Sato et al. 1995; Dimmelmeier et al. 2002; Fryer & Warren 2004; Sekiguchi et al. 2005; Kotake et al. 2006; Couch et al. 2009). But what happens when the shock phase, which underpins the the core-bounce outburst model, stalls?

Many theoretical studies of the core-collapse mechanism suggest that breaking spherical symmetry may be an essential ingredient in overcoming this stalled shock and producing a successful supernova (SN) explosion (Blondin et al. 2003; Buras et al. 2006a, 2006b; Burrows et al. 2006, 2007). An extreme case of breaking spherical symmetry involves jet-driven explosions (Khokhlov et al. 1999; Wheeler et al. 2000; Hoflich et al. 2001; Maeda & Nomoto 2003; Couch et al. 2009). Strongly collimated jets that expel the surrounding stellar envelopes may arise from accretion onto newly-formed black holes as in the collapsar model (MacFadyen & Woosley 1999; MacFadyen et al. 2001), or by magnetohydrodynamic (MHD) mechanisms in the collapse and spin-down of highly magnetized and rapidly rotating neutron stars, or magnetars (LeBlanc & Wilson 1970; Bodenheimer & Ostriker 1974; Wheeler et al. 2000; Thompson et al. 2004; Bucciantini et al. 2006; Burrows et al. 2007; Komissarov & Barkov 2007; Dessart et al. 2008; Metzger et al. 2010; Piro & Ott 2011).

Some models present the idea of an outwardly acting nuetrino luminosity (Bethe & Wilson 1985), sometimes referred to as the neutrino-heating mechanism. Heating in the sense that the intense flux of neutrinos radiated from the nascent neutron star might deposit the energy needed to reverse the stellar collapse. The viability of this neutrino-heating mechanism has recently been demonstrated for stars near the lower mass end of SN progenitors (Kitaura et al. 2006; Janka et al. 2007a, 2007b, 2008). However, for high-mass stars ~25, this method has not been verified and some feel that is not viable for such scenarios. Hence, another mechanism is needed and is supplied in the form of a jet-driven explosion (MacFadyen et al. 2001 & Heger et al. 2003).

A recent study by Smith et al. (2012) focused on confirming the jet model by looking at a specific supernova event: SN 2010jp (a.k.a PTF10aaxi). They found that the spectra of SN2010jp displays a triple-peaked Hα profile corresponding to a narrow (FWHM > 800km s^-1) central component that suggests strong shock interaction with a dense circumstellar medium (CSM); blue and red emission features centered at −12,600km s^-1 and +15,400km s^-1; and very broad wings extending from −22,000km s^-1 to +25,000km s^-1. Smith et al. (2012) found that these high-velocity features persist over multiple epochs during the 100 days after explosion, and that the features indicate a bipolar jet-driven explosion, with the central component produced by normal SN ejecta and CSM interaction at mid and low latitudes, while the high-velocity bumps and broad line wings arise in a non-relativistic bipolar jet. But, as ever, more study is needed to confirm the interpretation of the results presented by Smith et al. (2012)  are indeed correct.

The detection of a collimated jet in a Type II explosion is unprecedented. Models of jet-powered SNe have been published for fully stripped-envelope progenitors, which yield SNe Ibc and GRBs. Theoretical models also predict jet driven SNe II for a wide range of initial masses exceeding 25 (e.g., Heger et al. 2003), from collapsars that yield black holes. However, no clear case of a jet-driven SN II has yet been seen. For normal red supergiants (RSGs) with massive H envelopes, the general expectation is that that the collimated jet is largely destroyed while imparting its kinetic energy to a spherical envelope (Wheeler et al. 2002; Couch et al.2009). Hence, the evidence presented by Smith et al. (2012) is of significance to the future study of supernovae core-collapse events and GRBs.

Journal References:

• MacFadyen, A. I.; Woosley, S. E. (1999) Collapsars: Gamma-Ray Bursts and Explosions In “Failed Supernovae”. The Astrophysical Journal, 524 (1): pp. 262-289.

• Heger, A. et al. (2003) How Massive Single Stars End Their Life. The Astrophysical Journal, 591 (1): pp. 288-300.

• Burrows, A. et al. (2006) A New Mechanism For Core-Collapse Supernova Explosions. The Astrophysical Journal, 640 (2): pp. 878-890.

• Couch, S.M. et al. (2009) Aspherical Core-Collapse Supernovae In Red Supergiants Powered By Non-Relativistic Jets. The Astrophysical Journal, 696 (1): pp. 953-970.

Is RXJ1713.7-3946 The Remnant Of The AD393 Guest Star?

Since the middle of the 19th century, astronomers have known about pre-Tychonic bright new or ‘temporary’ stars recorded in ancient Asian, Arabic, and European texts chiefly through the works of Biot (1846) and Chambers (1867). After the acceptance of Zwicky & Baade’s (1934) seminal work “On Super-nova” and later by Pacini (1968) a concerted effort was made to link all known historical (pre-1900s) supernovae (SN) to their accompanying remnants (Hsi 1955; Clark & Stephenson 1976, 1977). Recent work by Stephenson & Green (2002) has given a greater understanding of the locations of many historical Galactic supernovae events, as well as providing details to the coupling of the pulsar wind to the rotation kinematics of the remnant to allow for a better technique to couple known supernovae compact remnants and nebulae. Clark & Stephenson focused on three Chinese guest stars* each observed within a 25 year margin of each other in the 4th century A.D.: namely, 369, 386, and 393 AD. Of these three guest stars the most likely SN event is the one seen in 393 A.D.

