Gaia BH2

{{Short description|Binary system in Centaurus}} {{Starbox begin | name = Gaia BH2 }} {{Starbox image | image = [[File:Gaia BH2 decaps dr2.jpg|250px]] | caption = [[Dark_Energy_Survey#Other_surveys_using_DECam|DECaPS]] image of Gaia BH2 (star at the center) }} {{Starbox observe | epoch = [[J2000]] | constell = [[Centaurus]]<ref name=constellation/> | ra = {{RA|13|50|16.748}}<ref name="GaiaDR3"/> | dec = {{DEC|-59|14|20.33}}<ref name="GaiaDR3"/> | appmag_v = 12.711<ref name="ucac4"/> }} {{Starbox character | component1 = Red giant | type = [[Red giant branch]]<ref name="GaiaDR3"/> | class = | component2 = Black hole | type2 = [[Stellar black hole]]<ref name="elbadry"/> }} {{Starbox astrometry | radial_v = {{val|-5.04|2.08}}<ref name="GaiaDR3"/> | prop_mo_ra = {{val|−10.48|0.1}} | prop_mo_dec = {{val|−4.61|0.06}} | pm_footnote = <ref name="elbadry"/> | parallax = 0.859 | p_error = 0.018 | parallax_footnote = <ref name="elbadry"/> | absmag_v = | absmag_bol = }} {{Starbox orbit | reference = <ref name="elbadry"/> | period_unitless = {{val|fmt=commas|1276.7|0.6|ul=d}} | axis_unitless = {{val|4.96|0.08|ul=AU}} | eccentricity = {{val|0.5176|0.0009}} | inclination = {{val|34.87|0.34}} | node = {{val|266.9|0.5}} | periastron = {{val|2457438.3|1.4}} | periarg = {{val|130.9|0.4}} | k1 = {{val|25.23|0.04}} }} {{Starbox detail | source = | component1 = Red giant | mass = {{val|1.17|0.08}}<ref name=Daniel2025/> | radius = {{val|8.55|0.20|0.15}}<ref name=Daniel2025/> | luminosity_bolometric = {{val|24.6|1.6}}<ref name="elbadry"/> | gravity = {{val|2.71|0.24}}<ref name="elbadry"/> | temperature = {{val|fmt=commas|4604|87}}<ref name="elbadry"/> | metal_fe = {{val|−0.22|0.02}}<ref name="elbadry"/> | rotation = {{val|398|5}}<ref name=Daniel2025/> days (preliminary estimate) | rotational_velocity = <1.5<ref name="elbadry"/> | age_gyr = {{Val|5.1|1.2|1.8}}<ref name=Daniel2025/> | component2 = Black hole | mass2 = {{val|8.94|0.34}}<ref name="elbadry"/> }} {{Starbox catalog | names = Gaia BH2, [[UCAC4]] 154-126202, [[2MASS]] J13501675-5914203, [[Gaia DR3]] 5870569352746779008 }} {{Starbox reference | Simbad = UCAC4+154-126202 }} {{Starbox end}}

'''Gaia BH2''' (Gaia DR3 5870569352746779008) is a [[binary system]] consisting of a [[red giant]] and a [[stellar-mass black hole]]. Gaia BH2 is located about 3,800 [[light year]]s away ({{val|1.16|ul=kpc}} away) in the constellation of [[Centaurus]], making it {{as of|2024|lc=y}} the [[List of nearest known black holes|third-closest known]] black hole system to Earth. Gaia BH2 is the second black hole discovered from [[Gaia DR3]] astrometric data.<ref name="elbadry"/>

The black hole and red giant orbit the system [[barycentre]] every 1,277 days, or around 3.5 years, with a moderate eccentricity of 0.518. The black hole's mass is around {{Solar mass|8.94|link=y}}, which means its [[Schwarzschild radius]] should be about {{convert|26.4|km|mi|abbr=on}}.<ref name="elbadry" /> The red giant has a mass of {{Solar mass|1.17|link=y}} and a radius of {{Solar radius|8.6|link=y}}.<ref name=Daniel2025/> Its temperature is estimated at {{convert|4604|K|C F|lk=on}}.<ref name="elbadry" /> The star is enriched in [[alpha elements]], and thus is believed to have undergone mass transfer with another star.<ref name=Daniel2025/>