N.B. Historical guest stars are now considered to be due to either a novae, a dwarf novae or a supernovae event.

393 A.D. is somewhat difficult to classify. The translated texts state that during the second month of the 18th year (27 Feb – 28 Mar of 393 AD) a guest star appeared within the tail of Scorpius and lasted until nine months. However, work to classify it as either a nova, a supernova or a recurrent nova has been problematic. For example, Pskovskii (1972) argued that the 393 star was likely a recurrent nova since Chinese records also reported a star within the tail of Scorpius in 1600 AD. A nova interpretation was also suggested by van den Bergh (1978) due to the lack of a supernova remnant (SNeR) in that region. Hence, identification, or lack of, of an SNeR that coincides with the AD393 guest star would allow a definitive classification to either be made, or ruled out as the case may be.

Identifying the remnant of a historic SN is often difficult and the guest star of 393 is no exception, with nearly a dozen Galactic supernova remnants located within the tail of Scorpius (Green 2009). The remnants of G348.5+0.1 (CTB 37A) and G348.7+0.3 (CTB 37B) were initially seen as possible SNR candidates to the 393 guest star due to their small angular sizes of 15′ and 17′, respectively (Clark & Stephenson 1977; Stephenson & Green 2002). However, these remnants lie ~10 kpc away (Aharonian et al. 2008; Nakamura et al. 2009) and such large distances near the Galactic center likely imply considerable interstellar extinction decreasing the chance of an associated visually bright guest star. The same is true for the apparently very young SNR G350.1-3.0 whose distance is only ∼3.4 kpc but lies behind an estimated 20 magnitudes of visual extinction (Gaensler et al. 2008). Hence, Chinese astronomers would have been extremely unlikely to record an such supernovae at these remnant locations.

In 1996 Pfeffermann & Aschenbach (1996) announced the ROSAT discovery of the Galactic remnant RX J1713.7-3946 (G347.3-0.5) in Scorpius. The remnant’s location near the SN 393 reported position along with Pfeffermann and Aschenbach’s estimated remnant distance of 1.1 kpc and 2100 year age led Wang et al. (1997) to suggest it as the likely remnant of SN393. However, Fesen et al. (2012) have recently shown that the search for the remnant of the 393 guest star is far from certain and may not be as conclusive as Wang et al. (19797) report.

At a distance less than 2kpc for the remnant RX J1713.7-3946 raises problems with the expected supernova maximum apparent brightness and durations which appear in conflict with the Chinese records. Although, the remnant RX J1713.7-3946 exhibits several properties suggesting a relatively young age, probably less than a few thousand years and thus potentially consistent with a SN event around 393 AD estimated absolute visual magnitude, , values between −12 and −14 imply a very sub-luminous core-collapse SN event.

Neither the Chinese nor the Roman descriptions are easily reconciled with an expected RX J1713.7-3946 supernova brightness and duration. Fesen et al. (2012)  note that if RX J1713.7-3946 were the SN 393 remnant, it would then rank as having been the nearest of all the known historic Galactic supernovae during the last 2000 years. It’s relatively small distance of around 1kpc plus a moderate amount of optical extinction also means its supernova would have likely have been a visually brilliant object, certainly as bright as Jupiter and maybe as bright as Venus. Hence, the link between RX J1713.7-3946 and the guest star of AD393 first proposed by Wang et al. (1997) seems to have been killed.

Journal References:

• Wang, Z.R. et al. (1997) Is RX J1713.7-3946 The Remnant Of The AD393 Guest Star? Astronomy & Astrophysics, 318 pp.L59-L61.

• Fesen, R.A. et al. (2012) The SN 393-SNR RX J1713.7-3946 (G347.3-0.5) Connection. The Astronomical Journal, 143 (2): Article I.D.: 27.

The Link Between RE J1034+396 & Seyfert I 1H0707-495 Galaxy: X-Ray Reverberations

RE J1034+396 is a Narrow Line Seyfert 1 galaxy (NLS1, z = 0.042), a sub-class of active galactic nucleus. Seyfert galaxies are characterized by extremely bright nuclei, and spectra which have very bright emission lines. These emission lines exhibit strong Doppler broadening (Capriotti et al. 1982; Matt et al. 1992; Fabian et al. 1995; Molina et al. 2007), as the emitting material is revolving around the black hole with high speeds, emitting photons at varying Doppler shifts. Such broadening implies velocities ranging from 500 to 4000 kms^-1, and are believed to originate near an accretion disc surrounding the central black hole (Antonucci 1993).

RE J1034+396 has an unusual multi-wavelength spectrum, with no significant big blue bump in the UV/Optical bands. It is very bright in the EUV and soft X-rays making it one of the brightest objects with a so-called ‘soft excess’ (Puchnarewicz et al. 1995; Pounds et al. 1995). The soft excess below  2 keV is a common feature in the X-ray spectra of many AGN, particularly of the NLS1 class. It is too hot to be thermal blackbody emission from the accretion disc, while the similarity of its shape in objects with different masses argues against an origin in a cool comptonised region existing along with the hot corona emitting at higher energies.

A corona of this type is expected to depend on the seed photons from the disc, which in turn depend on the mass of the black hole (Gierlínski & Done 2004). Atomic processes are more likely to be at work through line-of-sight absorption or reflection. The smoothness of the observed emission rules out the former (Schurch & Done 2008), while the latter works if the emission originates very close to the black hole where strong relativistic effects acts to smear out sharp atomic features (Crummy et al. 2006; Nardini et al. 2011).

One other object, 1H0707-495, also shows a strong soft excess below 1 keV. It was shown recently that because of the high iron abundance, its soft excess is mainly due to a strong iron L emission (Fabian et al. 2009; Zoghbi et al. 2010) produced by partially ionised reflection (Ross & Fabian 2005; George & Fabian 1991). The compactness of the reflecting region and its proximity to the black hole produces strong relativistic broadening that smears out the reflection features. The iron L-line is observed together with the more commonly seen broad K-line (e.g. see Miller 2007 for a recent review).

1H0707-495 also shows a time delay between the hard band (1.0–4.0 keV) dominated by the direct power-law emission, and the soft band (0.3–1.0 keV) dominated by the
reflected emission (Fabian et al. 2009; Zoghbi et al. 2010). The soft lag is the first to be detected, and is a confirmation that the soft band is dominated by reprocessed emission. The magnitude of the lag itself (30–50 seconds) is consistent with light travel time at  2 gravitational radii from the black hole, as inferred from the energy spectrum. Detailed energy-dependent lag spectra show that the lag traces the shape of the reflection spectrum, further confirming its interpretation (Zoghbi et al. 2011).

Zoghbi & Fabian (2011) have recently concluded that the lag in RE J1034 extends over a wide frequency range, similar to 1H0707-495. The frequency of the QPO(a narrow frequency band by definition) is smaller than the frequency where the lag is maximum. The accretion disc around the super massive black hole in RE J1034 is reflecting the random stochastic signal of the illuminating source, which is this case happens to have a quasi-periodic oscillation (QPO), but Zoghbi & Fabian (2011) conclude that generally it does not. Zoghbi & Fabian (2011) found that the difference in frequency between the QPO and the lag is possibly an indication that the corona producing the QPO is slightly extended, while the compact reflection from the disc is emitted in a very small region and dominated by light-bending effects.

Zoghbi & Fabian (2011)’s latest timing results confirm the inner reflector model, which also gives a self-consistent spectral model. The X-ray spectra and timing of both RE J1034+396 and 1H0707-495 are dominated by emission and reflection from just a few gravitational radii around the central black hole within both active galaxies.

Journal References:

• Zoghbi, A.; Uttley, P. & Fabian, A. C. (2011) Understanding Reverberation Lags In 1H0707-495. Royal Astronomical Society Monthly Notices, 412 (1): pp. 59-64.

• Zoghbi, A. & Fabian, A.C. (2011) X-ray Reverberation Close To The Black Hole In The Active Galaxy RE J1034+396. Royal Astronomical Society Monthly Notices, 418 (4): pp. 2642-2647.

The Binary Be2-Pulsar PSR B1259-63/ SS 2883 System: A VHE γ-Ray Candidate

PSRB1259−63 / SS 2883 is a binary system consisting of a ∼48ms pulsar in orbit around a massive B2e companion star (Johnston et al. 1992a, 1992b). Due to the highly eccentric orbit of the pulsar, approximately every 3 and a half years it is just 10⁹ m (some 6210000 miles) apart from its Be-class* companion during their periastron period. Be stars are known to have non-isotropic stellar winds forming an equatorial disk with enhanced mass outflow (e.g.Waters et al. 1988). In the case of PSR B1259−63, timing measurements suggest that the disk is inclined with respect to the orbital plane (Wex et al. 1998), probably because the neutron star received a substantial birth kick after the core-collapse of its progenitor, causing the pulsar to cross the disk two times near periastron. Such unique properties make the binary system PSRB1259−63 an excellent cosmic laboratory for the study of pulsar winds interacting with a changing environment in the presence of an extremely intense photon field.

N.B. Be stars are typically variable and can either be classified as Gamma-Cassiopeiae variables due to the transient nature of the disk and the scattering processes, or as Lambda-Eridani variables on account of their rapid pulsational nature and subsequent rotational distortion.

The intense photon field provided by the Be-type companion star not only plays an important role in the cooling of relativistic electrons but also serves as perfect target for the production of high energy (HE) γ-rays through inverse Compton scattering (Tavani et al. 1996; Kirk et al. 1999; Ball & Kirk 2000; Ball & Dodd 2001; Murata et al. 2004). Some of these emission models predict wind powered shock acceleration of electrons to multi-TeV energies, radiating predominantly through the synchrotron and inverse Compton channels, with the main energy release in the X- and high energy γ-ray bands, respectively.

The unpulsed non-thermal X-ray emission detected from PSRB1259−63 throughout its orbital phase in 1992 to 1996 by  the ROSAT and ASCA satellites (Cominsky et al. 1994; Kaspi et al. 1995; Hirayama et al. 1996) generally supports the synchrotron origin of X-rays. The spectrum of the synchrotron radiation seems to extend to hard X-rays/low energy γ-rays as shown by OSSE (Grove et al. 1995) and recently confirmed by observations with the INTEGRAL satellite (Shaw et al. 2004).

Since the companion star provides the dominant source of photons for inverse Compton scattering, the target photon density is well known throughout the entire orbit. Therefore, the ratio of X-ray flux to high energy γ-ray flux depends only on the strength of the ambient magnetic field (Aharonian et al. 2005). Although the latter can be estimated within a general magneto-hydrodynamic treatment of the problem, it contains large uncertainties.

Kirk et al. (1999) studied the light curves of very-high-energy (VHE) γ-rays under the assumption of a  dependence of the magnetic field which implies that the ratio of the energy density of the photon field to that in the magnetic field, , is independent of orbital phase, .