== Discovery == Gaia BH2 was originally discovered as a black hole binary candidate in 2022, found via [[Astrometry|astrometric]] observations with [[Gaia (spacecraft)|Gaia]], along with [[Gaia BH1]]. At that time it was not clear if Gaia BH2 did definitely harbour a black hole, but it was the only plausible candidate in the Gaia data other than Gaia BH1.<ref name=eb2023/><ref name="Tanikawa2022"/> Later radial velocity observations confirmed this black hole system and refined its orbital parameters.<ref name="elbadry" />

== See also == * [[List of nearest known black holes]] * [[Gaia BH1]] * [[Gaia BH3]]

== References == <references>

<ref name=constellation>{{cite constellation|UCAC4 154-126202}}</ref>

<ref name=eb2023>{{Cite journal |last1=El-Badry |first1=Kareem |last2=Rix |first2=Hans-Walter |last3=Quataert |first3=Eliot |last4=Howard |first4=Andrew W. |last5=Isaacson |first5=Howard |last6=Fuller |first6=Jim |last7=Hawkins |first7=Keith |last8=Breivik |first8=Katelyn |last9=Wong |first9=Kaze W. K. |last10=Rodriguez |first10=Antonio C. |last11=Conroy |first11=Charlie |last12=Shahaf |first12=Sahar |last13=Mazeh |first13=Tsevi |last14=Arenou |first14=Frédéric |last15=Burdge |first15=Kevin B. |date=2023-01-01 |title=A Sun-like star orbiting a black hole |bibcode=2023MNRAS.518.1057E |arxiv=2209.06833 |journal=Monthly Notices of the Royal Astronomical Society |volume=518 |issue=1 |pages=1057–1085 |doi=10.1093/mnras/stac3140 |doi-access=free |issn=0035-8711}}</ref>

<ref name=ucac4>{{Cite journal |last1=Zacharias |first1=N. |last2=Finch |first2=C. T. |last3=Girard |first3=T. M. |last4=Henden |first4=A. |last5=Bartlett |first5=J. L. |last6=Monet |first6=D. G. |last7=Zacharias |first7=M. I. |date=2012-07-01 |title=VizieR Online Data Catalog: UCAC4 Catalogue (Zacharias+, 2012) |bibcode=2012yCat.1322....0Z |journal=VizieR Online Data Catalog |pages=I/322A}}</ref>

<ref name=Daniel2025>{{Cite arXiv |last1=Hey |first1=Daniel |last2=Li |first2=Yaguang |last3=Ong |first3=Joel |title=Asteroseismology of the red giant companions to Gaia BH2 and BH3 |date=2025-03-12 |class=astro-ph.SR |eprint=2503.09690}}</ref>

<ref name=elbadry>{{Cite journal |last1=El-Badry |first1=Kareem |last2=Rix |first2=Hans-Walter |last3=Cendes |first3=Yvette |last4=Rodriguez |first4=Antonio C. |last5=Conroy |first5=Charlie |last6=Quataert |first6=Eliot |last7=Hawkins |first7=Keith |last8=Zari |first8=Eleonora |last9=Hobson |first9=Melissa |last10=Breivik |first10=Katelyn |last11=Rau |first11=Arne |last12=Berger |first12=Edo |last13=Shahaf |first13=Sahar |last14=Seeburger |first14=Rhys |last15=Burdge |first15=Kevin B. |date=2023-02-01 |title=A red giant orbiting a black hole |bibcode=2023MNRAS.521.4323E |arxiv=2302.07880 |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=521 |issue=3 |pages=4323–4348 |doi=10.1093/mnras/stad799|doi-access=free }}</ref>

<ref name="Tanikawa2022">{{cite journal |last1=Tanikawa |first1=Ataru |last2=Hattori |first2=Kohei |last3=Kawanaka |first3=Norita |last4=Kinugawa |first4=Tomoya |last5=Shikauchi |first5=Minori |last6=Tsuna |first6=Daichi |display-authors=2 |arxiv=2209.05632 |title=Search for a Black Hole Binary in Gaia DR3 Astrometric Binary Stars with Spectroscopic Data |date=September 2022 |journal=[[The Astrophysical Journal]] |volume=946 |issue=2 |pages=79 |doi=10.3847/1538-4357/acbf36 |bibcode=2023ApJ...946...79T |doi-access=free }}</ref>

</references>

[[Category:Centaurus]] [[Category:Red giants]] [[Category:Stellar black holes]] [[Category:Astrometric binaries]] [[Category:Astronomical objects discovered in 2023|20230215]]