They also assumed that the position of the termination shock as well as the strength of the magnetic field is not affected by the disk of the B2e star. Under such assumptions, they predicted an asymmetric γ-ray light curve with respect to periastron (because of the inclination of the orbit with respect to the line of sight and the dependence of the inverse Compton γ-ray emissivity on the scattering angle), with an increase towards periastron and monotonic decrease after the passage of periastron. However, one might possibly expect significant deviation from such a simplified picture given the apparent strong impact of the disk on the pulsar wind termination as seen in the X-ray light curve (Tavani & Arons 1997).

Moreover, during the time periods of interaction of the pulsar wind with the equatorial disk one may expect, in addition to the inverse Compton γ-rays, a new component of γ-radiation associated with interactions of accelerated electrons and possibly also protons with the dense ambient gas (Kawachi et al. 2004). Up to now, the theoretical understanding of the properties of this complex ystem, involving pulsar and stellar winds interacting with each other, is quite limited because of the lack of constraining observations.

Nevertheless, the fortunate combination of: (1) the high spin-down luminosity of the pulsar, , which is partially converted into populations of ultra-relativistic particles, (2) the presence of the intense target photon field provided by the companion star; and (3) the relatively small distance to the source () makes this object a very attractive candidate for VHE γ-ray emission.

Journal References:

• Kirk, J. G.; Ball, L.; Skjaeraasen, O. (1999) Inverse Compton Emission Of TeV Gamma-rays From PSR B1259-63. Astroparticle Physics, 10 (1): pp. 31-45.

• Abdo, A.A. et al. (2010) Fermi LAT Detection Of GeV Gamma-ray Emission From The Binary System PSRB1259-63. The Astronomer’s Telegram, #3085.

• Abdo, A.A. et al. (2011) Discovery Of High-Energy Gamma-ray Emission From The Binary System PSR B1259-63/LS 2883 Around Periastron. The Astrophysical Journal: Letters, 736 (1): Article I.D.: L11.

Discovery Of The Most Rapidly Rotating Magnetic B-Star: HR5907

Usually, strong magnetic fields are not expected to be observed in intermediate mass B-type stars (Grunhut et al. 2012). Such stars lack a significant convective outer envelope, and thus they do not generate the massive magnetic dynamos as observed on the Sun. At the thermally hotter end of the B-star class are the main sequence, Helium-strong stars (e.g. Bohlender et al. 1987) that show significant variability and enhancement in their Helium (He) lines, as is found in the archetypical star σ Orion E (e.g. Landstreet & Borra 1978).

Fig.1: This brilliantly vivid region (NGC 2170) in the constellation of Monoceros displays a wonderful mix of nebula types. The bluish areas are reflection nebulas, so-named as they reflect the light of nearby B-type stars.

In addition to this He variability, some He-strong stars show emission variability in Balmer lines, photometric brightness variations, variable UV resonance lines, and non-thermal radio emission, most of which vary with a single period, interpreted to be the rotational period of the star (e.g. Pedersen & Thomsen 1977; Shore & Brown 1990; Leone & Umana 1993). Many of these phenomena are thought to be due to the presence of a rigidly rotating, centrifugally supported magnetosphere – a region in the circumstellar environment where the stellar wind couples to the magnetic field and is forced to co-rotate with the star (Shore & Brown 1990; Townsend & Owocki 2005).

Although B-type stars are not expected to have strong magnetic fields, a theoretical consideration of electromagnetism would pertain to the fact that if a B-type star was rotating extremely fast (and such, charged particles were moving fast relative to an electric field) then a strong magnetic field could be observed. It was with this consideration of inducing magnetic fields which led Grunhut et al. (2012) to present their results on the detection of a largescale, organised magnetic field with a polar surface intensity of 10-16 kG in the B-type star HR5907 (Harvard spectral class B2.5V; Yerkes spectral classification Ib).

HR5907 (or Henry Draper Catalogue ”HD142184″, V1040 Scorpius), is a bright early type emission line star, located in the nearby Upper Scorpius OB association at a distance of 145pc from Earth (Hernandez et al. 2005). Additionally, archival Fibre-fed, Extended Range, Échelle Spectrograph (FEROS) spectra suggested that the observed He profile is morphologically more similar to the profiles of other magnetic He-strong stars (such as σ Orion E or HR7355) than to classical Be stars.

Combining newly measured Micro-variability and Oscillations of STars (MOST) survey photometry with archival Hipparcos measurements, Grunhur et al. (2012) obtained the photometric period of HD5907 to 0.508276 d, confirming its rapid movement. Their follow-up period search on the spectro-polarimetric data confirms this period is also present in the equivalent width variations of Hα as well as the longitudinal magnetic field measurements. As such, Grunhut et al. (2012) began investigation into a possible B-star with a strong magnetic field.

These results, as interpreted by the team, reveal the shortest period, non-degenerate, magnetic massive star known to date, making HD5907 one of the only known rapidly-rotating magnetic massive stars that show strong emission variations due to a magnetosphere. The only other massive star with a comparable rotation period is HR7355, which is believed to have a magnetic geometry more similar to σ Ori E, which is very different from this star. Grunthut et al. conclude that HR5907 is an ideal target for comparison with the predictions of rigidly rotating magnetosphere models as presented by Townsend & Owocki (2005) and again, by Townsend (2008). Hence, HR5907 provides a great testbed for studying the effects of the magnetic field orientation on angular momentum loss and magnetic spin-down.

In conjunction with a better understand of B-type stars, this work may give greater clues to the nature of pulsar formation and the properties of proto-neutron stars. Massive B-type stars such as HR5907, with masses in the range of   (Kiziltan 2011), are supernovae core-collapse candidates and as such will produce a compact stellar remnant of the neutron degenerate type (progenitor masses of more than approximately  (Shapiro & Teukolsky 1983) are expected to produce the more exotic black hole remnants).  As such, the discovery of an intense magnetic field within a neutron progenitor B-type star, and possible further discoveries of similar stars, could reveal greater information on the formation and evolution of the neutron star sub-class: rapidly-rotating and magnetic pulsar stars.

Journal References:

• Alecian, E. et al. (2011) First HARPSpol Discoveries Of Magnetic Fields In Massive Stars. Astronomy & Astrophysics, 536: pp.L6.

• Grunhut, J. H. et al. (2012) HR 5907: Discovery Of The Most Rapidly Rotating Magnetic Early B-type Star By The MiMeS Collaboration. Monthly Notices: Royal Astronomical Society, 419 (2): pp. 1610-1627.

Suggested Further Reading:

• Bohlender, D.A. et al. (1987) Magnetic Field Measurements Of Helium-Strong Stars. Astrophysical Journal, 323 (1): pp. 325-337.

Discovery Of An Unbound Hyper-Star In The Milky Way Halo: SDSS J0907.0+0245

If you’ve ever gazed at the night sky, you’ve probably wished upon a shooting star (which are really meteors). However, pioneering work by Brown et al. (2005) has shown that shooting stars do exist, and they’re as rare as one in one hundred million. They discovered that a hot blue horizontal branch star, SDSS J090745.0+024507, was leaving the Galaxy at an unprecedented rate. Currently situated at approximately 55kpc from the centre of our Galaxy, SDSS J090745.0+024507 is hurtling away at some 850 kilometres per second; 10 times faster than most ordinary stars orbit the centre of our Galaxy.

Corrected to the local standard of rest and for the solar reflex motion, the Galactic rest frame velocity of this star is +709 kms^-1. The observed radial velocity is only a lower limit to the star’s true space velocity, but the radial velocity alone substantially exceeds the escape velocity from the Galaxy. SDSS J090745.0+024507 is entering into the intergalactic abyss.

In Brown et al.’s (2005) survey of faint blue horizontal branch candidates in the Galactic halo, they discovered a star, SDSS J090745.0+024507 from the Sloan Digital Sky Survey catalog, traveling with a heliocentric radial velocity of +853 ± 12 kms^-1. This is a very unusual result given the age and proper motion of stars in the Galactic halo.

Unlike the galactic disc, the halo seems to be free of dust. In further contrast, stars in the galactic halo are of Population II, much older and with much lower metallicity than their Population I galactic disc counterparts (Benjamin et al. 2005). Thus, it seems something of a rarity to see an unbound blue star moving extremely fast through a sea of red stars. So how would such a star form and why is it traveling at this unprecedented rate?

The Milky Way, as well as most nearby elliptical galaxies and disk galaxies with spheroids, is believed to house a super-massive black hole (SMBH) at its center (Schodel et al. 2002, 2003; Genzel et al. 2003; Eisenhauer et al. 2003; Ghez et al. 2003a, 2003b, 2003c; Gebhardt et al. 2003). The deep gravitational potential well close to the BH enables relativistic phenomena, such as the relativistic motion of jets associated with some active galactic nuclei (Begelman, Blandford & Rees 1984).

Hills (1988) suggested that a stellar binary interaction with the Milky Way’s central black hole could eject one member of the binary with a velocity greater than 1000 kms^-1. Yu & Tremaine (2003) further develop Hill’s analysis and suggest two additional mechanisms to eject “hyper-velocity” stars (hyper-stars) from the Galactic center: close encounters of two single stars and three-body gravitational interactions between a single star and a binary black hole. Yu & Tremaine (2003) predict production rates for all three mechanisms. Even the discovery of a single hypervelocity star can place important constraints on the formation mechanism and the nature of the Galactic center.

Such mechanisms slingshot these stars into highly eccentric orbits around the central SMBH. However, as Yu & Tremaine (2003) outlined in their paper, it may be possible from the black hole at the centre of the galaxy to throw stars out at speeds greater than the escape velocity of the Milky Way: . Hence, as Hills (1988) first pointed out, this deep gravitational potential of the SMBH will result in stars being expelled from the vicinity of the SMBH with extremely high velocities; with SDSS J090745.0+024507 being a very likely candidate for just such a journey.

Journal References: 

• Yu, Q. & Tremaine, S. (2003) Ejection Of Hyper-Velocity Stars By The Black Hole In The Galactic Centre. The Astrophysical Journal, 599 (1): pp.1129–1138.

• Brown, W.R. et al. (2005) Discovery Of An Unbound Hyper-Velocity Star In The Milky Way Halo. The Astrophysical Journal, 622 (1): pp. L33-L36.

OB Associates & SNR G65.2+5.7: On The Cygnus Superbubble

The Cygnus superbubble is an area of space which presides over one of the most intense ISM enrichment regions in the sky. A number of interesting objects, such as OB associates (e.g. VI Cyg 8A, VI Cyg 9, and VI Cyg 12; Abbott et al. 1981) and huge Wolf-Rayet stars constantly pumping the local medium from the stripping of their outer layers via enormous stellar winds to supernovae explosions and remnants (e.g. G65.2+5.7, Cygnus Loop and HB21; Uyaniker et al. 2001) streaming huge shock waves into the ISM and the X-ray injections from binary transients (e.g. Cygnus X-6 and Cygnus X-7; Cash et al. 1980), all make it a tumultuous and exciting area of the sky to observe.

Fig. 1: The Cygnus superbubble near γ-Cygni which is a strong source of X-rays from Cygnus X-5, which coincides with SNR G074.0-08.6/the Cygnus Loop (Image Credit: John Gleason).

It would come as no surprise that the radiation and stellar winds from massive stars and supernova (SN) explosions determine the structure and energy content of the interstellar medium (ISM) (McKee & Ostriker 1977; Ferriere 2001). Hence, from a purely topological view, the ISM is a highly inhomogeneous on the small scale (Dyson 1997), due to the wealth of objects that constantly eject material and energy in a rather chaotic manner. But how would such a super structure form from this perceived chaos?

The most massive stars, with masses ranging from eight to roughly one hundred solar masses, are designated as spectral types O and B. These massive stars have been found to form in close proximity to each other called OB associations (Ambartsumian 1947); or, bigger still in the Wolf-Rayet (WR) category (Wolf & Rayet 1867). These massive stars have strong stellar winds (Castor,  Abbott & Klein 1975), and all of these stars are expected explode as supernovae at the ends of their lives, ejecting their outer hydrogen envelopes into the surrounding regions and thus, enriching the ISM. The strongest stellar winds from massive stars can release kinetic energy of the order of 10⁵¹ ergs (~10⁴⁴ J) over their lifetimes (Howarth & Prinja 1989;  Mokiem et al. 2007; Fehon-Gagné et al. 2011), which is equivalent to a supernova explosion. These winds can form stellar wind bubbles dozens of light years across (Castor et al. 1975).

Fig. 2: Left ~ Finder chart for the Cygnus region in Galactic coordinates. The thick dashed-dotted ellipse shows the location of the CSB and solid ellipses indicate the approximate position and extent of the OB associations. Thick dashed lines show the boundaries of the radio loops and plus signs mark the positions of the stars from the list of Garmany & Stencel (1992) and of candidate stars showing expansion (Comerón et al. 1998). The dotted circles denote the prominent H II regions. Right ~ The Cygnus superbubble as seen in the ROSAT 1.5 keV data. (Both Credit: Uyaniker, B. et al. (2001) The Cygnus Superbubble Revisited. A&A, 371 pp.675-697.)

Stars in OB associations are not gravitationally bound, but they drift apart at small speeds (of around 20 km s^-1), and they exhaust their fuel rapidly (after a few millions of years). As a result, most of their supernova explosions occur within the cavity formed by the stellar wind bubbles (Uyanıker et al. 2001). These explosions never form a visible supernova remnant, but instead expend their energy in the hot interior as sound waves. Both stellar winds and stellar explosions thus power the expansion of the superbubble in the interstellar medium. Inside OB associations and massive WR groups, such as the one found at the Cygnus nebula site (Abbott, Bieging & Churchwell 1981), the stars are close enough together that their wind bubbles merge together; forming a giant bubble. At this stage the bubble is starting to inflate as it pushes out into the ISM. What it needs is an extra kick.

The kick is provided via the deaths of our massive stars in the Cygnus OB group. Massive stars die in hugh supernovae core-collapse explosions, ejecting matter out at tremendous energies. This blast wave can cause the bubble reach even larger sizes, with expansion velocities up to several hundred km s^-1 (McKee & Cowie 1975; Mac Low & McCray 1988). This theory founded itself on the discovery of the supernovae remnant SNR G65.2+5.7 in the near vicinity.

What is special about this SNR is that it never became part of the Cygnus superbubble, yet providing interesting clues as to the nature of the super-inflation of such structures: they had to come via supernovae explosions. These explosions never formed a visible supernova remnant, other than SNR G65.2+5.7. Instead they expended their energy in the hot interior as sound waves (Bochkarev & Sitnik 1985). Thus, both stellar winds and stellar explosions power the expansion of the superbubble in the ISM from humbler beginnings.

So what is the eventual fate of such super-inflated cosmic structures? Large enough superbubbles can blow through the entire galactic disk (Tomisaka &Ikeuchi 1986; Mac Low & McCray 1988), releasing their energy into the surrounding galactic halo or even into the intergalactic medium (IGM). It is with this that the sheer scale, power and strength of these huge objects becomes apparent.

Journal References:

• Cash, W. et al. (1980) The X-ray Superbubble In Cygnus. The Astrophysical Journal: Letters, 238 (2): pp. L71-L76.

• Abbott, D. C.; Bieging, J. H. & Churchwell, E. (1981) Mass Loss From Very Luminous OB Stars & The Cygnus Superbubble. The Astrophysical Journal, 250 (1): pp. 645-659.

• Bochkarev, N. G. & Sitnik, T. G. (1985) On The Structure & Origin Of The Cygnus Superbubble. Astrophysics & Space Science, 108 (2): pp. 237-302.

• Uyanıker, B. et al. (2001) The Cygnus Superbubble Revisited. Astronomy and Astrophysics, 371 pp.675-697.

The Massive Progenitor Of The Type II-L SN2009hd In M66

There a two main classes of supernovae (SNe): Type Ia accretion event supernovae, usually associated with binary star systems and degenerate stars, and core-collapse supernovae (CC-SNe) of massive stars. The Type II core-collapse event, for lack of complication, is a result of the collapse of the degenerate Iron core of a massive star when it reaches the Chandrasekhar limit (Gilmore 2004).

This ‘type’ (which also includes Type Ib and Ic) can be sub-allocated either linear (L) or plateau (P) which are features attributed to their light curves.

Type II-Linear supernovae (SNe II-L) are among the least common and, therefore, most poorly studied subclasses of CC-SNe. They represent 6.4–10% of all CC-SNe (Smith et al. 2011; Li et al. 2011). The spectra of SNe II-L are similar to those of the much more common SNe II-Plateau (II-P), but SNe II-L are distinguished by the shapes of their light curves. While light curves of SNe II-P exhibit a relatively constant plateau for approximately 100 days after an initial peak in the first few days, the light curves of SNe II-L instead show a linear decline commencing just after the initial peak (e.g. Barbon, Ciatti & Rosino 1979).

A recent discovery of a Type II CC-SNe was made by Monard (2009) in July 2009: 2009hd, which was subsequently designated as a ‘reddened’ Type II SNe (Kasliwal, Sahu & Anupama 2009) and, now, perhaps wrongly as a Type II-P SNe by Kasliwal, Sahu & Anupama (2009). A recent paper by Filippenko et al. 2011 has now challenged the orginal Kasliwal, Sahu, & Anupama Type II-P designation, stating that it is, from their analysis, a Type II-L SNe.

After this re-designation from a Type II-P to a Type II-L Fillipenko et al. (2011) went on to constrain the properties (luminosity, mass and effective temperature) of the progenitor star for SN 2009hd.

Fillipenko et al. (2011) found that the inferred properties are certainly consistent with a red supergiant (RSG), the constraints also allow for the possibility that the star is more yellow than red. A yellow colour may indicate that it exploded in a post-RSG evolutionary state, or perhaps it is an RSG in a binary system with a bluish companion. The progenitor may have evolved back to the blue phase after reaching the RSG stage, for example, because it has lost mass (Meynet et al. 2011) perhaps through pulsationally induced and enhanced mass loss (Yoon & Cantiello 2010).

Fillipenko et al. (2011) then analysed the hydrogen lines and found them to be weak, possibly indicating that the SNe progenitor exploded with a low-mass outer hydrogen envelope which is consistent with mass loss via huge stellar winds.

Through comparison with high-mass stellar evolutionary tracks at solar metallicity they were are able to, all be it conservatively, constrain the initial mass of the progenitor to  This limit to the initial mass estimate for the SN2009hd progenitor is consistent with the range in mass found for the SN II-L 2009kr (Elias-Rosa et al. 2010) and SN2008cn (Elias-Rosa et al. 2009).

Ultimately, a very-late-time set of multi-band images of the SN 2009hd field should be obtained with HST and the Wide Field Camera 3 (WFC3) many years later, when the SN has greatly faded, as has been done for the SN IIn 2005gl (Gal-Yam & Leonard 2009) and for the SN IIb 1993J and SN II-P 2003gd (Maund & Smartt 2009), to better distinguish the constituents of the SN 2009hd environment and help decipher the true nature of the progenitor star.

Journal References:

• Filippenko, A.V.; Elias-Rosa, N. et al. (2011) The Massive Progenitor Of The Possible Type II-Linear Supernova 2009hd In Messier 66. The Astrophysical Journal, 742 (1) Article I.D.: 6.

• Elias-Rosa, N. et al. (2010) The Massive Progenitor Of The Type II-Linear Supernova 2009kr. The Astrophysical Journal: Letters, 714 (2) pp. L254-L259.

• Berger, E.; Foley, R.; Covarrubias, R. (2009) SN2009hd Is A Highly-reddened Type II-P Supernova. The Astronomer’s Telegram, #2118.

On The Cooling Of The Neutron Stars RX J1856-37 & IE1207.4-52

A neutron star, born in a gravitational collapse with an initial temperature of ~10¹¹K cools rapidly via neutrino emission from the core (Yakovlev & Pethick 2004). For about one minute following its birth, the star stays in a special proto-neutron star state: hot, opaque to neutrinos and larger that an ordinary neutron star (Pons et al. 2001). Later the star becomes transparent to neutrinos generated in its interior and transforms into an ordinary neutron star, where its temperature is independent of the structure (Shapiro & Teukolsky 1983).

The cooling of neutron stars is affected by many factors. Arguably the most important factor is the rate of neutrino emission from the interior of the neutron star (Yakovlev & Pethick 2004). Supplemented to this is heat capacity of the stellar interior and its associated thermal conductivity (Gnedin et al. 2001) as well as reheating mechanisms such as the frictional dissipation of rotational energy (Glen & Sutherland 1980).

Fig.1: Left ~ Theoretical considerations of a proton superfluid crust. Right ~ Temperature of neutron star surfaces as a function of age. (Credit: Yakovlev, D. G. & Pethick, C. J. 2004 Neutron Star Cooling. A&A, 42 pp.169-210.)

At a distance of 117pc RX J185635-3754 is one of the nearest neutron stars to us (Walter et al. 1996). Its identification was based on the thermal spectrum at a temperature of 57eV and the low luminosity of the extremely faint (25.7 mag) optical counterpart. Its associated spectrum can be akin to a pure blackbody spectrum at a temperature of 61eV. An altogether different object is 1E 1207.4-5209, an object located at the centre of the supernova remnant PKS 1209-52.  As opposed to RX J185635-3754, 1E 1207.4-5209 does show significant deviations from a standard black body spectrum.

So why the difference within the same class of compact objects that appear to have the same temperature? The answer probably, although this theory is untested, is quite simple. The spectrum from RX J1856-37 is the pure blackbody spectrum expected from a neutron star, whereas the light from 1E 1207.4-5209 has to escape via the gases and dust of the supernova remnant PKS 1209-52. What must also be taken into consideration is the magnetic structure, which will vary somewhat between different neutron stars (NS).

Since the age of a neutron star within a supernova remnant is sometimes well determined through the historical record these objects make good calibrators for comparison with cooling calculations (Seward & Charles 2010). Modeling of the complex atmosphere is important and, hence, there are many processes and properties that need to be taken into consideration in understanding the cooling rates of NS. In the future, as observational data supplements the wealth of theories surrounding NS structure, the cooling rates of neutron stars can be better appreciated.

Journal References:

• Drake, J.J. et al. (2002) Is RX J1856.5-3754 A Quark Star? The Astrophysical Journal, Volume 572,(2) pp.996-1001.

• Bignami, G. F. et al. (2004) 1E1207.4-5209 – A Unique Object. Memorie Della Società Astronomica Italiana, 75 (1) pp.448-454.

• Yakovlev, D. G.; Pethick, C. J. (2004) Neutron Star Cooling. Astronomy & Astrophysics, 42 (1) pp.169-210.

The Perseus Cluster (Abell 426): Cavities & Bubbles

The Perseus Cluster, designated as either A426 or Per X-1, is the brightest X-ray cluster in the sky (Edge, Stewart & Fabian 1992). The vastness of this cluster is such that it can be described as one of the most massive objects in the known universe, containing thousands of galaxies immersed in a vast cloud of immensly hot ionized gases; known as a plasma (Edge 2001). The cluster contains the radio source 3C 84 that is currently blowing bubbles of relativistic plasma into the core of the cluster (Conselice et al. 2001; Wilman et al. 2005; Sanders et al. 2005). These are seen as holes in an X-ray image of the cluster, as they push away the X-ray emitting gas. They are known as radio bubbles, because they appear as emitters of radio waves due to the relativistic particles from within a hollow spherical structure. The galaxy NGC 1275 is located at the centre of the cluster, where the X-ray emission is brightest (Fabian et al. 2000).

Fig.1: Left ~ Perseus X-Ray image with contours of constant radio surface brightness superposed. The radio fills the two cavities to the North and South-West of the central AGN. (Fabian et al. (2001) 318: pp.65-68). Right ~ A Chandra ACIS Observation of the centre of the Perseus Cluster. Prominent features include the central AGN engine and cavities to the North, South-West and North-West of the centre. (Credit: NASA/CXC/IoA).

The active galaxy NGC 1275 resides at the centre, and the intercluster medium (ICM) shows evidence that the black hole at the centre has been periodically depositing energy into the surrounding medium of gases (Seward & Charles 2010). This deposition is so large that it has been observed to offset radiative cooling: in a sense it is too hot to cool down.

There are two cavities in the ICM medium to the north and south/south-west of NGC 1275. Figure 1 overlays X-ray and radio images and shows that the X-Ray cavities are filled with radio-emitting relativistic particles (Fabian et al. 2000; Salomé et al. 2011). It can therefore be inferred from this that accretion onto the central black hole has produced jets of material and electromagnetic energy, inflating these bubble like structures. It has been theorised that in the future these bubbles will become more buoyant than the surrounding ICM and begin a journey outwards away from the central AGN (Hatch et al. 2006).

However, the bubble to the north-west of NGC 1275, with a lack of radio emission, does not contain any relativistic particles. It is therefore assumed that this is a ghost bubble of some sort that is much older than the other two that has perhaps cooled down as the thermal conduction of the surrounding ICM is very high (Seward & Charles 2010).

This is theorised by Fabian et al. (2006) to be due a cyclic process where a cooling flow in the ICM produces cold gas and stars which sink in towards the central region of NGC 1275. Some of this material then finds it way towards the central black hole, accreted and flung out as jets pumping huge amounts of energy into the surrounding ICM. This heating then turns of the cooling effect and therefore no more material is being accreted onto the black hole. Thus, cooling flows can be turned off and regulated periodically (Fabian et al. 2006; Tucker et al. 2007).

These observations provide astrophysicists with means to understanding the evolution of structures within galaxy clusters, as the are not confined to the Perseus Cluster. The largest deposition of energy observed thus far is within the cluster MS 0735.6+7421 which also exhibits this bubble and cavity behavior. Thus, understanding the Perseus Cluster will provide a dramatic picture of the relations between AGNs within clusters and the surrounding ICM.

Journal References:

• Edge A.C.; Stewart G.C.; Fabian A.C. (1992) Properties Of Cooling Flows In A Flux Limited Sample Of Clusters Of Galaxies. Monthly Notices Royal Astronomical Society, 258 (1) pp. 177-188.

• Hatch, N.A. et al. (2006) On The Origin & Excitation Of The Extended Nebula Surrounding NGC1275. Monthly Notices Royal Astronomical Society, 367 (2) pp. 433-448.

• Salomé, P. et al. (2011) A Very Extended Molecular Web Around NGC 1275. Astronomy & Astrophysics, 531 (1) Article I.D.: 85.

Suggested Further Reading:

• Seward, F.D.; Charles, P.A. (2010) Exploring The X-Ray Universe: 2nd Edition. Cambridge University Press, Cambridge: pp. 319-320.